Mycorrhizal Fungal Effects on Plant Growth, Osmolytes, and CsHsp70s and CsPIPs Expression in Leaves of Cucumber under a Short-Term Heat Stress.

Arbuscular mycorrhizal (AM) fungi enhance plant stress tolerance, but it is unclear whether AM fungi affect heat tolerance in cucumbers. This study aimed to analyze how an AM fungus, , affected growth, chlorophyll, five osmolytes, and plasma membrane intrinsic proteins () and heat shock protein 70 () gene expression in cucumber leaves after a short-term (80 h) heat stress. Heat treatment significantly reduced root AM fungal colonization rate (0.26 folds). Heat treatment also distinctly suppressed plant height, stem diameter, and biomass, whereas AM fungal inoculation improved these growth variables as well as the chlorophyll index, with the benefit being more obvious under heat than under no-heat stress conditions. Heat treatment triggered differential changes in osmolytes (sucrose, fructose, and betaine) of inoculated and uninoculated cucumbers, whereas inoculation with AM fungus significantly raised leaf sucrose, fructose, glucose, betaine, and proline levels when compared to non-AM fungal inoculation. Heat treatment increased the expression of two ( and ) of eight in inoculated and uninoculated plants, whereas AM fungal inoculation up-regulated the expression of , , and under heat stress conditions. expressed differently in inoculated and uninoculated plants under heat versus no-heat stress, with 6 of 11 down-regulated in inoculated plants. Under heat stress conditions, AM fungus only up-regulated expression in 11 , while another eight were down-regulated. Heat treatment and AM fungal inoculation both increased the expression of and . It was concluded that AM fungus-inoculated cucumbers have high levels of growth, chlorophyll, and osmolytes under heat stress and do not require high and expression to tolerate a short-term heat treatment.