Plastid G6PD3 and cytosolic G6PD5/6 are involved in the Ustilago sp. HFJ311-enhanced tolerance to cadmium stress in Arabidopsis

BACKGROUND: Cadmium (Cd) contamination in soils severely threatens agricultural security and productivity. Glucose-6-phosphate dehydrogenase (G6PD), a key rate-limiting enzyme in the pentose phosphate pathway, is crucial for plant growth and stress tolerance. The endophytic fungus Ustilago sp. HFJ311 enhances plant Cd tolerance, but the underlying mechanism, especially the regulatory roles of G6PD in this process, remains unclear. RESULTS: Here, we demonstrate that plastid G6PD3 and cytosolic G6PD5/G6PD6 synergistically govern the HFJ311-induced Cd tolerance in Arabidopsis. HFJ311 significantly enhanced seedling biomass, leaf area, stomatal density, and photosynthetic pigment content in wild type (Col-0) under control and Cd treatment conditions. Comparatively, compared with Col-0, the shoot fresh weight (FW) of g6pd5 and g6pd6 and the root FW of g6pd3 were significantly reduced under Cd+HFJ311 conditions. Under Cd stress, all the growth-promoting effects of HFJ311 were further attenuated in g6pd3/5/6 triple mutants. Mechanistically, HFJ311 up-regulated G6PD3/5/6 expressions, maintained nicotinamide adenine dinucleotide phosphate (NADPH) homeostasis, and enhanced antioxidant enzyme activities and ascorbate-glutathione (ASA-Glu) cycle efficiency to mitigate reactive oxygen species (ROS) burst and oxidative damage. Meanwhile, HFJ311 down-regulated the expression of Cd transporter genes (Iron-Regulated Transporter 1, IRT1; ZRT/IRT-like proteins, ZIPs; NATURAL RESISTANCE-ASSOCIATED MACROPHAGE PROTEIN 3/4, NRAMP3/4), ultimately reducing Cd transport from roots to shoots. The redox balance in the g6pd3/5/6 mutant was disrupted, thus aggravated Cd translocation and triggered a nitric oxide (NO) overproduction. However, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO), a NO scavenger, completely abolished the protective effects of HFJ311 under Cd stress, indicating that NO is involved in growth-promoting effect of HFJ311. CONCLUSION: Collectively, our findings uncover a novel mechanism whereby G6PD3/5/6 act as central hubs mainly integrating redox homeostasis, Cd transport and chelation to mediate endophyte-enhanced Cd tolerance.