Ciência e Agrotecnologia 41(4):347-358, Jul/Aug. 2017
http://dx.doi.org/10.1590/1413-70542017414000217
REVIEW
Date palm micropropagation: Advances and applications
Micropropagação de tamareira: Avanços e aplicações
Jameel Mohammed Al-Khayri1*, Poornananda Madhava Naik1
1
King Faisal University/KFU, College of Agriculture and Food Sciences, Department of Agricultural Biotechnology, Al-Hassa, Saudi Arabia
Corresponding author: jkhayri@kfu.edu.sa
Received in June 15, 2017 and approved in July 28, 2017
*
ABSTRACT
Date palm (Phoenix dactylifera L.) is a fruit tree resilient to adverse climatic conditions predominating in hot arid regions of the Middle East
and North Africa. The date fruit contains numerous chemical components that possess high nutritional and medicinal values. Traditional
propagation by offshoots is inefficient to satisfy current demands for date palm trees. Alternatively, micropropagation provides an efficient
means for large-scale propagation of date palm cultivars. Both somatic embryogenesis and organogenesis, either directly or indirectly though
the callus phase, have been demonstrated in date palm in vitro regeneration. Culture initiation commonly utilizes shoot-tip explants isolated
from young offshoots. Recently, the immature inflorescences of adult trees were utilized as an alternative nondestructive source of explants.
In addition to the nature of the explant used, successful plant regeneration depends on the cultivar, composition of the culture medium and
physical status. Challenges of date palm micropropagation include long in vitro cycle, latent contamination, browning, somaclonal variation as
well as ex vitro acclimatization and transplanting. A remarkable amount of research investigating these factors has led to optimized protocols
for the micropropagation of numerous commercially important cultivars. This has encouraged the development of several international
commercial tissue culture laboratories. Molecular characterization provides an assurance of genetic conformity of regenerated plantlets, a
key feature for commercial production. This article describes date palm micropropagation protocols and also discusses recent achievements
with respect to somaclonal variation, molecular markers, cryopreservation and future prospects.
Index terms: Cryopreservation; somatic embryogenesis; somaclonal variation; organogenesis; molecular marker.
RESUMO
A tamareira (Phoenix dactylifera L.) é uma arvore frutífera adaptada à condições climáticas adversas predominantemente em regiões áridas
do Oriente Médio e Norte Africano. As tâmaras possuem vários componentes químicos com alto valor medicinal e nutricional. A propagação
tradicional por estacas não é suficiente para satisfazer a demanda por mudas e assim, a micropropagação apresenta-se como uma alternativa
eficiente para a produção de mudas em larga escala. Embriogênese somática e organogênese, tanto direta quanto indireta via calos, tem
sido usada para obter a regeneração in vitro de tamareira. O inicio do cultivo in vitro normalmente utiliza meristemas excisados de brotações
jovens. Recentemente, inflorescências imaturas de árvores adultas são usadas como fonte alternativa de explantes não destrutiva. Além
da origem do explante, o sucesso da regenerção depende do cultivar, da composição do meio de cultura e de condições físicas. Desafios
na micropropagação de tamareira incluem um longo ciclo in vitro, contaminação, escurecimento do tecido, variação somaclonal além
do enraizamento e aclimatização ex vitro. Diversos estudos investigando esses fatores tem conduzido à otimização de protocolos de
micropropagação de inúmeros cultivares comerciais proporcionando o estabelecimento de vários laboratórios de cultura de tecidos de
plantas. A caracterização molecular permite uma segura conformidade genética do material regenerado, considerado uma característica
chave na produção comercial. Essa revisão descreve protocolos de micropropagação de tamareira e aborda as mais recentes conquistas
relacionadas à variação somaclonal, marcadores moleculatres, criopreservação e perspectivas futuras.
Termos para indexação: Criopreservação; embriogênese somática; variação somaclonal; organogênese; marcador molecular.
INTRODUCTION
Date palm (Phoenix dactylifera L.) is a ‘tree of life’
which belongs to the family Arecaceae; it is distributed
mainly in the Middle East and North Africa. Over the last
century, date palm agriculture has spread to Australia,
Southern Africa and the Americas. The distribution of date
palm is very peculiar because of the inherent requirement
for hot climate which is necessary for successful pollination
and fruit setting (Chao; Krueger, 2007). Date palm
cultivation has been recorded for thousands of years and
over that time selection procedures have developed more
than 3,000 known varieties of date palm around the world
(Johnson, 2011). Date fruits possess important nutritional
and medicinal values and as a part of religious practice, dates
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348
are consumed by people all over the world (Vayalil, 2012).
