Regulation of Drought Tolerance by the F-Box Protein MAX2 in Arabidopsis

Abstract

(E.H.).MAX2 (for MORE AXILLARY GROWTH2) has been shown to regulate diverse biological processes, including plant architecture, photomorphogenesis, senescence, and karrikin signaling. Although karrikin is a smoke-derived abiotic signal, a role for MAX2 in abiotic stress response pathways is least investigated. Here, we show that the max2 mutant is strongly hypersensitive to drought stress compared with wild-type Arabidopsis (Arabidopsis thaliana). Stomatal closure of max2 was less sensitive to abscisic acid (ABA) than that of the wild type. Cuticle thickness of max2 was significantly thinner than that of the wild type. Both of these phenotypes of max2 mutant plants correlate with the increased water loss and drought-sensitive phenotype. Quantitative real-time reverse transcriptionpolymerase chain reaction analyses showed that the expression of stress-responsive genes and ABA biosynthesis, catabolism, transport, and signaling genes was impaired in max2 compared with wild-type seedlings in response to drought stress. Double mutant analysis of max2 with the ABA-insensitive mutants abi3 and abi5 indicated that MAX2 may function upstream of these genes. The expression of ABA-regulated genes was enhanced in imbibed max2 seeds. In addition, max2 mutant seedlings were hypersensitive to ABA and osmotic stress, including NaCl, mannitol, and glucose. Interestingly, 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. Taken together, these results uncover an important role for MAX2 in plant responses to abiotic stress conditions.

Strigolactones are involved in sugar signaling to modulate early seedling development in <i>Arabidopsis</i>

“…As shown in Supplemental Figure 2, 6% glucose significantly delayed the germination of the max mutants compared with that of the WT, and the germination of the max2 mutant was retarded more severely than that of the max1 mutant. Ultimately, there were no obvious differences in sugar sensitivity in phenotypes of the max mutants and the WT (data are not shown), which is consistent with Bu's report that max2 mutants are hypersensitive to the osmotic stress induced by 4% glucose (Bu et al 2014). Our results suggest that the gin phenotype of the max mutants is easily masked in the screening assay for gin mutants.…”

“…In this study, mutants of the plant hormone SL clearly displayed the gin phenotype during seedling establishment under low-glucose conditions ( Figure 1A and B), which is often assumed to involve osmotic hypersensitivity, and have never been screened with the classic high-sugar method (Bu et al 2014;Shen et al 2012). Our results indicate that SL signaling interacts with a sugar-specific signaling pathway and also that osmotic stress does not always cooperate with sugar signaling in repressing seedling establishment.…”

Abscisic Acid Is a General Negative Regulator of Arabidopsis Axillary Bud Growth

“…Loss of MAX2 (Shen et al, 2012;Bu et al, 2014) or MAX2, MAX3, or MAX4 (Ha et al, 2014) function was reported to either increase (Shen et al, 2012;Bu et al, 2014) or decrease (Ha et al, 2014) the inhibitory effects of ABA on seed germination and early growth/development. Additionally, Bu et al (2014) concluded that strigolactones per se were not involved in altering ABA responsiveness because loss of the biosynthetic function of MAX1, MAX3, and MAX4 had no effect. In this study, the branching inhibition of max2 was similar to that of the wild type at low ABA levels but was possibly reduced at higher levels.…”

Osmotic stress represses strigolactone biosynthesis in Lotus japonicus roots: exploring the interaction between strigolactones and ABA under abiotic stress

“…However, besides this shared conclusion, the two reports are contradictory at several 145 levels. While some inconsistencies could be explained with the slightly different 146 experimental set-up, it is puzzling that in one instance biosynthetic mutants were found not 147 to be compromised in drought tolerance (Bu et al, 2014), while in the other they were as 148 drought-sensitive as max2 (Ha et al, 2014). Furthermore, exogenous SL treatment could 149 rescue the drought-sensitive phenotype of SL biosynthetic mutants and even increase 150 performances of drought-stressed WT plants in the latter report (Ha et al, 2014).…”