Physiological and molecular analysis on root growth associated with the tolerance to aluminum and drought individual and combined in Tibetan wild and cultivated barley

The drought-stimulated gene expression of NCED, SUS, and KS - DHN and ABA signal cross-talk with other phytohormones maintains barley root growth under drought stress at pH 4.0 plus polyethylene glycol plus aluminum. Aluminum (Al) toxicity and drought are two major factors that limit barley production. In this work, the individual and combined effects of Al/acid and polyethylene glycol (PEG 6000) induced drought stress that suppressed root growth and caused oxidative damage as characterized by increased H2O2 and accumulation. The wild-barley genotypes, XZ5 and XZ29, exhibited a higher tolerance than the two cultivars Dayton (Al tolerant) and Tadmor (drought tolerant) under combined stress (pH 4.0 + PEG + Al). The oxidative damage induced by PEG was more severe at pH 4.0 than at pH 6.0. In XZ29, the highest root secretion of malate and citrate was recorded, and the least Al uptake in the four genotypes. In XZ5, a peak accumulation of ABA and minor synthesis of zeatin riboside and ethylene were found being essential in maintaining primary root elongation and root hair development. PEG-induced drought stress repressed Al uptake in root tips, with a lower increase in callose formation and HvMATE (Hordeum vulgare multidrug and toxic compound exudation) expression compared to Al-induced callose production. Stress by pH 4.0 + PEG + Al up-regulated 9-cis-epoxycarotenoid dioxygenase (NCED) which is involved in ABA biosynthesis. Such treatment stimulated the regulation of ABA-dependent genes sucrose synthase (SUS) and KS-type dehydrin (KS-DHN) in root tips. Our results suggest that the tolerance ranking to pH 4.0 + PEG + Al stress in Tibetan wild barley by gene expression is closely correlated to physiological indices. The results show that acclimatisation to pH 4.0 + PEG + Al stress involves specific responses in XZ5 and XZ29. The present study provides insights into the effects of Al/acid and drought combined stress on the abundance of physiological indices in the roots of barley varieties.