Dates are a good source of essential minerals, which enhance
their nutritional value (Al-Shahib; Marshall, 2002, 2003;
Elleuch et al., 2008). The aqueous extracts of dates possess
antioxidant activity (Al-Farsi et al., 2007; Biglari; AlKarkhi;
Easa, 2008; Saafi et al., 2009). Phenolic compounds present
in dates, have beneficial effects on human health and act
against cancer and cardiovascular diseases (Vayalil, 2012).
Dates are a high energy source consisting of carbohydrates
44-88% in addition to proteins 2.3-5.6% and fats 0.2-9.3%
(El Hadrami; El Hadrami, 2009). Palm syrup, palm sugar,
vinegar, wine and honey are made from the date fruits
(Chao; Krueger, 2007). The annual international market
value (including import and export) of the date crops
reached nearly 1.9 billion USD (FAOSTAT, 2013), from
which a country can build a strong economic platform. The
vegetative part of the date tree is used as a raw material for
roof coverings for houses, wooden boats, timber for wood
industry and making numerous handicrafts (Al-Khayri; Jain;
Johnson, 2015a, 2015b).
In recent years, because of overexploitation,
the diversity of the date palm groves has declined. The
production and utilization of the date fruits also varies
from country to country due to the influence of current
environmental conditions. There are a number of elements
which hinder the production of date palm, such as major
pests and diseases, salinity and drought, poor harvest
and postharvest practices (Al-Khayri; Jain; Johnson,
2015a, 2015b). Date palm is conventionally propagated
vegetatively by offshoots. Due to their heterozygous
nature, seeds cannot be used for the propagation of
commercially elite cultivars because they produce off-type
propagules (Abahmane, 2011). The number of offshoots
produced by a date palm tree during their life span is only
about 20-30 (Zaid; El-Korchi; Visser, 2011). The survival
rate of offshoots in the field is low and chances of disease
transmission are high (Abahmane, 2011). These factors
hinder meeting the increasing agro-industrial demand
for propagules. The use of micropropagation techniques
eliminates these restrictions and allows for large-scale
production of healthy, disease-free, true-to-type plants.
Since the early reports of date palm in vitro
regeneration (Tisserat, 1979, 1982) numerous researchers
have described different approaches to achieve date
palm micropropagation: somatic embryogenesis (AlKhayri, 2003, 2005; Fki et al., 2003, 2011a; Mazri et
al., 2017; Naik; Al-Khayri, 2016; Othmani et al., 2009a;
Roshanfekrrad et al., 2017) and organogenesis (Bekheet,
2013; Jazinizadeh et al., 2015; Khan; Bi, 2012; Khierallah;
Bader, 2007; Meziani et al., 2015, 2016). In addition to
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AL-KHAYRI, J. M. and NAIK, P. M.
providing a means for rapid clonal propagation of date
palm (Khierallah; Bader, 2007), tissue culture techniques
can be utilized for the production of synthetic seeds
(Bekheet et al., 2002), cell suspension culture (Othmani et
al., 2009b), cryopreservation (Fki et al., 2013), somaclonal
variation introduces stress tolerant, disease resistance and
with high quality fruits (El Hadrami; El Hadrami, 2009;
Jain, 2001), the production of secondary metabolites (ElSharabasy, 2004) and commercial production of date palm
gives considerable profit to both public-sector and private
agencies (Zaid; El-Korchi; Visser, 2011).
The present article gives an update of the current
approaches of date palm micropropagation with emphasis on
the plant regeneration through somatic embryogenesis and
organogenesis and use of a bioreactor. It highlights key factors
that influence in vitro differentiation and culture growth
including tissue browning, hyperhydricity, explant type,
light and composition of culture medium. Also addressed
are concerns related to potential somaclonal variation,
molecular detection techniques, utilization of tissue culture
in germplasm conservation and commercial production.
MICROPROPAGATION
The decades of efforts on date palm micropropagation
by scientists achieved the goal of using different parameters
which affect the in vitro development of date palm such as
explant source (Figure 1A), age of explant, size of explant,
intensity and quality of light, temperature, pH of the medium,
plant hormones, culture medium and age of culture (AlKhayri, 2013; Mazri, 2015; Mazri; Meziani, 2015; Mazri et
al., 2016). This is an efficient alternative to the conventional
means of propagation. Somatic embryogenesis and
organogenesis are the two main modes of micropropagation
of date palm, which are widely accepted throughout the tissue
culture arena in the world (Bhansali, 2010). The utilization of
bioreactors is also making a significant contribution toward
micropropagation of different plant species (Frómeta et al.,
2017; Gomes et al., 2016; Othmani et al., 2011).
