Root hydrotropism is controlled via a cortex-specific growth mechanism

Abstract

2Plants can acclimate by using tropisms to link the direction of growth to 41 environmental conditions. Hydrotropism allows roots to forage for water, a process 42 known to depend on abscisic acid (ABA) but whose molecular and cellular basis 43 remains unclear. Here, we show that hydrotropism still occurs in roots after laser 44 ablation removed the meristem and root cap. Additionally, targeted expression 45 studies reveal that hydrotropism depends on the ABA signalling kinase, SnRK2.2, and 46 the hydrotropism-specific MIZ1, both acting specifically in elongation zone cortical 47 cells. Conversely, hydrotropism, but not gravitropism, is inhibited by preventing 48 differential cell-length increases in the cortex, but not in other cell types. We conclude 49 that root tropic responses to gravity and water are driven by distinct tissue-based 50 mechanisms. In addition, unlike its role in root gravitropism, the elongation zone 51 performs a dual function during a hydrotropic response, both sensing a water 52 potential gradient and subsequently undergoing differential growth. 53 3 Tropic responses are differential growth mechanisms that roots use to explore the 54 surrounding soil efficiently. In general, a tropic response can be divided into several steps, 55 comprising perception, signal transduction, and differential growth. All of these steps have 56 been well characterized for gravitropism, where gravity sensing cells in the columella of the 57 root cap generate a lateral auxin gradient, whilst adjacent lateral root cap cells transport 58 auxin to epidermal cells in the elongation zone, thereby triggering the differential growth that 59 drives bending [1][2][3][4] . In gravi-stimulated roots, the lateral auxin gradient is transported 60 principally by AUX1 and PIN carriers [3][4][5] . 61Compared with gravitropism, the tropic response to asymmetric water availability, i.e., 62 hydrotropism, has been far less studied. Previously, it was reported that surgical removal or 63 ablation of the root cap reduces hydrotropic bending in pea [6][7][8] and Arabidopsis thaliana 9 , 64suggesting that the machinery for sensing moisture gradients resides in the root cap. It has 65 also been reported that hydrotropic bending occurs due to differential growth in the 66 elongation zone 7,10 . However unlike gravitropism, hydrotropism in A. thaliana is independent 67 of AUX1 and PIN-mediated auxin transport 11,12 . Indeed, roots bend hydrotropically in the 68 absence of any redistribution of auxin detectable by auxin-responsive reporters 13,14 . 18,19 . 83However it is unclear whether this broad expression pattern is necessary for MIZ1's function 84 in hydrotropism or whether ABA signal transduction components in general have to be 85 expressed in specific root tip tissues for a hydrotropic response. The present study describes 86 a series of experiments in A. thaliana designed to identify the root tissues essential for a 87 hydrotropic response. We report that MIZ1 and a key ABA signal-transduction component 88SnRK2....

Asymmetric distribution of cytokinins determines root hydrotropism in Arabidopsis thaliana

“…S1a-c). 12,13,15,[22][23][24] We found that in wild-type Col-0 seedlings, the lower water potential side of the roots contain more actively dividing cells and more meristematic cortex cells compared to the higher water potential side ( Fig. 1a, b, e, Supplementary information, Fig.…”

“…Expression of MIZ1 in different root tip cell layers suggested that MIZ1 functions mainly in the cortex of transition area between meristem and elongation zones. 12 More recently, it was found that an asymmetric cytosolic Ca 2+ signal in the phloem tissue of elongation zone is associated with root hydrotropism. 13 The Ca 2+ signal is likely originated from the root cap and slowly moves to the elongation zone where the asymmetric distribution is finally established.…”

Going with the Flow: Multiscale Insights into the Composite Nature of Water Transport in Roots

“…The proposed enrichment of the composite transport model sheds new light on the field of plant water relations. Cutting-edge research on hydropatterning (Bao et al, 2014), hydrotropism (Dietrich et al, 2017), and apoplastic barriers (Barberon et al, 2016;Doblas et al, 2017) recently highlighted the need for a quantitative hydraulic framework to address questions related to pressure distribution and water flow direction at the cell scale. The model compatibility with a root anatomical software (Pound et al, 2012) and functionalstructural plant models (Javaux et al, 2008) may open avenues to investigate the relationships between root architecture, anatomy, and water availability.…”

A Spatiotemporal DNA Endoploidy Map of the Arabidopsis Root Reveals Roles for the Endocycle in Root Development and Stress Adaptation

“…Endoploidy-specific gene expression profiles were obtained from sorted nuclei of the cortical cells of the Arabidopsis thaliana pCO2:YFP-H2b line (Heidstra et al, 2004). Tissue-specific endoploidy measurements were obtained via flow cytometric analysis of the 13 Arabidopsis marker lines listed in Supplemental Data Set 16 (Brady et al, 2007;Dietrich et al, 2017). Endoploidy boundary positions were confirmed using SIM:GUS and SMR1:GFP-GUS reporter lines (Yi et al, 2014).…”