Arabidopsis thaliana mutants with altered low water potential response

Low water potential (psiw) is the major stress component affecting plant performance during times of drought; yet plant responses to low psiw are not understood at the molecular or genetic level. Using an experimental system of PEG-infused agar plates to impose a steady low psiw stress, a genetic screen was undertaken to find Arabidopsis thaliana mutants with altered low-psiw response. A negative selection using allyl alcohol was used to find seedlings that had reduced expression of an le25-ADH construct at low psi w. Allyl alcohol resistant lines were then screened for altered low psi w-induced proline accumulation in two subsequent generations. After backcrossing, twenty-two mutant lines with increased allyl alcohol resistance and either increased or decreased proline accumulation were isolated. Three of these mutant lines (l&barbelow;ow w&barbelow;ater potential r&barbelow;esponse 1 [lwr1], lwr2 and lwr3) were characterized in more detail. In addition to higher proline content, lwr1 had increased solute accumulation and osmotic adjustment. lwr2 had decreased proline accumulation and decreased osmotic adjustment. The phenotypes of these mutants show that proline accumulation is regulated in part through changes in cellular osmoregulation. lwr3 acts to modulate abscisic acid (ABA) responsiveness at low psi w and this led to increased proline accumulation, decreased leaf water loss in lwr3 and an increased response to exogenous ABA at low psiw in a lwr3/aba2-1 double mutant. Both lwr3 and aba2-1 had wild type osmotic adjustment; suggesting that this process is regulated independently of ABA. The low psi w responses of Arabidopsis ABA-insensitive ( abi) mutants were also quantified. abil-1 and abi2-1 had higher ABA contents than wild type suggesting an altered ABA homeostasis in these two mutants. This was true both under low psi w stress and in response to exogenously applied ABA, suggesting that abi1-1 and abi2-1 affect ABA homeostasis through a mechanism other than ABA synthesis. abi4 has twice the wild type level of proline when exposed to low psiw in the absence of exogenous sugar but does not differ from wild type in the presence of high sucrose. This suggests a role for abi4 in mediating an interaction of psiw, ABA and carbohydrate status in controlling proline accumulation.