Somatic embryogenesis
Somatic embryogenesis can be defined as the
development of somatic embryos from somatic cells, which
undergo a series of morphological and biochemical changes
(Quiroz-Figueroa et al., 2006). Somatic embryogenesis
includes a series of stages viz. embryogenic callus initiation,
formation of somatic embryo, development of somatic
embryo, maturation and plantlet formation. In date palm
micropropagation somatic embryogenesis is considered
as the most potent tool to achieve large-scale production.
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Date palm micropropagation: Advances and applications
Figure 1: Micropropagation of date palm. (A) Shoot tip culture, (B) Embryogenic callus induction, (C) Embryogenic
callus proliferation, (D) Embryo formation, (E) Rooting and (F) Transplanted plant.
Embryogenic callus initiation and embryo formation
The embryogenic calli initiation process in the
date palm is very slow; this is mainly due to the tree
nature of the plant. The embryogenic callus initiation
and proliferation (Figure 1B and 1C) process depends
on various parameters like the type of genotype, explant
source, plant hormones, and culture condition. To induce
embryogenic callus in most of the cases for either shoot
tip or inflorescence, a high concentration of auxins is used.
Most researchers suggest 2,4-dichlorophenoxyacetic acid
(2,4-D) as the most efficient auxin to induce embryogenic
callus used at 100 mg L-1 concentration (Al-Khayri, 2005;
Al-Khayri, 2010; Eshraghi; Zaghami; Mirabdulbaghi,
2005; Naik; Al-Khayri, 2016). Fki et al. (2011a) reported
that high concentrations of 2,4-D causes somaclonal
variations. Many researchers applied low concentration
of 2,4-D such as 1.5, 5 and 10 mg L-1 to induce somatic
embryo (Aslam et al., 2011; El Hadrami; Cheikh; Baaziz,
1995; Othmani et al., 2009a). Depending on the genotype
the somatic embryo formation (Figure 1D) period also
varies from a few to several months (Eshraghi; Zaghami;
Mirabdulbaghi, 2005; Hassan; Taha, 2012; Othmani et
al., 2009a). Recently Mazri et al. (2017) reported somatic
embryogenesis from the adventitious bud of date cv.
Najda, where the MS medium was supplemented with
2,4-D, kinetin (KN) or 6-dimethylallylamino purine (2iP).
Proximal leaf segment showed embryogenesis only in the
medium supplemented with 2,4-D or picloram.
Development of somatic embryo and maturation
Various factors are responsible for the development
of somatic embryo and maturation. Date palm has the
capacity to develop as a mature embryo in both semisolid
and liquid medium. The authors reported in the date
palm cv. Deglet Nour, 1-month old suspension culture
produces 200 embryos from 100 mg fresh weight callus
inoculum, on the other hand it produces 10 embryos on
semisolid medium (Fki et al., 2003). In the date cv. Khalas,
application of thiamine and biotin showed an increased
number of embryos and also helps in the elongation
process (Al-Khayri, 2001). The embryogenic calli with fine
chopping and partial desiccation significantly improves
the embryo maturation in cv. Boufeggous as reported by
Othmani et al. (2009a).
Plantlet formation
Somatic embryo germination or plantlet formation
depends on different factors. The size of the somatic
embryos also varies the germination percentage in date
palm (Al-Khayri; Al-Bahrany, 2012). Al-Khayri (2003)
also reported that the somatic embryo germination is
influenced by the strength of the medium and concentration
Ciência e Agrotecnologia 41(4):347-358, Jul/Aug. 2017
350
of the 1-naphthalenacetic acid (NAA) and indole-3-butyric
acid (IBA) used. Somatic embryo germination with 81%
plantlet conversion was achieved with 1 mg L-1 NAA
concentration (Othmani et al., 2009b). The hormone
combination of 6-benzylamino purine (BAP), IBA and
NAA also influences the plantlet formation or rooting
(Figure 1E) in dates (Zouine; El Hadrami, 2007). However,
Mazri et al. (2017) achieved somatic embryo germination
in hormone-free MS medium.
