Functional Analysis Of MAX2 In Regulating ABA Signaling And Drought Stress Response In Arabidopsis

MAX2(More Axillary growth 2) is an important component in the plant hormone-Strigolacton signal, whose functions has been investigated in several plant species including Arabidopsis, Oryza sativa and Petunia hybrida. MAX2 was involved in many aspects of plant growth and development such as plant architecture, photomorphogenesis, senescence. However, the functions in response to abiotic stress and ABA signaling pathway was never reported.ABA(Abscisic acid) as an important phytohormone, mediates diverse biological processes of plant growth, development and defense, it especially plays an vital roles in responses to abiotic stress such as chilling, drought and salt stress.Here we showed that MAX2 positively regulated plant responses to drought stress during seedling stage max2 mutant is strongly hypersensitive to drought stress and loss water faster compared with wild type. Stomatal closure of max2 was less sensitive to ABA and drought stress and cuticle thickness of max2 was significantly thinner than that of the wild type. q RT-PCR showed that the expression of stress-inducible gene and ABA biosynthesis, catabolism,transport, and signaling genes was impaired in the max2 mutant compared with wild type seedlings.Meanwhile, MAX2 negatively regulated the responses to ABA during seed germination and early seedling development. max2 mutant is strongly hypersensitive to ABA compared with wild-type Arabidopsis during seed germination, cotyledon greening and root elongate. q RT-PCR analyses showed that The expression of ABI3, ABI5 and EM1 which were regulated by ABA was enhanced in imbibed max2 seeds. Treatment with ABA can reduced the expression of MAX2 during seedling stage. Double mutant analysis of max2 with the ABA-insensitive mutants abi3 and abi5 indicated that MAX2 may function upstream of these genes. In addition, max2 mutant seedlings were hypersensitive to osmotic stress including Na Cl, mannitol and glucose. Taken together, these results uncover an important role for MAX2 in plant responses to abiotic stress conditions.Further study found that ABA, osmotic stress, and drought-sensitive phenotypes were restricted to max2, and the strigolactone biosynthetic pathway mutants max1, max3 and max4 did not display any defects in these responses during seed germination.In summary, all of these results revealed molecular mechanism of MAX2 involved ABA and drought stress besides SL, Karrikin, GA and Auxin. Our investigation provides novel insight into function of MAX2 protein and has putative application value in improving plant drought resistance.