Role of the strigolactones in the repeated drought stress

Plants are frequently exposed to repeated cycles of similar stresses; responding differently to them, thus displaying “stress memory”, is an adaptive strategy to cope with fluctuating environmental conditions. The so-called “after-effect” of drought, for example, is a feature of stress memory seen at the stomatal level: an incomplete recovery of conductance after drought, even when water potential has fully recovered. Besides the likely dependence on abscisic acid (ABA), the molecular and physiological mechanisms driving the after-effect are not clear yet. So, we set to investigate whether the phytohormones strigolactones may contribute to it. Interestingly in fact, among other roles, strigolactones promote stomatal closure in both ABA-dependent and independent manners, and are needed for effective drought acclimation processes. We investigated their physiological and molecular contributions to the after-effect, by contrasting stomatal conductance and transcriptome of wt versus strigolactone biosynthetic/signalling mutants of Arabidopsis and tomato in repeated dehydration cycles. In both model plants, strigolactones were needed for a sustained after-effect during recovery from the first stress. They were also indispensable in tomato for a full after-effect during recovery in the second cycle, in which stress avoidance by stomatal closure was overall stronger. On the other hand, stomatal conductance was instead consistently higher in Arabidopsis during the second stress and recovery cycle than during the first, highlighting how in order to cope with repeated drought spells, Arabidopsis rather adopts a drought tolerance strategy (e.g. osmotic adjustment) than avoidance by stomatal closure, as in tomato. Since in both models however, strigolactones had been proven necessary for the after effect from the first stress, we took genome-wide RNA-seq and ATAC-seq approaches to investigate the links between strigolactones, chromatin accessibility and transcriptome changes in both species under recovery conditions. Results will be presented that highlight how strigolactone-dependent chromatin remodelling may underlie stomatal memory of drought.