Acclimatization of plantlet
The success of acclimatization is hidden in the nature
of the substrate which should be high in organic matter,
possess optimal water-holding capacity and proper aeration
attributes (Hegazy, 2008). Plantlets derived from date palm
somatic embryos have been successfully acclimatized in
many cultivars. Kurup et al. (2014) found nearly 60%
survival rate of cv. Kheneizi when transferred to pots with
a peat/vermiculite mixture of 2:1. Othmani et al. (2009a,
2009b) reported 60 and 80% survival rates in date palm
cvs. Boufeggous and Deglet Nour, respectively. Al-Khayri
(2010) observed 72-84% survival rate in cvs. Khasab and
Nabout Saif plantlets after ex vitro transfer (Figure 1F).
Organogenesis
AL-KHAYRI, J. M. and NAIK, P. M.
with 1 mg L-1 BAP, 2 mg L-1 2iP, 1 mg L-1 NAA and 1 mg
L-1 2-naphthoxyacetic acid (NOA) (Khierallah; Bader,
2007). Bekheet (2013) reported that MS medium fortified
with 2 mg L-1 2iP and 1 mg L-1 NAA promotes buds in cv.
Zaghlool, while Al-Mayahi (2014) used cv. Hillawi which
induced buds in the MS medium supplemented with 1 mg
L-1 BAP and 0.5 mg L-1 thidiazuron (TDZ).
Multiplication of shoot buds
Shoot bud multiplication is influenced by various
factors such as medium composition, genotype and plant
hormones. Khierallah and Bader (2007) mentioned that
the date palm cv. Maktoom showed higher shoot-bud
multiplication in MS medium with a hormone combination
of 1 mg L-1 NAA, 1 mg L-1 NOA, 4 mg L-1 2iP and 2 mg L-1
BAP. Mazri and Meziani (2013) found that half-strength
MS medium augmented with 0.5 mg L-1 NOA and 0.5 mg
L-1 KN produced 23.5 shoot buds per explant after 3 months
of multiplication in cv. Najda. Al-Mayahi (2014) reported
production of an average of 18.2 buds per culture in cv.
Hillawi, in the MS medium containing 1 mg L-1 BAP and
0.5 mg L-1 TDZ.
Shoot Elongation and rooting
Organogenesis refers to the development of organ
tissue with a vascular connection and finally which influences
the plant formation from an explant with or without the
intermediate callus stage. Date palm micropropagation
favors the direct organogenesis of an explant without callus
formation stage. To produce elite cultivars from rapid clonal
propagation, direct organogenesis techniques have been
widely adopted (Khierallah; Bader, 2007). There are various
steps involved in date palm organogenesis: adventitious bud
formations, multiplication of shoot bud, shoot elongation,
and rooting (Abahmane, 2011; Bekheet, 2013; Mazri;
Meziani, 2013, 2015).
The medium with or without hormone promotes
shoot elongation and rooting in date palm. Mazri and
Meziani (2013) found that in the cv. Najda shoot elongation
was faster in the medium supplemented with hormones
when compared to hormone free medium, which also
adds high frequency of root formation, the hormone free
medium showed wider and green leaves with optimum
survival rates. Bekheet (2013) suggested 1 mg L-1
NAA induces better and optimum rooting at the same
concentration IAA or IBA. Meziani et al. (2015) reported
cv. Mejhoul shoots grew an average of 13.4 cm with an
average 4.6 roots number per shoot with wide and green
leaves from 3 months old hormone-free half MS medium.
Adventitious bud formations
Acclimatization of plantlet
The formation of adventitious buds from date
palm explants depends on several factors. Based on
the findings of Al-Khateeb (2006) a high concentration
of plant hormones induces abnormal growth of the
tissue without bud formation, and low concentrations
influence the formation of adventitious buds. Many
researchers have found that different auxin and cytokinin
combinations promote bud formation in different date
palm cultivars. Cultivar Maktoom induced buds in the
MS medium (Murashige; Skoog, 1962) supplemented
Acclimatization of plantlets is the final step of
micropropagation. Mazri and Meziani (2013) achieved
good survival frequency in cv. Najda when a peat-gravel
mixture was used as the substrate. Meziani et al. (2015)
reported after 3 months of observation in the greenhouse
the cv. Mejhoul plantlets showed 88% survival rate. The
cv. Boufeggous shoots from semisolid medium, found the
highest survival rate up to 92.5% whereas the shoots from
stationary liquid media culture showed a survival rate of
50% after 3 months in the greenhouse (Mazri, 2015).
