The signalling of drought stress in Arabidopsis

Abiotic stresses, drought in particular, adversely affect plant growth and development and have long been considered as limiting factors of plant growth and crop production (Boyer, 1982). Understanding the mechanisms by which plants perceive and transfer these stress signals to initiate adaptive responses is essential for engineering stress tolerant crop plants. The Arabidopsis thaliana mutant, altered APX2 expression 8 (alx8), was identified during a screen for altered expression of ASCORBATE PEROXIDASE2 (APX2) (Rossel et al., 2006). The alx8 mutant is drought tolerant, exhibits improved water-use efficiency and a number of stress responsive genes are up-regulated. This mutant contains a genetic lesion in SAL1, a bifunctional enzyme that has 3'(2'),5'-bisphosphate nucleotidase and inositol polyphosphate 1-phosphatase activities (Wilson et al., 2009). Both inositol 1,4,5-triphosphate (IP{u2083}) and 3'-phosphoadenosine 5'-phosphate (PAP) have been considered as potential substrates for SAL1. However, the in vivo substrate for SAL1 is still debated. This study investigates the physiological and molecular changes underlying the drought tolerance mechanisms in alx8. Ion leakage experiments indicated that cell membranes were more stable to PEG-induced water stress in alx8 than in the wild-type. Additionally, quantification of reactive oxygen species (ROS) showed that less ROS accumulated in alx8 compared to wild-type following high light (HL) exposure. Despite this, no significant differences in stomatal conductance and ABA accumulation were observed between alx8 and wild-type plants under normal or drought conditions. A newly developed HPLC analyses revealed that alx8 accumulated PAP 10-fold higher than the wild-type, indicating that PAP is a primary in vivo substrate of SAL1. The role of SAL1 in ABA signalling was also explored by crossing alx8 with ABA insensitive mutants. The mutation in SAL1 improved the drought tolerance to the ABA insensitive mutants, open stomata 1 (ost1-2) and ABA insensitive 1 (abil-1) and restored ABA responses. Removal of nitric oxide (NO), an inducer of stomatal closure, did not promote stomatal opening in alx8. To investigate the contribution of the altered rosette morphology on alx8 drought tolerance, transgenic lines were produced to silence SAL1 in an inducible manner. Upon induction, the SAL1 inducible RNAi lines displayed wild-type phenotype but appeared to have survived longer than the wild-type during drought, and this also correlated with higher PAP. Taken together, the findings presented here have provided a better understanding of the role of SAL1 during drought stress in plants demonstrating that tolerance is probably due to increased detoxification of ROS, reduced membrane damage and increased PAP. In addition, SAL1-PAP possibly mediates a novel ABA-dependent pathway for stomatal regulation that is different to the well-characterised ABI1/OST1 and NO pathways. Finally, the results of experiments using inducible silencing of SAL1 appear to indicate that drought tolerance is not a consequence of the altered plant morphology of alx8, but probably due to an increase in PAP. It is hoped that this study can contribute to the future generation of drought tolerant crop plants