Does the brassinosteroid signal pathway in photomorphogenesis overlap with the gravitropic response caused by auxin?

Abstract

Brassinosteroid (BR) and auxin co-regulate plant growth in a process termed cross-talking. Based on the assumption that their signal transductions are partially shared, inhibitory chemicals for both signal transductions were screened from a commercially available library. A chemical designated as NJ15 (ethyl 2-[5-(3,5-dichlorophenyl)-1,2,3,4-tetrazole-2-yl]acetate) diminished the growth promotion of both adzuki bean epicotyls and Arabidopsis seedlings, by the application of either BR or auxin. To understand its target site(s), bioassays with a high dependence on the signal transduction of either BR (BR-signaling) or auxin (AX-signaling) were performed. NJ15 inhibited the photomorphogenesis of Arabidopsis seedlings grown in the dark, which mainly depends on BR-signaling, while NJ15 also inhibited their gravitropic responses mainly depending on AX-signaling. On the study for the structure–activity relationships of NJ15 analogs, they showed strong correlations on the inhibitory profiles between BR- and AX-signalings. These correlations imply that NJ15 targets the downstream pathway after the integration of BR- and AX-signals.

The chemical NJ15 affects hypocotyl elongation and shoot gravitropism via cutin polymerization

“…In our previous report [3], we showed that the brassinosteroid biosynthesis inhibitor brassinazole (Brz) decreased the hypocotyl length but not the gravitropic response of etiolated seedlings. Conversely, treatment with the auxin transport inhibitor NPA inhibited only the gravitropic response of etiolated seedlings, but not their hypocotyl length.…”

“…To elucidate the common downstream pathways of auxin and brassinosteroid signaling, we attempted to find novel chemicals that inhibit both auxin and brassinosteroid signaling simultaneously [3]. We finally obtained a compound, named NJ15, which showed inhibitory effects on the two hormones in Arabidopsis.…”