Marker Assisted Selection for Striga Resistance

A B S T R A C T Sorghum, the second most important cereal crop in Kenya is often attacked by Striga hermonthica weed with grain yields being reduced up to 100%. In the marginal and semi-arid areas, there is urgent need to enhance the genetic resistance to Striga hermonthica in local varieties. The aim of this study was to introgress Striga resistance from a documented resistant donor line N13, into Ochuti, a susceptible farmer preferred variety through molecular marker assisted selection (MAS). Two backcross populations namely, BC2F1 and BC3F1 were generated by crossing N13, the donor parent to Ochuti, the recurrent parent line and the resultant backcrossed Striga resistant progenies were subjected to phenotypic selection initially. At the BC3F1 stage, foreground selection for the Striga resistance Quantitative Trait Loci (QTLs) was conducted through Polymerase Chain Reaction (PCR) and N13 and Ochuti alleles sized through capillary electrophoresis. Eleven polymorphic markers identified at least three Striga resistance QTLs, in five plants of BC3F1//F2 generations. Eight progenies from BC2F1 and BC3F1 backcross populations were evaluated in field trials under artificial Striga inoculation in two locations and for two seasons. The backcrossed genotypes with Striga resistance allowed fewer Striga plants to germinate though in certain cases Ochuti genotypes performed equally the same. Marker assisted Selection (MAS) can successfully be utilized to transfer Striga resistance QTLs from a resistant donor source to a susceptible sorghum variety but the transfer should be complimented by field evaluation of the resistant progenies under artificial Striga infestation over several seasons, locations and replications.

Sorghum grain is being used predominantly as a food grain crop and the lion share of the grain production is consumed at the household level. Ethiopia is demanding huge amount of malt barley for the national beer production; unfortunately, the production area is not proportionally increasing with demand and the supply of malt barley is by far lower than the demand. Currently, however, there is a room to accommodate market oriented activities such as the development of malt sorghum varieties. It has been speculated that the demand for malting type sorghum will be increased for the reason that sorghum is climate resilient crop and the area for barley production is not growing with malt demand. Hence, selecting genotypes meeting the specific malt quality to save foreign currency used to import malt barley and improve livelihood of sorghum farmers. There was an attempt to develop malt sorghum varieties by national sorghum improvement program. Currently, a variety called Debir, which was identified as a potential candidate for adaptation trial. This candidate variety has good grain yield potential with acceptable protein content which is really very important trait for beer making quality. Moreover, Meta brewery factory has been identified this candidate variety as suitable for malting and have great interest to use as an ingredient for the brewery industry in Ethiopia. Therefore, in 2017 crop season the grain yield performance of the variety has been tested in six major sorghum testing sites and twenty one (21) locations both on farm and on station. Considering the potential use of the variety for the brewery industry and grain yield the national sorghum research program presented so that the field performance and adaptability of the candidate genotypes along with the standard check were evaluated by the national variety releasing technical committee. Finally, 2018 G.C, the standing committee approved the release of Dabir variety for commercial production. In general, this variety was recommended for lowland areas (500-1600masl) of Ethiopia with annual rainfall of 500-870mm and farmers can be benefited from production and sell of this variety to the brewery factories.

Sorghum (Sorghum bicolour (L.) Moench), the second most important staple crop in Sub-Saharan Africa (SSA) after maize, is well adapted to marginal environments of drought stress and high temperatures. But besides drought stress, the obligate root-parasitic flowering plant Striga hermonthica is an equally economically important biotic stress in agro-ecological zones where soils are marginal. Notwithstanding widespread and intense Striga infestation, genetic variations in defence mechanisms against the parasite have been reported. Sorghum variants, producing low levels of chemical stimulants such as sorgolactones that deter the advance of Striga seed germination and are therefore deemed resistant to the parasite, have been also reported in a few studies. But the existence of sorghum genetic variation for this resistance especially among farmers' landraces is yet to be demonstrated. The objective of this study was therefore to determine the levels of Striga germination stimulants in response to each of the 111 collected sorghum landraces and their progenies from Eri

A series of laboratory experiments were carried out in order to investigate the effect of plant growth regulators (PGRs) and NaCl on germination and haustorium initiation of Striga hermonthica. Therefore, manipulation of seed germination and pre-attachment stages are mandatory for successful management. In the present study, conditioned Striga seeds in water treated with 2, 4-D produced high germination 76% in response to GR24. Induction of germination by BAP varied from low (30%) to moderate (50%). Moreover, conditioned seeds treated with the combination of auxins and NaCl reduced germination by 90-100% as compared to the control.
Furthermore, germination of Striga was relatively higher in the presence of kinetine and BAP in presence or absence of the GR24. In addition, both the cytokinies in combine with NaCl induced haustorium initiation in a dose dependent manner. Both axuins in combination with NaCl displayed negative response for haustorium initiation, in absence of haustorium factor. Striga seeds conditioned in presence of PGRs plus NaCl and subsequently treated with germination stimulant displayed considerable germination reduction (35-100%).
Kinetine in combination with NaCl completely inhibited Striga germination in response to the synthetic germination stimulants GR24 in a concentration dependent manner. Withrespect to haustorium, all treatments reduced Striga germilings, significantly as compared to control (88%) in response to DMBQ. Auxin, 2, 4-D applied to Striga seeds after seeds conditioned in water was completely inhibited (100%) haustoria as compared to the control. Furthermore, Osmotic potential may significantly affect germination and haustorium initiation of Striga hermonthica. The results are consistent with a model in which both germination and subsequent
morphogenesis in Striga are associated with exogenous and endogenous phytohormones