Role of Xanthoceras sorbifolium MYB44 in tolerance to combined drought and heat stress via modulation of stomatal closure and ROS homeostasis

Yellowhorn (Xanthoceras sorbifolium) is an important edible woody oil tree species that is endemic to China. Drought and heat stresses are factors severely limiting the high-quality development of the yellowhorn industry. Transcription factors (TFs) play critical roles in regulating the response of woody plant species to water deficit or high temperature. However, the MYB TFs that respond to combined drought and heat stress in yellowhorn remain unclear. Here, we first investigated the physiological changes in 5 yellowhorn varieties in response to combined stress treatments. We observed significant changes in antioxidant enzyme activities and photosynthesis. The Maigaiti variety yielded the best results and was selected for subsequent experiments. An R2R3-type MYB TF, designated XsMYB44, was isolated from the leaves of yellowhorn. XsMYB44 expression was strongly induced by combined stress. Suppression of XsMYB44 expression via virus-induced gene silencing weakened yellowhorn tolerance to both individual and combined drought and heat stress, and the increased susceptibility was coupled with decreased plant height, fresh weight and relative water content and inhibited stomatal closure. Moreover, compared with the individual stresses, the combined stress caused increased reactive oxygen species levels and decreased antioxidant enzyme activities and proline content in XsMYB44-silenced plants. Furthermore, the expression levels of several defense-related genes were reduced in the XsMYB44-silenced plants. Overall, we studied the physiological characteristics of 5 yellowhorn varieties, and the results demonstrated that XsMYB44 acts as a positive regulator in the yellowhorn response to combined stress by triggering stomatal closure to maintain water levels and by modulating ROS homeostasis.

Arbuscular mycorrhizal fungi mitigate negative effects of combined drought and heat stress on tomato plants

Arbuscular mycorrhizal (AM) symbiosis can alleviate drought and temperature stresses in plants, but it is unknown whether the benefits can be maintained when the plants are exposed to combined drought and heat stress. In this study, the impacts of AM fungi, Septoglomus deserticola and Septoglomus constrictum on tomato plant tolerance to combined drought and heat stress were investigated. No substantial differences in physiological parameters were found in all plants under non-stress conditions, except a higher expression of SlLOXD and SlPIP2.7 in plants + S. constrictum. Under drought, heat and drought + heat stress, both fungal symbionts could moderate oxidative stress by decreasing the lipid peroxidation, hydrogen peroxide level and improving leaf and root antioxidant enzyme activities, however better performance in plants + S. constrictum. Under drought and the combined stress, inoculation with S. constrictum enhanced stomatal conductance, leaf water potential and relative water content, elevated F_v/F_m and biomass production of the hosts as compared to non-inoculated plants whilst these improvements in plants + S. deserticola were not obvious. Under the combined stress inoculation of S. constrictum did not change the expression of SlNCED and SlPIP2.7 in roots as under heat stress. Expression of SlLOXD in root were upregulated in plants + S. contrictum under drought + heat stress as in mycorrhizal roots under drought stress. Altogether, our results indicated that AM inoculation, particularly with S. constrictum had a positive influence on the tomato plant tolerance to drought + heat stress. Further studies are essential to add some light on molecular mechanisms of mycorrhizal plant tolerance to this combined stress.