Gibberellic Acid Inhibits - Symbiosis by Regulating the Expression of Cell Wall Metabolism Genes.

Orchid seeds lack endosperms and depend on mycorrhizal fungi for germination and nutrition acquisition under natural conditions. is a mycorrhizal fungus that promotes seed germination and seedling development in epiphytic orchids, such as . To understand the impact of on seed germination, we examined endogenous hormone levels by using liquid chromatography-mass spectrometry. We performed transcriptomic analysis of protocorm at two developmental stages under asymbiotic germination (AG) and symbiotic germination (SG) conditions. The result showed that the level of endogenous IAA in the SG protocorm treatments was significantly higher than that in the AG protocorm treatments. Meanwhile, GA was only detected in the SG protocorm stages. IAA and GA synthesis and signaling genes were upregulated in the SG protocorm stages. Exogenous GA application inhibited fungal colonization inside the protocorm, and a GA biosynthesis inhibitor (PAC) promoted fungal colonization. Furthermore, we found that PAC prevented fungal hyphae collapse and degeneration in the protocorm, and differentially expressed genes related to cell wall metabolism were identified between the SG and AG protocorm stages. Exogenous GA upregulated and expression, leading to decreased fungal colonization. Meanwhile, GA inhibitors upregulated , , and expression, leading to increased fungal colonization. Our findings suggest that GA regulates the expression of cell wall metabolism genes in , thereby inhibiting the establishment of mycorrhizal symbiosis.