Bacillus velezensis promotes growth and flavonoid accumulation in Anoectochilus roxburghii via phytohormone signaling

Beneficial Bacillus agents play crucial roles in promoting crop growth and metabolic processes by modulating phytohormone levels and increasing the availability of soil nutrients. This study investigated the phytohormone signaling pathways to elucidate the underlying mechanisms by which Bacillus velezensis promotes root development, nutrient uptake, and the accumulation of bioactive compounds in Anoectochilus roxburghii Lindl. It was found that B. velezensis promoted fine root development by upregulating genes involved in the synthesis of 3-indoleacetic acid (IAA) and downregulating genes involved in the synthesis of abscisic acid (ABA). In addition, the application of B. velezensis significantly increased the soil available phosphate content, promoted the expression of P transporter, and significantly increased the absorption of P in A. roxburghii. Furthermore, it upregulated key enzymes, such as CYP73A, CHS and LDOX, in the flavonoid synthesis pathway. And the addition of 0.5 mg/L IAA significantly up-regulated the expression of flavonoid synthesis gene LDOX, while the addition of 0.5 mg/L ABA significantly suppressed the expression of the flavonoid synthesis gene F3'H. This study demonstrates that B. velezensis promotes flavonoid synthesis in A. roxburghii by regulating IAA and ABA levels, via a sequence beginning with enhanced phosphorus uptake, followed by root growth and activation of specific flavonoid-related genes. The findings of this study contribute to a better understanding of how beneficial microorganisms can be utilized to develop efficient cultivation practices for horticultural crops.