The Xerobranching Response Represses Lateral Root Formation When Roots Are Not in Contact with Water.

Efficient soil exploration by roots represents an important target for crop improvement and food security [1, 2]. Lateral root (LR) formation is a key trait for optimizing soil foraging for crucial resources such as water and nutrients. Here, we report an adaptive response termed xerobranching, exhibited by cereal roots, that represses branching when root tips are not in contact with wet soil. Non-invasive X-ray microCT imaging revealed that cereal roots rapidly repress LR formation as they enter an air space within a soil profile and are no longer in contact with water. Transcript profiling of cereal root tips revealed that transient water deficit triggers the abscisic acid (ABA) response pathway. In agreement with this, exogenous ABA treatment can mimic repression of LR formation under transient water deficit. Genetic analysis in Arabidopsis revealed that ABA repression of LR formation requires the PYR/PYL/RCAR-dependent signaling pathway. Our findings suggest that ABA acts as the key signal regulating xerobranching. We conclude that this new ABA-dependent adaptive mechanism allows roots to rapidly respond to changes in water availability in their local micro-environment and to use internal resources efficiently.

Repression of early lateral root initiation events by transient water deficit in barley and maize.

The formation of lateral roots (LRs) is a key driver of root system architecture and developmental plasticity. The first stage of LR formation, which leads to the acquisition of founder cell identity in the pericycle, is the primary determinant of root branching patterns. The fact that initiation events occur asynchronously in a very small number of cells inside the parent root has been a major difficulty in the study of the molecular regulation of branching patterns. Inducible systems that trigger synchronous lateral formation at predictable sites have proven extremely valuable in Arabidopsis to decipher the first steps of LR formation. Here, we present a LR repression system for cereals that relies on a transient water-deficit treatment, which blocks LR initiation before the first formative divisions. Using a time-lapse approach, we analysed the dynamics of this repression along growing roots and were able to show that it targets a very narrow developmental window of the initiation process. Interestingly, the repression can be exploited to obtain negative control root samples where LR initiation is absent. This system could be instrumental in the analysis of the molecular basis of drought-responsive as well as intrinsic pathways of LR formation in cereals.