Characteristics and functions of volatile organic compounds in the tripartite symbiotic system of <i>Gastrodia elata-Armillaria gallica-Rahnella aceris</i> HPDA25

Tripartite interactions among plants, fungi, and bacteria are critical for maintaining plant growth and fitness, and volatile organic compounds (VOCs) play a significant role in these interactions. However, the functions of VOCs within the niche of mycoheterotrophic plants, which represent unique types of interactions, remain poorly understood. <i>Gastrodia elata</i>, a mycoheterotrophic orchid species, forms a symbiotic relationship with specific <i>Armillaria</i> species, serving as a model system to investigate this intriguing issue. <i>Rahnella aceris</i> HPDA25 is a plant growth-promoting bacteria isolated from <i>G. elata</i>, which has been found to facilitate the establishment of <i>G. elata-Armillaria</i> symbiosis. In this study, using the tripartite symbiotic system of <i>G. elata-Armillaria gallica-R. aceris</i> HPDA25, we investigate the role of VOCs in the interaction among mycoheterotrophic plants, fungi, and bacteria. Our results showed that 33 VOCs of HPDA25-inducible symbiotic <i>G. elata</i> elevated compared to non-symbiotic <i>G. elata</i>, indicating that VOCs indeed play a role in the symbiotic process. Among these, 21 VOCs were accessible, and six active VOCs showed complete growth inhibition activities against <i>A. gallica</i>, while <i>R. aceris</i> HPDA25 had no significant effect. In addition, three key genes of <i>G. elata</i> have been identified that may contribute to the increased concentration of six active VOCs. These results revealed for the first time the VOCs profile of <i>G. elata</i> and demonstrated its regulatory role in the tripartite symbiotic system involving <i>G. elata</i>, <i>Armillaria</i>, and bacteria.