The biosynthesis of SA takes place from chorismic acid via the enzymatic reaction of ICS or PAL Wildermuth et al. [38]. From chorismic acid, the ICS or PAL is the committed step for further SA biosynthesis [30], therefore, the genes of ICS and PAL were selected to determine the expression levels for the de novo synthesis of SA by the infection of P. putida 229. When the expression levels of the ICS gene were measured, the ICS gene appeared to be highly expressed in soybean sprouts grown under red light (0 h + 0h) (Fig. 2A); therefore, the gene was responsible for the higher levels of SA in sprouts grown under red light. The expression levels in the soybean sprouts grown in darkness or under red light decreased after being incubated with P. putida 229 (3 h + 3 h and 8 h + 8 h), but returned to the original expression levels in soybean sprouts grown under red light after 24 h (Fig. 2A).
Expression of the PAL gene did not differ significantly among soybean sprouts grown in darkness or under red light, or in response to infection with P. putida 229 (Fig. 2A).
To confirm that the increased levels of SA affected expression of the PR gene, we measured the expression level of the PR-1 gene, which is a downstream marker of SA accumulation. After incubation for 24 h (8 h + 24 h), the gene was up-regulated to a greater degree in soybean sprouts grown under red light than in those grown in darkness, but gene expression was not detected in un-infected sprouts (0 h + 0 h) (Fig. 2A).