In plants, GH3 gene family is widely considered to be involved in a broad range of plant physiological processes, through modulation of hormonal homeostasis. Multiple GH3 genes have been functionally characterized in several plant species, however to date, limited works to study the GH3 genes in tomato have been reported. Here, we characterize the expression and regulatory profiles of six tomato GH3 genes, SlGH3.2, SlGH3.3, SlGH3.4, SlGH3.7, SlGH3.9 and SlGH3.15 in response to different phytohormone applications and arbuscular mycorrhizal (AM) fungal colonization. All the six GH3 genes showed inducible responses to external indole-3-acetic acid (IAA), and three members were significantly upregulated in response to AM symbiosis. Particularly, SlGH3.4, of which the transcripts were barely detectable under normal growth condition, was strongly activated in the IAA-treated and AM fungal-colonized roots. A comparison of the SlGH3.4 expression in wild-type plants and M161, a mutant with defection in AM symbiosis confirmed that the SlGH3.4 expression is highly correlated to mycorrhizal colonization. Histochemical staining demonstrated that a 2258-bp SlGH3.4 promoter fragment could drive the GUS expression strongly in root tips, steles and cortical cells of IAA-treated roots, but predominantly in the fungal-colonized cells of mycorrhizal roots. A truncated promoter with 654 bp in length failed to direct the GUS expression in IAA-treated roots, but maintained the symbiosis-induced activity in mycorrhizal roots. In summary, our results propose that a mycorrhizal signaling pathway that is at least partially independent to the auxin signaling pathway have been evolved in the co-regulation of the auxin- and mycorrhiza-activated GH3 genes in plants.
Via Jean-Michel Ané, Christophe Jacquet