Kihara Institute for Biological Research, Yokohama City University Kihara Institute for Biological Research, Maiokacho 641-12, Totsuka, Yokohama, Kanagawa, 244-0813 Japan.
RIKEN Plant Science Center, Suehirocho 1-7-22, Tsurumi, Yokohama, 230-0045 Japan.
Plant Cell Physiol. 2020 Jul 1;61(7):1239-1251. doi: 10.1093/pcp/pcaa053.
Although brassinosteroids (BRs) have been proposed to be negative regulators of photomorphogenesis, their physiological role therein has remained elusive. We studied light-induced photomorphogenic development in the presence of the BR biosynthesis inhibitor, brassinazole (Brz). Hook opening was inhibited in the presence of Brz; this inhibition was reversed in the presence of brassinolide (BL). Hook opening was accompanied by cell expansion on the inner (concave) side of the hook. This cell expansion was inhibited in the presence of Brz but was restored upon the addition of BL. We then evaluated light-induced organ-specific expression of three BR biosynthesis genes, DWF4, BR6ox1 and BR6ox2, and a BR-responsive gene, SAUR-AC1, during the photomorphogenesis of Arabidopsis. Expression of these genes was induced, particularly in the hook region, in response to illumination. The induction peaked after 3 h of light exposure and preceded hook opening. Phytochrome-deficient mutants, hy1, hy2 and phyAphyB, and a light-signaling mutant, hy5, were defective in light-induced expression of BR6ox1, BR6ox2 and SAUR-AC1. Light induced both expression of BR6ox genes and petiole development. Petiole development was inhibited in the presence of Brz. Our results largely contradict the early view that BRs are negative regulators of photomorphogenesis. Our data collectively suggest that light activates the expression of BR biosynthesis genes in the hook region via a phytochrome-signaling pathway and HY5 and that BR biosynthesis is essential for hook opening and petiole development during photomorphogenesis.
虽然油菜素内酯（BRs）被认为是光形态建成的负调控因子，但它们在其中的生理作用仍不清楚。我们研究了在 BR 生物合成抑制剂油菜素唑（Brz）存在下的光诱导光形态建成发育。Brz 的存在抑制了钩的张开；而在油菜素内酯（BL）的存在下，这种抑制作用被逆转。钩的张开伴随着钩内侧（凹面）细胞的扩张。Brz 的存在抑制了细胞的扩张，但添加 BL 后恢复了。然后，我们评估了光诱导的三个 BR 生物合成基因 DWF4、BR6ox1 和 BR6ox2，以及一个 BR 反应基因 SAUR-AC1 在拟南芥光形态建成过程中的器官特异性表达。这些基因的表达在光照下被诱导，特别是在钩区。诱导在光照 3 小时后达到峰值，先于钩的张开。光敏色素缺陷突变体 hy1、hy2 和 phyAphyB 以及光信号突变体 hy5 在 BR6ox1、BR6ox2 和 SAUR-AC1 的光诱导表达中存在缺陷。光诱导 BR6ox 基因的表达和叶柄发育。Brz 的存在抑制了叶柄的发育。我们的结果在很大程度上与 BRs 是光形态建成的负调节剂的早期观点相矛盾。我们的数据共同表明，光通过光敏色素信号通路和 HY5 激活钩区 BR 生物合成基因的表达，BR 生物合成对于光形态建成过程中钩的张开和叶柄发育是必不可少的。
