Figure 1.

Auxin-regulated root-adaptive responses to water availability. (A) X-ray microCT image of a maize root growing down a macropore reveals lateral branches only form on the side in contact with soil employing the hydropatterning response. (B) Hydropatterning is regulated via the posttranslational modification of ARF7 by SUMO (blue hexagon “S”) in cells on the side of the root exposed to the air-filled macropore (denoted by bubble). SUMOylated ARF7 recruits the repressor Aux/IAA3, blocking auxin-dependent transcription and lateral root development. On the wet side of the root, ARF7 is not SUMOylated and therefore can activate LBD16 gene expression, leading to lateral root initiation. (C) X-ray microCT image of a barley root growing through an air-filled space, causing branching to cease until later reentering soil due to the xerobranching response. (D) A transient reduction in water uptake causes abscisic acid (ABA) to accumulate in root tip tissues during growth through the air space (denoted by bubble). ABA triggers its receptor PYR/PYL, which results in an increase of indole-3-acetic acid (IAA) conjugation to aspartate (Asp) and glutamate (Glu) by GH3 enzymes. This leads to a drop in free IAA levels, negatively affecting lateral root initiation.