Satyanarayana Gouthu

Ripening initiation-associated hormonal changes and sugar accumulation for individual fruits differed by seed content and did not depend heavily on flowering time or duration from anthesis to clusters' onset of ripening. For Vitis vinifera, the ripening initiation of individual fruits in a cluster occurs unevenly. This developmental period is called véraison. Why individual fruits initiate ripening at different times is not well studied, though differences in seed content and unequal developmental durations that arise from asynchronous flowering within a cluster have been proposed. This study examined how much both variables contribute to individual fruits' ripening progress by mid-véraison, when half of berries in a cluster have initiated ripening, and whether either or both factors affect the timing of characteristic, ripening-initiation associated changes in abscisic acid and auxin before, at, and after véraison. Overall, developmental duration and flowering time did not sufficiently explain how far berries had progressed into the ripening stage because fruits did not require a fixed amount of time to initiate ripening. Fruits from early and late flowers but of similar seed content were able to initiate ripening at the same time despite differences in chronological age. This suggests either an early developmental enhancement occurred for late-initiated fruits or that flowering time is an inappropriate "day zero". Ultimately, only seed content was linked to the timing and magnitude of ripening-related hormone changes, supporting that seeds have a comparatively larger influence than flowering time on the ripening initiation of individual berries. More specifically, if the fraction of berry weight occupied by seed was high, then the initiation of ripening for that berry and its associated hormone changes were delayed relative to berries with less seed weight versus total berry weight.

Bud dormancy in grapevine is an adaptive strategy for the survival of drought, high and low temperatures and freeze dehydration stress that limit the range of cultivar adaptation. Therefore, development of a comprehensive understanding of the biological mechanisms involved in bud dormancy is needed to promote advances in selection and breeding, and to develop improved cultural practices for existing grape cultivars. The seasonally indeterminate grapevine, which continuously develops compound axillary buds during the growing season, provides an excellent system for dissecting dormancy, because the grapevine does not transition through terminal bud development prior to dormancy. This study used gene expression patterns and targeted metabolite analysis of two grapevine genotypes that are short photoperiod responsive (Vitis riparia) and non-responsive (V. hybrid, Seyval) for dormancy development to determine differences between bud maturation and dormancy commitment. Grapevine gene expression and metabolites were monitored at seven time points under long (LD, 15 h) and short (SD, 13 h) day treatments. The use of age-matched buds and a small (2 h) photoperiod difference minimized developmental differences and allowed us to separate general photoperiod from dormancy specific gene responses. Gene expression profiles indicated three distinct phases (perception, induction and dormancy) in SD-induced dormancy development in V. riparia. Different genes from the NAC DOMAIN CONTAINING PROTEIN 19 and WRKY families of transcription factors were differentially expressed in each phase of dormancy. Metabolite and transcriptome analyses indicated ABA, trehalose, raffinose and resveratrol compounds have a potential role in dormancy commitment. Finally, a comparison between V. riparia compound axillary bud dormancy and dormancy responses in other species emphasized the relationship between dormancy and the expression of RESVERATROL SYNTHASE and genes associated with C3HC4-TYPE RING FINGER and NAC DOMAIN CONTAINING PROTEIN 19 transcription factors.

Background: Indole-3-acetic acid (IAA), the most abundant auxin, is a growth promoter hormone involved in several developmental processes. Auxin homeostasis is very important to its function and this is achieved through the regulation of IAA biosynthesis, conjugation, degradation and transport. In grapevine, IAA plays an essential role during initial stages of berry development, since it delays fruitlet abscission by reducing the ethylene sensitivity in the abscission zone. For this reason, Continuous polar IAA transport to the pedicel is required. This kind of transport is controlled by IAA, which regulates its own movement by modifying the expression and localization of PIN-FORMED (PIN) auxin efflux facilitators that localize asymmetrically within the cell. On the other hand, the hormone gibberellin (GA) also activates the polar auxin transport by increasing PIN stability. In Vitis vinifera, fruitlet abscission occurs during the first two to three weeks after flowering. During this time, IAA and GA are present, however the role of these hormones in the control of polar auxin transport is unknown. Results: In this work, the use of radiolabeled IAA showed that auxin is basipetally transported during grapevine fruitlet abscission. This observation was further supported by immunolocalization of putative VvPIN proteins that display a basipetal distribution in pericarp cells. Polar auxin transport and transcripts of four putative VvPIN genes decreased in conjunction with increased abscission, and the inhibition of polar auxin transport resulted in fruit drop. GA 3 and IAA treatments reduced polar auxin transport, but only GA 3 treatment decreased VvPIN transcript abundance. When GA biosynthesis was blocked, IAA was capable to increase polar auxin transport, suggesting that its effect depends on GA content. Finally, we observed significant changes in the content of several IAA-related compounds during the abscission period.