Fig. 3

Factors contributed to plant-growth-promoting activities in SQR9. Indole-3-acetic acid (IAA), 2,3-butanediol and phytase synthesis are shown in the diagram. Genes are shown in italics. The acetolactate synthase AlsS ① and the decarboxylase AlsD ② catalyze the two-step conversion from pyruvate to acetoin. The following enzymes are proposed to be involved in tryptophan-dependent IAA biosynthesis in SQR9 based on a combination of chemical and genetic analyses: ③ tryptophan transaminase (PatB), ④ indole-3-pyruvate decarboxylase (YclC and YclB) and ⑤ indole-3-acetaldehyde dehydrogenase (DhaS) in the IPyA pathway; ⑥ tryptophan decarboxylase (YclC and YclB) and ⑦ amine oxidase in the TAM pathway; ⑧ oxidoreductase and acetaldoxime dehydratase and ⑨ nitrilase (YhcX) in the IAN pathway; and ⑩ tryptophan acetyltransferase (YsnE) in a predicted tryptophan-dependent IAA production pathway. The three genes shown in red, patB, yclC and dhaS, were predicted as a complete IPyA pathway for IAA synthesis, and both homologous expression in B. amyloliquefaciens SQR9 and heterologous expression in B. subtilis 168 successfully increased IAA production. The genes predicted to be involved in IAA synthesis in SQR9 were knocked out in this study. Underlines indicate that the generated knockout mutants produced less IAA than wild type