Light on perenniality: Para-dormancy is based on ABA-GA antagonism and endo-dormancy on the shutdown of GA biosynthesis

Perennial para- and endo-dormancy are seasonally separate phenomena. Whereas para-dormancy is the suppression of axillary buds (AXBs) by a growing shoot, endo-dormancy is the short-day elicited arrest of terminal and AXBs. In hybrid aspen (Populus tremula x P. tremuloides) compromising the apex releases para-dormancy, whereas endo-dormancy requires chilling. ABA and GA are implicated in both phenomena. To untangle their roles, we blocked ABA biosynthesis with fluridone (FD), which significantly reduced ABA levels, downregulated GA-deactivation genes, upregulated the major GA3ox-biosynthetic genes, and initiated branching. Comprehensive GA-metabolite analyses suggested that FD treatment shifted GA production to the non-13-hydroxylation pathway, enhancing GA(4) function. Applied ABA counteracted FD effects on GA metabolism and downregulated several GA(3/4)-inducible alpha- and gamma-clade 1,3-beta-glucanases that hydrolyze callose at plasmodesmata (PD), thereby enhancing PD-callose accumulation. Remarkably, ABA-deficient plants repressed GA(4) biosynthesis and established endo-dormancy like controls but showed increased stress sensitivity. Repression of GA(4) biosynthesis involved short-day induced DNA methylation events within the GA3ox2 promoter. In conclusion, the results cast new light on the roles of ABA and GA in dormancy cycling. In para-dormancy, PD-callose turnover is antagonized by ABA, whereas in short-day conditions, lack of GA(4) biosynthesis promotes callose deposition that is structurally persistent throughout endo-dormancy.

Automated summary

Perennial para- and endo-dormancy are seasonally separate phenomena. Para-dormancy is the suppression of axillary buds (AXBs) by a growing shoot, while endo-dormancy is the short-day elicited arrest of terminal and AXBs. ABA and GA play important roles in both phenomena. Blocking ABA biosynthesis reduced ABA levels, upregulated GA3ox-biosynthetic genes, and initiated branching, while blocking GA(4) biosynthesis promoted callose deposition and endo-dormancy. The results provide new insights into the roles of ABA and GA in dormancy cycling.