Abstract
Cytokinin (CK) is the primary hormone that positively regulates axillary bud outgrowth. However, in many woody plants, such as Jatropha curcas, gibberellin (GA) also promotes shoot branching. The molecular mechanisms underlying GA and CK interaction in the regulation of bud outgrowth in Jatropha remain unclear. To determine how young axillary buds respond to GA3 and 6-benzyladenine (BA), we performed a comparative transcriptome analysis of the young axillary buds of Jatropha seedlings treated with GA3 or BA. Two hundred and fifty genes were identified to be co-regulated in response to GA3 or BA. Seven NAC family members were down-regulated after treatment with both GA3 and BA, whereas these genes were up-regulated after treatment with the shoot branching inhibitor strigolactone. The expressions of the cell cycle genes CDC6, CDC45 and GRF5 were up-regulated after treatment with both GA3 and BA, suggesting they may promote bud outgrowth via regulation of the cell cycle machinery. In the axillary buds, BA significantly increased the expression of GA biosynthesis genes JcGA20oxs and JcGA3ox1, and down-regulated the expression of GA degradation genes JcGA2oxs. Overall, the comprehensive transcriptome data set provides novel insight into the responses of young axillary buds to GA and CK.
Highlights
The plant above-ground architecture is primarily due to the complex spatial-temporal regulation of shoot branching
We previously reported that exogenous treatment with gibberellin A3 (GA3) or BA effectively promote the axillary bud outgrowth in J. curcas (Fig. S1), by suppressing negative regulators of shoot branching, such as JcBRC1, JcBRC2, and JcMAX219
Our results showed that the expressions of JcBEL1 (Cluster-26.17792) and JcTFL1b (Cluster-26.17723) in J. curcas were significantly decreased in the buds after GA3 or BA treatment (Fig. 3C and J), as in the activated axillary buds of hybrid aspen, where TERMINAL FLOWER 1 (TFL1) ortholog CENL1 is down-regulated during axillary bud activation[65], suggesting that down-regulation of JcBEL1 and JcTFL1b was correlated with bud outgrowth in J. curcas
Summary
The plant above-ground architecture is primarily due to the complex spatial-temporal regulation of shoot branching. GA biosynthesis is crucial in axillary bud formation and activation[2] These findings demonstrated that in many perennial woody plants, the regulation of shoot branching may differ from that in annual, herbaceous plants, such as Arabidopsis, pea and rice. Quantitative real-time PCR analyses have shown that GA3 and BA have common target genes, such as JcBRC1, JcBRC2 and JcMAX219, key inhibitors of bud outgrowth in pea, rice and Arabidopsis[12, 16, 37,38,39,40,41,42] These results suggest that the downstream regulatory networks of bud outgrowth mediated by these two hormones could interact in J. curcas[19]. Further detailed analyses of the function of these genes could provide novel insights into the regulatory network of phytohormones that control axillary bud outgrowth
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