Garden pea (Pisum sativum) was prominent in early studies investigating the genetic control of flowering and the role of mobile flowering signals. In view of recent evidence that genes in the FLOWERING LOCUS T (FT) family play an important role in generating mobile flowering signals, we isolated the FT gene family in pea and examined the regulation and function of its members. Comparison with Medicago truncatula and soybean (Glycine max) provides evidence of three ancient subclades (FTa, FTb, and FTc) likely to be common to most crop and model legumes. Pea FT genes show distinctly different expression patterns with respect to developmental timing, tissue specificity, and response to photoperiod and differ in their activity in transgenic Arabidopsis thaliana, suggesting they may have different functions. We show that the pea FTa1 gene corresponds to the GIGAS locus, which is essential for flowering under long-day conditions and promotes flowering under short-day conditions but is not required for photoperiod responsiveness. Grafting, expression, and double mutant analyses show that GIGAS/FTa1 regulates a mobile flowering stimulus but also provide clear evidence for a second mobile flowering stimulus that is correlated with expression of FTb2 in leaf tissue. These results suggest that induction of flowering by photoperiod in pea results from interactions among several members of a diversified FT family.

A major environmental signal influencing day-time stomatal aperture is the vapour pressure deficit between the leaf and atmosphere (VPD). In angiosperms, increased VPD triggers biosynthesis of abscisic acid (ABA), prompting rapid stomatal closure. Altered cell turgor has been proposed as the trigger for ABA biosynthesis, but the timing and nature of the genetic signals linking these processes have remained uncertain. We investigated this in Arabidopsis by examining changes induced by a decrease in leaf turgor, simulating a natural increase in VPD. We found that the rate-limiting gene within the de novo ABA biosynthesis pathway, 9-cis-epoxycarotenoid dioxygenase 3 (NCED3), was induced and ABA levels increased within just 5 min of decreased leaf turgor. This rapid induction matches the time-frame for initiation of stomatal closure in response to a doubling in VPD. We further examined Arabidopsis histidine kinase1 (AHK1) as the most likely candidate for the turgor-sensing receptor involved, but found no significant difference between wild-type and an ahk1 null mutant in the induction of ABA-biosynthetic genes, ABA production, or stomatal behaviour. We show that decreased leaf turgor triggers de novo ABA biosynthesis within the time-frame of the stomatal response to VPD, but that AHK1 does not fulfil a critical role as a turgor-sensing receptor within this pathway.

Rapid biosynthesis of abscisic acid (ABA) in the leaf, triggered by a decrease in cell volume, is essential for a functional stomatal response. However, it is not known whether rapid biosynthesis of ABA is also triggered in other plant tissues. Through the application of external pressure to flower, root, and leaf tissues, we test whether a reduction in cell volume can trigger rapid increases in ABA levels across the plant body in two species, Solanum lycopersicum and Passiflora tarminiana. Our results show that, in contrast to rapid ABA synthesis in the leaf, flower and root tissue did not show a significant, increase in ABA level in response to a drop in cell volume over a short time frame, suggesting that rapid ABA biosynthesis occurs only in leaf, not in flower or root tissues. A gene encoding the key, rate-limiting carotenoid cleavage enzyme (9-cis-epoxycarotenoid dioxygenase, NCED) in the ABA biosynthetic pathway in S. lycopersicum, NCED1, was upregulated to a lesser degree in flowers and roots compared with leaves in response to applied pressure. In both species, floral tissues contained substantially lower levels of the NCED substrate 9’-cis-neoxanthin than leaves, and this ABA precursor could not be detected in roots. Slow and minimal ABA biosynthesis was detected after 2 h in petals, indicating that floral tissue is capable of synthesizing ABA in response to sustained water deficit. Our results indicate that rapid ABA biosynthesis predominantly occurs in the leaves, and not in other tissues.

Sexual reproduction in animals and plants shares common elements, including sperm and egg production, but unlike animals, little is known about the regulatory pathways that determine the sex of plants. Here we use mutants and gene silencing in a fern species to identify a core regulatory mechanism in plant sexual differentiation. A key player in fern sex differentiation is the phytohormone abscisic acid (ABA), which regulates the sex ratio of male to hermaphrodite tissues during the reproductive cycle. Our analysis shows that in the fern Ceratopteris richardii, a gene homologous to core ABA transduction genes in flowering plants [SNF1-related kinase2s (SnRK2s)] is primarily responsible for the hormonal control of sex determination. Furthermore, we provide evidence that this ABA–SnRK2 signaling pathway has transitioned from determining the sex of ferns to controlling seed dormancy in the earliest seed plants before being co-opted to control transpiration and CO₂ exchange in derived seed plants. By tracing the evolutionary history of this ABA signaling pathway from plant reproduction through to its role in the global regulation of plant–atmosphere gas exchange during the last 450 million years, we highlight the extraordinary effect of the ABA–SnRK2 signaling pathway in plant evolution and vegetation function.