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Date palm micropropagation: Advances and applications
Bioreactor
In recent years micropropagation protocols have
used liquid media instead of semisolid media. The
production of somatic embryos in liquid culture systems
was 17-fold higher than on semisolid medium (Othmani
et al., 2011). The Temporary Immersion Bioreactor
(TIB) showed that shoot clusters were regenerated 5.5fold faster as compared to semisolid medium (Othmani
et al., 2011). The bioreactor system may be used in the
micropropagation optimization process, which needs
less manipulation of medium, reduces the cost of gelling
agents, experienced labor, laboratory space and induces
the multiplication frequency of the particular plant species
(Gomes et al., 2016). Recently, Frómeta et al. (2017)
successfully achieved the regeneration of the Barberton
daisy (Gerbera jamesonii) using TIB system. RamírezMosqueda and Iglesias-Andreu (2016) reported the
micropropagation of the vanilla orchid (Vanilla planifolia)
in different bioreactor systems such as TIB, Gravity
Immersion Bioreactors (GIB) and Recipient for Automated
Temporary Immersion (RITA). The high number of
shoots observed in the TIB followed by RITA and GIB
and optimum rooting was found in TIB compared to GIB
and RITA. Gomes et al. (2016) evaluated the regeneration
of somatic embryos of oil palm (Elaeis guineensis) by
using semisolid, RITA and Temporary Immersion System
(TIS) and found TIS was most suitable for regeneration.
Whereas, the use of TIB system in date palm (cv. Deglet
Bey) in vitro regeneration was reported by Othmani et
al. (2009b, 2011). Improved embryogenic callus and
proliferation of regenerated shoots was observed in TIB
system compared to semi-solid medium.
MAJOR PROBLEMS AND FACTORS
AFFECTING THE DATE PALM
MICROPROPAGATION
In date palm micropropagation various problems
are encountered like tissue browning, hyperhydricity and
genetic variation. The major factors are as follows: nature
of the explant, light and medium composition used. The
problems and factors affecting micropropagation are
briefly discussed in this section.
Tissue browning
The browning of the explant is the most frequent
problem in date palm tissue culture which finally leads
to the death of the tissue. During micropropagation both
somatic embryogenesis (Abohatem; Zouine; El Hadrami,
2011) and organogenesis (Mazri, 2015) encounter this
problem. The reason is that date tissue contain high levels
of phenolic compounds which are toxic to the tissue and
finally causes its death (Loutfi; El Hadrami, 2005). During
the surface sterilization procedure, ascorbic acid and citric
acid are used to control browning in the explant tissue (AlKhayri, 2010; Khierallah et al., 2015; Naik; Al-Khayri,
2016). In order to prevent this browning many researchers
add activated charcoal and polyvinylpyrrolidone (PVP) in
the culture medium (Al-Khayri, 2010; Mazri; Meziani,
2013; Naik; Al-Khayri, 2016).
Hyperhydricity
Hyperhydricity refers to the accumulation of water
content in the cultured tissue. This physiological disorder is
common in somatic embryogenesis and organogenesis of
date palm (Mazri, 2015; Mazri; Meziani, 2013; McCubbin;
Zaid, 2007). Kriaa et al. (2012) reported when hydrated
calli were cultured on the low concentration of 2,4-D
for a long period, calli will induce somatic embryos or
shoots. The factors such as plant hormones, liquid media
and concentration of ammonia used are responsible for
hyperhydricity (Al-Khateeb, 2008; Mazri; Meziani, 2015).
The use of high agar concentration in the semisolid media
and low concentration of plant hormones and ammonium
concentration will help in the reduction of formation of
hyperhydricity in the cultured tissue (Al-Khateeb, 2008).
Nature of explant
In the beginning era of date palm tissue culture,
researchers used different kind of explants: embryos,
immature fruits, roots, leaf petioles, lateral buds,
shoot tips, pieces of stem and rachilla tissue (Sharma;
Deepak; Chowdhury, 1986; Tisserat, 1979). After
decades of research it has been proved that date palm
micropropagation is most responsive with explants having
meristematic origin and these includes apical shoot tips,
lateral buds and leaf primordial isolated from shoot tip
(Aslam et al., 2011; Khan; Bi, 2012). The meristematic
cells or tissue respond quickly to the components of the
culture medium. However, Khan and Bi (2012) reported
the multiplication of date palm cv. Dhakki through direct
shoot regeneration where different explants such as shoot
tips, leaf primordia and apical meristem were used, among
these shoot tips emerged as a most promising explant with
highest capacity for direct shoot regeneration. In recent
years use of the inflorescence as an explant has increased,
where harvesting of the offshoots is not required.
