Divining Roots: Linking Growth and Development with Water Availability

Soil resources required by plants are often distributed in a highly heterogeneous pattern. To aid foraging, root growth and development is highly responsive to soil signals. As a result, root architecture is shaped by myriad environmental signals to ensure resource capture is optimised. Plant roots use tropisms to acclimate to changes in environmental conditions and link their direction of growth to cues such as water.

Hydrotropism allows roots to forage for water, a process known to depend on abscisic acid (ABA) but whose molecular and cellular basis remains unclear. We recently reported that hydrotropism still occurs in roots after laser and manual ablation removed the meristem and root cap (Dietrich et al, 2017, Nature Plants). Additionally, targeted expression studies reveal that hydrotropism depends on the ABA signalling kinase, SnRK2.2, and the hydrotropism-specific MIZ1, both acting specifically in elongation zone cortical cells. Conversely, hydrotropism, but not gravitropism, is inhibited by preventing differential cell-length increases in the cortex, but not in other cell types. We conclude that root tropic responses to gravity and water are driven by distinct tissue-based mechanisms. In addition, unlike its role in root gravitropism, the elongation zone performs a dual function during a hydrotropic response, both sensing a water potential gradient and subsequently undergoing differential growth.

Non-invasive X-ray microCT imaging has recently revealed that soil-water contact profoundly impacts root architecture. Novel root adaptive responses revealed using microCT include hydropatterning (Bao et al, 2014, PNAS) and xerobranching (Orman et al, under review), where lateral roots form only when in direct contact with moisture. I will describe how the hydropatterning response requires core components of the auxin signalling machinery to be controlled via an environmentally regulated post-translational modification to pattern root branching.