Phytochrome B (phyB) can adjust morphological and physiological responses according to changes in the red : far-red (R:FR) ratio. phyB-driven acclimation of plants to open environments (high R:FR ratio) increases carbon gain at the expense of increased water loss. This behaviour alleviates stressful conditions generated by an excess of light, but increases the chances of desiccation. Here we evaluated how phyB modulates this droughttolerance response by comparing wild-type Arabidopsis thaliana adult plants to the null phyB in response to water shortage. phyB wilted before the wild type, and this was due to phyB maintaining open stomata under a reduction in soil water availability. Although phyB presented enhanced ABA levels under well-watered conditions, this mutant was less sensitive than the wild type in diminishing stomatal conductance in response to exogenous ABA application. Reduced sensitivity to ABA in phyB correlated with a lower expression of ABCG22, which encodes a putative ABA influx transporter, and PYL5, which encodes a soluble ABA receptor. Furthermore, the expression of RAB18 and RD29A, both typical ABA-induced genes, was lower in phyB than the wild type after ABA treatment. We propose that phyB contributes to the acclimation of plants to open environments by enhancing ABA sensitivity when soil water becomes limiting.

In Arabidopsis seeds, germination is promoted only by phytochromes, principally phytochrome B (phyB) and phytochrome A (phyA). Despite the abundant information concerning the molecular basis of phyB signaling downstream of PIF1/PIL5, the signaling network inducing germination by phyA is poorly known. Here, we describe the influence of phyA on the transcriptome of Arabidopsis seeds when germination is induced by a far-red (FR) pulse. The expression of 11% of the genome was significantly regulated by phyA. Most of the genes were up-regulated and the changes noted late (i.e. 5 h after a FR pulse), whereas changes in down-regulated genes were more abundant earlier (i.e. 0.5 h after a FR pulse). Auxin- and GA-associated elements were overrepresented in the genes that were modified by phyA. A significant number of genes whose expression was affected by phyA had not been previously reported to be dependent on PIL5. Among them, homozygotic mutant seeds of MYB66, a SAUR-like protein, PIN7, and GASA4 showed an impaired promotion of germination by phyA. Natural variation at the transcriptional level was found in early signaling and GA metabolic genes, but not in ABA metabolic and expansin genes between Columbia and Landsberg erecta accessions. Although phyA and phyB/PIL5 signaling pathways share some molecular components, our data suggest that phyA signaling is partially independent of PIL5 when germination is promoted by very low fluences of light.