Micropropagation from inflorescence was achieved in date
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352
palm cvs. such as Gulistan, Aseel, Dedhi, Gajar, Kashoowari, Khar, Kharbline, Khormo, Deglet Noor, Blombek,
Menakher and Barhee (Abahmane, 2013; Abul-Soad,
2012; Abul-Soad; Mahdi, 2010; Kriaa et al., 2012).
Effect of light
Light serves as an external factor to regulate the
growth and development of in vitro plants. The broad
spectrum of fluorescent lamps with a wavelength range
from 380-750 nm are used as a light source for tissue
culture (Kim et al., 2004). Light intensity and type of light
affect date palm micropropagation (Al-Mayahi, 2016;
Meziani et al., 2015). Meziani et al. (2015) evaluated the
organogenesis of date palm cv. Mejhoul using different
levels of light intensities. The results explained that the
2000-3000 lux light intensity enhances shoot elongation
and greening but reduction in the shoot bud proliferation
was observed. Darkness and low light intensity (500 lux)
significantly induced advanced rooting. The 1000 lux light
intensity during the multiplication stage showed optimal
growth of culture with respect to shoot buds per explant,
greening an advanced rooting. Recently Al-Mayahi (2016)
conducted an experiment to test the effect of combinations
of red + blue light emitting diode (18:2) (CRB-LED)
and white fluorescent light on direct organogenesis
by induction of adventitious buds from shoot tip and
multiplication shoots of date palm cv. Alshakr. The results
showed CRB-LED performed better in the production of
shoot numbers than white fluorescent light. CRB-LED
also significantly increased the total soluble carbohydrate,
starch, free amino acids, peroxidase activity, potassium,
magnesium and sodium contents of the in vitro shoots.
Medium composition
Tissue culture media is composed of a wide range
of micro and macro elements, these compositions of
basal media regulates the growth and development of
tissue in the in vitro culture. The date palm tissue culture
also showed different growth patterns with respect to the
various media composition (Al-Khayri, 2011). Al-Khayri
(2011) tested five different types of media such as MS
medium, W medium (White, 1963), NN medium (Nitsch;
Nitsch, 1969), SH medium (Schenk; Hildebrandt, 1972),
Woody Plant Medium, WPM (Lloyd; McCown, 1981) to
evaluate the callus growth and somatic embryogenesis.
Optimal callus growth was attained in cv. Barhee using SH,
W and MS media, cv. Berny using SH and NN medium,
in cv. Khusab using W and WPM media. The best embryo
production was seen in cv. Barhee on W and SH media, cv.
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AL-KHAYRI, J. M. and NAIK, P. M.
Berny using WPM and MS media and cv. Khusab using
W and SH media. The optimum regeneration percentage
obtained in the cv. Barhee occurred on W and WPM media,
cv. Berny on WPM medium and cv. Khusab using W
medium. Mazri et al. (2016) evaluated the effect of various
concentrations of major mineral salts including ammonium
nitrate, potassium nitrate, calcium chloride dehydrate,
potassium dihydrogen phosphate and magnesium sulfate
heptahydrate on shoot bud proliferation of date palm cv.
Mejhoul, they also tested the different concentration of
L-glutamine and myo-inositol. The results showed that
maximum number of shoot buds per explant obtained on
the medium fortified with 825 mg L-1 NH4NO3, 1900 mg
L-1 KNO3, 220 mg L-1 CaCl2.2H2O, 170 mg L-1 KH2PO4,
370 mg L-1 MgSO4.7H2O as well as 1 g L-1 L-glutamine,
2 g L-1 myo-inositol and 30 g L-1 sucrose. The media
supplemented with 1650 mg L-1 NH4NO3 significantly
influenced the frequency of hyperhydricity. The results
also showed that the number of shoot buds per explant was
significantly affected by the concentration of L-glutamine,
and myo-inositol present in the medium.
In addition to the mineral salts in tissue culture
medium, the carbon source also plays an important role
in the tissue growth. In the tissue culture of different plant
species and also in the date palm the major carbon source
used is sucrose at 30 g L-1 (Al-Khayri, 2013). In addition to
the sucrose, other carbon source such as sorbitol, maltose,
mannitol or commercial granulated sugar are also being used
to improve the callus growth and shoot bud proliferation in
date palm (Al-Khayri, 2013; Mazri et al., 2016). Mazri et
al. (2016) evaluated the various concentrations of carbon
sources to improve the shoot bud multiplication in date
palm cv. Mejhoul. The experiments revealed the medium
supplemented with 30 g L-1 sucrose obtained the optimal
number of shoot buds per explant when compared to
sorbitol, mannitol and commercial granulated sugar. Low
and high concentrations of sucrose did not induce optimum
number of shoot buds. The superior morphology of the shoot
buds was generated from sucrose supplemented media when
compared to buds obtained from the media augmented with
sorbitol, mannitol and commercial granulated sugar.
