Research progresses on GH3s, one family of primary auxin-responsive genes

Auxin plays a very important role in plant growth and development. Those genes that are specifically induced by auxin within minutes of exposure to the hormone are referred to as early/primary auxin-responsive genes, mainly including the auxin/indole-3-acetic acid (Aux/IAA), the small auxin-up RNA (SAUR), and the GH3 gene families. So far, GH3 genes have been identified in various plant species including soybean, Arabidopsis, rice, tobacco, pungent pepper, sweet orange, pine, and moss. Twenty members of GH3 family were identified in Arabidopsis and these genes were classified into three groups (Group I-III) based on their sequence similarities and substrate specificities. GH3s belong to acyl adenylate-forming firefly luciferase superfamily and can catalyze adenylation of specific substrates. Group I adenylates jasmonic acid (JA), and Group II adenylates indole-3-acetic acid (IAA) and salicylic acid (SA), respectively. Because of the presence of Auxin-Responsive Elements (AuxRE) in the GH3s' promoter regions, Auxin Response Factors (ARFs) are able to bind to the AuxRE and regulate expression of some GH3s, which in turn modulate the auxin homeostasis. Identification of GH3 mutants in Arabidopsis reveals the function of GH3s in hypocotyl elongation under different light conditions, root growth, stress adaptation, sensitivity to MeJA, or susceptibility to P. syringae. Taken together, GH3s may be linkers among auxin, JA, SA and light signal transduction pathways.