Osmotic adjustment and maintenance of the redox balance in root tissue may be key points to overcome a mild water deficit during the early growth of wheat

In this investigation we analyzed in detail the consequences of water deficit during the first 4 days of wheat development, focusing on root growth as affected by eventual changes in cell cycle regulation and oxidative processes. Root elongation decreased under water restriction in correlation with the intensity of this limitation, but the total number of cells between the quiescent center and the start of the rapid elongation zone in the root apex did not vary. Neither lipid peroxidation nor protein carbonylation increased in the roots of water-starved seedlings (psi(w): -0.6 MPa); accordingly, catalase activity increased, and transcript levels of cat2 gene were enhanced. Superoxide dismutase activity rose at day 2 and 3 and, unlike catalase, displayed quite similar levels on comparing roots and coleoptiles. Proline and total soluble carbohydrates increased in the roots of water-starved seedling. Total conductivity and osmolality were also augmented. No changes in the transcript levels of the markers related to G1-S transition phase of cell cycle could be detected. However, two expansin genes (TaEXPB8 and TaEXPA5) were up-regulated in roots under water deficit. We conclude that wheat root elongation in water-deprived seedlings was simply hampered by lack of water income to cells. The enhanced expression of two root expansin genes is probably related to the eventual need of a quick cell wall expansion to allow the existing root cells to recover normal turgor, in case of sudden rewatering. Maintenance of redox balance, osmotic adjustment and modification of cell wall plasticity may be key points for wheat seedlings to overcome a water restriction during early growth.