VARIATION IN MICROPROPAGATED PLANTS
Somaclonal variation
The morphological and genotypical differences
observed in micropropagated plants are called somaclonal
variations. Sometimes micropropagated plants carry
advantageous characters with respect to commercial and
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Date palm micropropagation: Advances and applications
agronomical interest which finally leads to some new
varieties (El Hadrami et al., 2011). Somaclonal variation act
as a raw material for the mass propagation, cryopreservation
and synthetic seed production. Date palm micropropagation
and somatic variation introduces new genotypes with stress
tolerant, disease resistant and with high quality fruits (El
Hadrami; El Hadrami, 2009; Jain, 2001). Research has
showed that somaclonal variation frequency is depend on
the age of the tissue cultured date palm (Saker et al., 2000).
The high concentration of 2,4-D induced 25% somaclonal
variation and observed change in the leaf morphology and
also cause poor flower pollination leads to low quality date
fruit (Fki et al., 2011b). To reduce the risk of somaclonal
variation researchers suggest using juvenile explants,
low auxin concentration, especially 2,4-D and minimum
numbers of subcultures (El Hadrami et al., 2011; Fki
et al., 2011a, 2011b; Khan; Bi, 2012). The use of high
concentration of plant growth regulators induces somaclonal
variation in date palm (Al-Mazroui et al., 2007).
Molecular characterization
The date palm is heterozygous and its outbreeding nature creates progeny with 50% male and
50% female trees that are not true-to-type (Othmani et
al., 2010). In commercial micropropagation it is very
important to test the genetic conformity/true-to-type of
the regenerated plants. Several researchers have used
molecular markers (AFLP, ISSR and RAPD) to confirm
the genetic conformity of the tissue-cultured plants
(Kumar et al., 2010; Othmani et al., 2009c, 2010). The
use of RAPD primers showed micropropagated date cv.
Ferhi with high level of polymorphism and 37.8% of
variability (Moghaieb; Abdel-Hadi; Ahmed, 2011). The
variation in tissue-cultured plants is also observed in cvs.
Barhee and Khalas which display dwarfism and abnormal
flower development (Al-Kaabi; Zaid; Ainsworth, 2007;
Zaid; Al-Kaabi, 2003). Kumar et al. (2010) evaluated
27 micropropagated date plants with 160 RAPD and 21
ISSR primers, in which 30 RAPD and 12 ISSR primers
produced a total of 347 reproducible monomorphic band;
the results suggested that the micropropagated plants are
true-to-type. Aslam, Khan and Naqvi (2015) established
the somatic embryo derived regeneration protocol for the
six cvs. Barhee, Zardai, Khalasah, Muzati, Shishi and
Zart, in which the RAPD profile showed a similar banding
pattern for micropropagated plants and mother plants and
confirmed the genetic stability. Abass, Al-Utbi and AlSamir (2017) screened the genetic toxicity of hormones
on date palm callus using RAPD and protein profile.
Low concentration of 2,4-D and Dicamba showed no
polymorphism in RAPD primers and protein profile when
compared to the control profile. The high concentration of
hormones induced more polymorphism in both the tested
markers compared to control.
GERMPLASM CONSERVATION
Micropropagation methods help in the germplasm
conservation of many plant species. The disappearance of
date palm farms due to urbanization, and which also leads
in the declining of the genetic diversity of the date palm.
Shortage of arable land, adverse climatic conditions, diseases
and natural disasters are some major factors affecting the
diversity of date palm (Bekheet; Taha, 2013; Zaid et al.,
1999). There is a crucial need for the conservation of date
palm germplasm and better utilization of available genetic
resources. Although farmers are playing an important role
in the conservation of the date palm germplasm, through
cultivating different varieties in their traditional groves
(Bekheet; Taha, 2013), they tend to propagate a limited
number of commercially desired cultivars. The use of
modern techniques are necessary to conserve date palm
germplasm. The establishment of a germplasm bank based
on available in vitro technologies for this important species
is yet to be realized.
Cryopreservation
One of the modern conservation method is
cryopreservation, where genetic materials like shoot tips,
callus, cell suspension, microspore and somatic embryos are
stored long-term under super-low temperatures (-196 ºC
in liquid nitrogen). A genuine micropropagation method is
the primary requirement of cryopreservation of germplasm
and in vitro conservation. Over decades researchers have
standardized date palm micropropagation and started the
cryopreservation of date palm tissue (Bekheet et al., 2007).
