Root growth adaptation under water deficit

Abstract

Growth and development of the root system require the coordinated regulation of developmental programs and environmental signals; however, the knowledge about its interconnection is scarce. The balance between cell division, regulated cellular expansion, and differentiation in the root apical meristem directs primary root growth in Arabidopsis. Cellular expansion requires cell wall controlled relaxation, which ensures cell integrity during the expansion process. In field conditions, the root faces different kinds of stress, including osmotic stress. Mutations in the Arabidopsis Tetratrico-peptide Thioredoxin-Like 1 (TTL1) cause hypersensitivity to osmotic stress evidenced by root tip swelling, making it an attractive model to explore how root growth is regulated under osmotic stress conditions. We found that osmotic stress decelerates root growth by reducing first cell elongation in the elongation zone and second the number of cortical cells in the proximal meristem. Using atomic force microscopy, we measure the stiffness of epidermal cell walls in the root elongation zone of ttl1 mutants, and we found that the mean apparent elastic modulus was 448% higher for live Col-0 cell walls than for ttl1 (88.1 ± 2.8 vs. 16.08 ± 6.9 kPa) in plants grown in control conditions. Furthermore, cell walls of epidermal cells in the elongation zone increase their stiffness 87% and 84% for Col-0 and ttl1, respectively, in response to seven days of osmotic stress. These findings suggest that TTL1 may play a role in controlling cell expansion orientation during root growth, necessary for osmotic stress adaptation.