Gibberellin Regulates Bud Outgrowth In Jatropha Curcas

Plant shoot architecture is mainly determined by the appropriate shoot branching,which is a reflection to the environmental conditions,such as soil nutrients,radiation,photoperiod and the growth density.In the model plants pea,Arabidopsis and rice,the manipulation of the lateral bud outgrowth is determined by both the environmental clues and the endogenous hormone signals.In the hormonal level,the lateral bud outgrowth is positively regulated by CK,while inhibited by auxin and the recently identified hormone-strigolactones.Sugar was also identified as an important signal molecular that positively regulates the lateral bud outgrowth in pea.The interactions among CK,auxin and SLs decide the fate whether the lateral bud should start to grow or stay in dormant.The study of gibberellin on the control of the lateral bud was few.In Arabidopsis,rice and poplar,GA seems to inhibit the lateral bud outgrowth.However,in this study,we demonstrated that GA acts as a positive regulator in the control of the lateral bud outgrowth in a perennial woody plant Jatropha curcas.The main results in this study are listed as follows:1.Gibberellin promotes the lateral bud outgrowth in J.curcas.In the adult plants or the seedlings,both GA and CK could effectively promote the lateral bud outgrowth in a dose-dependent manner.GA and CK also act differently when applied onto the branch or bud.More lateral bud could be stimulated by GA application while BA treatment only can promote the bud outgrowth.Moreover,GA treatment can stimulate the secondary lateral bud outgrowth while BA can't.Furthermore,BRC1 and BRC2 are key regulators of the lateral bud outgrowth,the expression of which was inhibited by both GA3 and BA in the J.curcas seedlings.2.GA and CK corporately regulate the lateral bud outgrowth.Our physiological experiments showed that the combined treatment with both GA and CK was more effective in promoting the lateral bud outgrowth in the seedlings than that by single GA3 or BA treatment.The inhibition of gibberellin biosynthesis by paclobutrozal treatment significantly inhibited the bud outgrowth promoted by BA treatment indicating GA maybe involved in the regulation of bud outgrowth by CK.Accordingly,when the seedling root,which acts as the main CK biosynthesis place,was removed,the promotion of the lateral bud outgrowth at the shoot by GA3 treatment was also inhibited.The addition of BA in the liquid culture or in the GA3 solution could rescue the bud outgrowth.These results demonstrated that both CK and GA could be required for promoting the bud outgrowth.3.Auxin and SLs inhibit the bud outgrowth promoted by GA and CK.Our results showed that both auxin and SLs inhibit the bud outgrowth promoted by GA3 or BA.Furthermore,GA3 and BA inhibit the expression of the SLs biosynthesis gene CCD7.MAX2,the key regulator of SLs signaling,was also down-regulated by GA3 or BA treatment.It indicated that the regulation of SLs biosynthesis and signaling could contribute to the lateral bud outgrowth by GA or CK treatment.4.The transcriptomic study of the lateral bud exposed to GA3 and BA treatment.Our results showed that,there are 872 genes regulated by BA treatment,while only 342 genes by GA3 treatment.Nevertheless,there are 259 genes were commonly regulated both by BA or GA3.Among these genes,a group of NAC family genes were identified to be down-regulated both by GA3 or BA treatment.Interestingly,most of these NAC genes were conversely regulated by GR24.There are some other transcription factors that have been reported to be involved in the regulation of shoot development or bud growth.such as JcDRM1,JcDRM1-LIKE,JcDEAR2 and JcBEL1,were also regulated by GA3 and BA.Furthermore,our results also showed that BA treatment actually up-regulate most of the GA biosynthesis genes(JcGA20ox and JcGA3ox),while down-regulate all the JcGA2ox family genes which are involved in GA anabolism.This is in accord with the physiological results that GA is required in the promotion of bud outgrowth by BA treatment.5.GA20ox1 is key regulator in the GA biosynthesis pathway.In this study,we generated the transgenic J.curcas overexpressing JcGA20ox1.Compared with the WT plants,the transgenic plants showed accelerated shoot growth,and also the bud outgrowth.Our results also showed that the over-produced GAs in the transgenic plants also showed some other phenotypes,such as enhanced lignification,larger inflorescence,female flower and fruits.decreased seed oil content.6.GA promotes the bud outgrowth in some other perennial woody plants.Many studies of the bud outgrowth regulation by GA and CK were also carried in another economic plant,papaya.Similar to the results in J.curcas,GA could also effectively promote the lateral bud outgrowth not only in the adult trees,but also in the seedlings.Further,the positive role of GA on the bud outgrowth regulation was found on many other perennial plants,such as Morus alba,Jatropha gossypifolia,nihot esculenta,Eeutherococcuss senticsus,Elaeagnus conferta,Elaeagnus loureirii,Uvaria microcarpa,Bauhinia variegate,Murraya exotica,Casimiroa edulis.These results demonstrate that GA may act as a positive regulator of the lateral bud outgrowth ubiquitously existed in the perennial woody plants.