LAUSR

Brassinosteroids (BRs) are essential plant hormones. In angiosperms, brassinolide, the most active BR and castasterone, the second most active BR, are synthesised by CYP85A2 and CYP85A/A1, respectively. BRs function through an essential receptor BRI1 (BR Insensitive 1) in angiosperms. In addition, some angiosperms have nonessential BRI1-like 1/3 (BRL1/3). In conifers, BRs promote seed germination under drought-stress. Yet, how BRs function in gymnosperms is unknown. We addressed this problem by functional complementation of BR biosynthesis and receptor genes from Picea abies (Pa) with respective Arabidopsis mutants. Here, we report that Picea possesses functional PaCYP85A and PaBRL1 but not PaCYP85A2 or PaBRI1 for weak BR signaling. We found that both PaBRL1 and PaCYP85A were abundantly expressed in Picea. However, neither BR treatment of Picea seedlings nor expression of PaBRL1 in Atbri1 promoted plant height, yet BR responsive genes were activated. Importantly, chimeric AtBRI1 replaced with the BR-binding domain of PaBRL1 complemented Atbri1 phenotypes. Furthermore, PaBRL1 had less kinase activity than BRI1 in vitro. Altogether, Picea has weaker but active BR signaling, explaining an aspect of its slow growth and high stress-tolerance. Our study sheds light on the functional and evolutionary significance of distinct BR signaling independent of BRI1 and brassinolide in plants.