Response and degradation of indole-3-aceticacid by the plant growth-promoting rhizobacteria Variovorax boronicumulans

Plant growth-promoting rhizobacteria (PGPR) of the genus Variovorax facilitate plant growth through beneficial microbe-plant interplay. Unlike most PGPRs, Variovorax boronicumulans CGMCC 4969 utilizes indole-3-acetonitrile as a precursor to generate indole-3-acetic acid (IAA), which is subsequently metabolized by itself. In this study, IAA enhanced the growth of V. boronicumulans CGMCC 4969 in minimal salt medium (MSM), whereas it inhibited bacterial growth when glucose was added to the MSM broth. IAA was rapidly degraded within 12 h in MSM broth despite glucose appeared or not. Notably, in LB broth, the cell growth was significantly inhibited by IAA concentration beyond 1 mmol/L, while the IAA degradation capability of CGMCC 4969 was significantly increased following exposure to IAA-dosed LB medium. V. boronicumulans CGMCC 4969 degraded IAA to yield a new intermediate 3-hydroxy-anthranilate. An iad gene cluster was identified in V. boronicumulans CGMCC 4969, and co-expression of the iadD and iadE genes endows Escherichia coli with the capacity to degrade IAA. This degradation efficiency is augmented when the iadC gene is expressed simultaneously. Subsequent proteomics and bioinformatics analyses highlighted that the addition of IAA induced a significant up-regulation of ABC transporter proteins, in particular IadK3 and IadK2. Interestingly, there was also a significant increase in protein expression associated with group-sensing metabolism. Collectively, this research helps our understanding of the intricate regulatory mechanisms of IAA within Variovorax own metabolism and expands our knowledge of its complex role in plant-microbe interactions. KEY POINTS: The iad gene cluster degraded IAA to a previously uncharacterized intermediate. Adding IAA during the cell culture period enhances IAA-degrading activity. Proteomics defined the adaptive response to IAA.