Arbuscular mycorrhizal symbiosis enhances perennial ryegrass growth during temperature stress through the modulation of antioxidant defense and hormone levels

Abstract

Perennial ryegrass (Lolium perenne L.) refers to a vital species of grass grown extensively as a biofuel and biochemical production across temperate regions. However, both low temperature (LT) and high temperature (HT) conditions are unfavorable for the development of this species, causing reduced forage yield. Arbuscular mycorrhizal (AM) fungi are recognized for promoting host plants’ resistance to abiotic stresses via different mechanisms. However, the mechanism underlying the alleviation of plant damage induced by high-temperature stress and low-temperature stress by AM fungi remains to be elucidated so far. Therefore, the current work attempted to explore how the symbiosis with AM fungus Rhizophagus irregularis (R. irregularis) affects the antioxidant activity and hormone levels in ryegrass under LT and HT stresses and reveals the molecular mechanisms. In addition, the contents of non-enzymatic antioxidants, and hydrogen peroxide (H2O2), the activities of antioxidant enzymes, the genes encoding antioxidant enzymes, and the endogenous hormone-related genes in the perennial ryegrass inoculated with or without R. irregularis were analyzed upon exposure to LT and HT stresses. The results showed that inoculating AM plants with R. irregularis alleviated growth inhibition caused by LT and HT stress, as evidenced by significantly higher plant height, shoot and root dry weight, leaf chlorophyll and carotenoid content, net photosynthesis rate, and photochemical efficiency (Fv/Fm) in comparison to non- mycorrhizal (NM) plants under LT and HT stress. Under LT and HT conditions, AM symbiosis decreased electrolyte leakage (EL), malondialdehyde (MDA), H2O2, and abscisic acid (ABA) contents while significantly increasing the activities of our analyzed enzymes (with the exception of peroxidase (POD) in shoots and ascorbate peroxidase (APX) in roots), AsA/DHA and GSH/GSSG ratio, gibberellic acid 3 (GA3), jasmonic acid (JA) and indoleacetic acid (IAA) contents in shoots and roots. R. irregularis inoculation significantly upregulated almost all genes encoding antioxidant enzyme we tested (except for LpGPX1, LpGPX3, and LpGR in shoots, which were unaffected by AM symbiosis in 3d LT treatment) and some related genes in JA biosynthetic pathway, gibberellin synthesis as well as auxin transport and signaling while down-regulated the biosynthesis and signaling genes involved in ABA.In general, the findings suggested that AM symbiosis regulated the antioxidant defense and the hormone levels in ryegrass, which consequently alleviated the temperature stress-induced plant inhibition.