Molecular responses to abiotic stress and the development of drought tolerance in plants

Water availability is a major limiting factor in the yield of agricultural crops in marginal rainfall areas. One solution is the development of crops that are drought tolerant, either through traditional breeding or genetic engineering. To permit a targeted approach to developing drought tolerance, the molecular basis of drought tolerance must first be elucidated. Previously in our group, a forward genetics screen was undertaken to identify mutants, in the model plant species Arabidopsis thaliana, with an altered response to high light stress (Rossel et al, 2006). This study describes the positional cloning of one of these mutations, namely alx8 (altered APX.2 expression 8). The location of the alx8 mutation was identified as a known stress response regulator, SALJ (At5g63980). This alx8 mutation appears to cause a loss of function of the SALl protein, as the reported SALJ null mutant,fiyl-1 (Xiong et al, 2001), and the SALJ T-DNA insertion line, salk_020882, were found to have similar phenotypes to alx8. DespitefryJ-J being reported as drought sensitive (Xiong et al, 2001), here it is shown that this mutant, as well as alx8 and salk_020882, are drought tolerant in soil based experiments. This drought tolerance was investigated by the characterisation of the morphological, physiological and molecular changes resulting from the alx8 mutation. It was established that alx8 's drought tolerance was not due to a change in cuticular water loss or a decreased number of stomata. Likewise, alx8's drought tolerance was not due to its delayed growth and development. The stomatal dynamics of alx8 were investigated by a number of methods. Previously, alx8 was shown to have an increased water use efficiency (WUE) in a light response curve, but in this study the carbon isotope ratio indicated no change in WUE. However, stomatal function is altered in alx8, as it is shown that the response of stomata to light and high C02 is delayed in alx8 delayed in alx8. Furthermore, double mutants, of alx8 with ostl-2, abil-1 and abi2-1, all of which have impaired stomatal control, were drought tolerant. This indicates that SALJ is involved in the regulation of stomatal signalling pathways. Microarray analysis of alx8 revealed the constitutive up-regulation of 5413 genes involved in a wide range of functions including a number of stress response genes. Other expression changes were genes involved in the regulation of flowering, and growth and development. There were also large changes in the metabolic profile of alx8 including increased accumulation of sugars and polyamines and decreased accumulation of proline. This increased sugar content corresponded with a decrease in transitory starch accumulation in alx8. Despite increased levels of the stress hormone Abscisic Acid (ABA) in alx8, the production of ABA-deficient and ABA-independent double mutants indicated that the delayed development, altered leaf morphology and drought tolerance of alx8 are not ABA-dependent. This is supported by few ABA induced genes being up-regulated in alx8. Hence, SAL1 is a key regulator of stress signalling pathways in Arabiodpsis whose modification in alx8 causes a number of morphological, molecular and physiological changes, and results in drought tolerance