The transition to flowering is characterized by a shift of the shoot apical meristem (SAM) from leaf production to the initiation of a floral meristem. The flowering process is of vital importance for agriculture, but the associated events or regulatory pathways in the SAM are not well understood, especially at a system level. To address this issue, we have used a GeneChip® containing 37 744 probe sets to generate a temporal profile of gene expression during the floral initiation process in the SAM of the crop legume, soybean (Glycine max). A total of 331 transcripts displayed significant changes in their expression profiles. The in silico and RT-PCR analysis on differentially regulated transcripts implies the intriguing involvement of sugar, auxin or abscisic acid (ABA) in events prior to the induction of floral homeotic transcripts. The novel involvement of ABA in the floral transition is further implicated by immunoassay, suggesting an increase in ABA levels in the SAM during this developmental transition. Furthermore, in situ localization, together with in silico data demonstrating a marked enhancement of abiotic stress-related transcripts, such as trehalose metabolism genes in SAMs, points to an overlap of abiotic stress and floral signalling pathways.

Molecular processes underlying the floral transition in the soybean shoot apical meristem

Flowering process governs seed set and thus affects agricultural productivity. Soybean, a major legume crop, requires short-day photoperiod conditions for flowering. While leaf-derived signal(s) are essential for the photoperiod-induced floral initiation process at the shoot apical meristem, molecular events associated with early floral transition stages in either leaves or shoot apical meristems are not well understood. To provide novel insights into the molecular basis of floral initiation, RNA-Seq was used to characterize the soybean transcriptome of leaf and micro-dissected shoot apical meristem at different time points after short-day treatment. Shoot apical meristem expressed a higher number of transcripts in comparison to that of leaf highlighting greater diversity and abundance of transcripts expressed in the shoot apical meristem. A total of 2951 shoot apical meristem and 13,609 leaf sequences with significant profile changes during the time course examined were identified. Most changes in mRNA level occurred after 1short-day treatment. Transcripts involved in mediating responses to stimulus including hormones or in various metabolic processes represent the top enriched GO functional category for the SAM and leaf dataset, respectively. Transcripts associated with protein degradation were also significantly changing in leaf and SAM implicating their involvement in triggering the developmental switch. RNA-Seq analysis of shoot apical meristem and leaf from soybean undergoing floral transition reveal major reprogramming events in leaves and the SAM that point toward hormones gibberellins (GA) and cytokinin as key regulators in the production of systemic flowering signal(s) in leaves. These hormones may form part of the systemic signals in addition to the established florigen, FLOWERING LOCUS T (FT). Further, evidence is emerging that the conversion of shoot apical meristem to inflorescence meristem is linked with the interplay of auxin, cytokinin and GA creating a low cytokinin and high GA environment.

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The Dynamics of Soybean Leaf and Shoot Apical Meristem Transcriptome Undergoing Floral Initiation Process

Environmental, Chemical, and Genetic Control of Flowering

Plant growth regulators applied in vitro affected strawberry plant performance ex vitro for a period of up to 4 months. Benzyl-adenine and gibberellin enhanced juvenile characteristics; in general, more runners and monofoliolate leaves were produced, net photosynthesis was reduced, leaf diffusive resistance was increased and, on occasion, flowering was delayed. In contrast, abscisic acid and a gibberellin biosynthesis inhibitor, paclobutrazol, resulted in a more adult phenotype; specifically, flowering was earlier, net photosynthesis and leaf diffusive resistance rates were equal to those of adult plants, and fewer leaves were monofoliolate. Tissue culture-produced plants reacquire the adult phenotype earlier than seedlings mature. Abscisic acid application to seedlings also resulted in earlier and increased flowering. Endogenous abscisic acid concentrations were greater in adult plants and less in tissue culture and seedling plants at 3 and 7 weeks ex vitro, after germination or after adult runner propagation. No flowering occurred at 3 weeks in any propagation type and only runner-produced plants flowered at 7 weeks. At 15 weeks, no statistical difference in abscisic acid concentrations existed among propagation treatments and all propagation types flowered. The endogenous concentrations of abscisic acid in plants propagated by all three methods were much higher at three weeks compared to concentrations at 7 or 15 weeks.

The role of abscisic acid and plant growth regulators in tissue culture-induced rejuvenation of strawberry ex vitro

Introduction: After several subcultures, grapevines growing in vitro show rejuvenation (MULLINS et al. 1979). The juvenile characters can be induced by the manipulation of culture conditions. By changing the CO2 concentration, two distinct morphological patterns were obtained (FOURNIOUX 1995): adult (phyllotaxy 1/2 and presence of tendrils) and juvenile (phyllotaxy 2/5 and lack of tendrils) micropropagated plants. Some juvenile characteristics may be preserved after acclimatization and transfer into the vineyard: leaves are more jagged, the anthocyanin content is higher and lower fertility is observed in most cultivars (GRENAN 1982). As it is difficult to determine the degree of juvenility with morphological characteristics only (FOURNIOUX et al. 1998), the use of biochemical markers should be added. The abscisic acid (ABA) content of a plant depends on age and varies during ontogenesis. ABA acts as an antagonist of gibberellic acid by maintaining the adult stage in Hedera helix (ROGLER and HACKETT 1975). It seems to be one of the factors involved in the control of flowering of Xanthium strumarium and as high levels of ABA are supposed to characterize the adult stage, low levels wouldmaintain the juvenile stage (PODOLNYI et al. 1989). We have quantified the ABA content of juvenile and adult plants cultivated in situ (greenhouse) or in vitro, with the aim to investigate the possible relationship between ABA changes and the process of juvenilisation.

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Endogenous abscisic acid in juvenile and adult grape (Vitis vinifera L. cv. Pinot noir)

The cotyledons ofXanthium strumarium plants are of low sensitivity to photoperiodic treatment and contain only trace amounts of ABA under long-day conditions The first pair of leaves, very sensitive to photoperiodic treatment, contains a higher level ofABA, decreasing with age of the plant Prolonged short-day photoperiodic treatment increases the ABA level in the cotyledons but this is still 10times lower than in the first two leaves Exogenous 10−4M ABA increases the ABA level in the cotyledons to the level corresponding to that in the first leaves, and enhances the photoperiodic sensitivity of cotyledons In contrast to cotyledons, the photoperiodic treatment affects the ABA level in the first pair of leaves only slightly The authors propose that a high ABA level supports the transition of plants to flowering, while a low ABA level may be responsible for a low photoperiodic sensitivity of cotyledons inXanthium plants

S. P. Verenchikov

Papers

Abscisic acid as a potent regulator of the transition from juvenile to mature stage inXanthium strumarium