Embryogenic calli of date palm were treated with cryogen
and stored for several months under ultra-low temperature
(-196 ºC). Tisserat (1982) reported date palm regeneration
from latent calli revived after 4-8 weeks. The successful
cryopreservation of date palm callus after 4 months at
-25 ºC was achieved by Mater (1987) by treating with
cryoprotectant mixture. The normal growth of date palm
embryos was achieved by Mycock et al. (1997) by pretreating the embryos with glycerol and sucrose and dried
to a water content of 0.4-0.7 g g-1. Bekheet, Taha and Saker
(2002b) reported a long-term preservation method for in
vitro shoot bud and callus culture with optimum percent of
viability after 12 months treatment at 5 ºC. Al-Bahrany and
Ciência e Agrotecnologia 41(4):347-358, Jul/Aug. 2017
354
AL-KHAYRI, J. M. and NAIK, P. M.
Al-Khayri (2012) described the germplasm conservation
by providing reliable cryoprotectant solution for date palm
(cv. Khalas) cell suspension. Fki et al. (2011b) reported the
successful cryopreservation method for proembryogenic
masses (PEMs) of date palm variety Barhee and the
morphology of the regenerated plants confirmed the stability
of the clonal material. Furthermore, Fki et al. (2013)
reported the regeneration of cryopreserved callogenic
meristems of date palm cv. Khenizi. Cryopreserved date
palm tissue with optimum regeneration capacity can boost
the micropropagation process.
COMMERCIAL PRODUCTION
Date palm micropropagation is an economical
means to achieve rural development (Rajmohan, 2011).
It has been proven that date palm micropropagation
protocols have been developed and achieved commercial
production (Hoop, 2000). Numbers of cultivars are being
developed and multiplied throughout the world. It has been
reported that United Arab Emirates University developed
micropropagation protocol for about 50 date palm cultivars
(Rajmohan, 2011). In India Atul Rajasthan Date Palms Ltd,
has mounted a project to grow tissue cultured plants in the
desert area using appropriate cultivars. As commercial
production of date palm involves considerable profit, a
number of private and public-sector agencies are involved
in developing protocols for micropropagation of date palm.
Some of the countries and their leading agencies/companies
are producing millions of micropropagated plants annually
are as follows: Marionnet G.F.A. in France, Palmdat in
France and Namibia, Atul Rajasthan Date Palms Ltd. in
India, Rahan Meristem in Israel, Domaine Agricole El
Bassatine in Morocco, Al Rajhi Tissue Culture Laboratory
in Saudi Arabia, Sapad Tissue Culture Date Palm Co. in
Saudi Arabia, United Arab Emirates University - Date
Palm Development Research Unit in U.A.E. and Date Palm
Developments in United Kingdom. Most of the institutions
and laboratories are focused on cv. Medjool because of its
high economic importance; recently cv. Barhee also has
been a focus for micropropagation, selling for USD 24-26
per plant. The price of the plant depends on the growth stage,
cultivar and quantity ordered.
time-consuming and tedious process. The application of
the tissue culture techniques gave date palm an improved
efficiency compared to other crops. Micropropagation
of date palm through somatic embryogenesis and
organogenesis reduces labor cost and time. Date palm
micropropagation depends on various factors which
include tissue browning, hyperhyricity, explant source,
age of explant, size of explant, cultivars, intensity
and quality of light, temperature, pH of the medium,
plant hormones, culture medium composition and
age of culture. The various researchers all over the
world are working on the factors which control date
palm micropropagation via somatic embryogenesis
and organogenesis. Optimization of all these factors
in a particular elite cultivar will enhance the chances
of biotechnological application such as somaclonal
variation selection, cryopreservation, synthetic seeds,
cell culture, protoplast cultures, haploid production,
mutation studies, secondary metabolite production, use
of bioreactor, assessment of genetic fidelity and genetic
transformation (Al-Khayri; Jain; Johnson, 2015a, 2015b;
Aslam; Khan; Azad, 2015; Jain; Al-Khayri; Johnson,
2011). Thus, there is a need for the optimization of date
palm micropropagation in elite cultivars to encourage
the development of commercial production of date via
tissue culture laboratories in various parts of the world.
ACKNOWLEDGEMENT
The authors are grateful to Prof. Dennis V.
Johnson (Cincinnati, Ohio, USA) for critical reading and
commenting on the manuscript.
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