Are Strigolactones Relevant for Arabidopsis Co-exposed to Heat and Salt Stress? – Exploring the Role of MAX2 and MAX3 in Stress Responses and Physiological Performance

Under the current climatic context, understanding the complex dynamics of plant responses to stress conditions is a matter of special interest to properly develop sustainable mitigation strategies. The exogenous application of strigol actones (SLs) has been recently associated with improved stress tolerance. However, there has been limited focus on the role of endogenous SLs in modulating plant stress responses, especially under a combined stress scenario. To address this, two Arabidopsis T-DNA mutants deficient in either SL signaling ( max2-4 ) or synthesis ( max3-11 ) were exposed to salt (75 mM NaCl), heat (42 °C for 4 h d − 1 ) and their combination for 14 days. The results highlight that while max3 presented the highest growth inhibition in response to heat, both genotypes presented an overall disruption in redox status, responding differently to the stressors, when compared to the wild-type. Thermotolerance appeared to be closely linked to SL biosynthesis, as max3 seedlings demonstrated higher susceptibility to both heat-related treatments, which was associated with increased membrane damage and a lower induction of antioxidant defenses. On the other hand, max2 mutants appeared more sensitive to salt conditions, lacking the activation of the SOS pathway and showing signs of lipid peroxidation, which were accompanied by a high accumulation of proline and the upsurge of the seedlings’ enzymatic antioxidant defenses. Overall, although the importance of endogenous SLs in regulating plant response to combined heat and salt is corroborated by the present research, the results also highlight the complexity of this regulation, with mutations at different points of the MAX pathway being associated with different responses.