Research On The Function Of Strigolactones In The Regulation Of Shoot Branching In Tomato

The greenhouse vegetable industry is developing rapidly in our country,and it plays an important role in ensuring the annual production and stable supply of vegetables.Plant architecture is one of the main factors affecting "crop yield per unit area",which is of great significance to the agricultural practices like agricultural mechanization.Plant architecture is plastic and consists of two parts: the shoot architecture and the root architecture.Shoot branching is the important part of the shoot architecture,and it differs among plant species and growing stages.Tomato(Solanum lycopersicum)is one of the model plants for the research on shoot branching which affects yields,quality and production efficiency.Therefore,it is of great theoretical and practical values to study the mechanisms and the efficient regulation means of tomato shoot branching.Shoot branching of tomato is a comprehensive result of genetic factors,environment,plant hormones and many other factors,among which plant hormones mediate genetic and environmental signals and play an important role in regulating branching.The classical hormones that affect branching are auxin and cytokinin(CKs).Strigolactones(SLs)as a new hormone regulating shoot branching is one of the research hotspots in recent years.However,it is still unclear as for the mechanisms of signal crosstalk between strigolactones and other hormones in the regulation of shoot branching in tomato.Here we use tomato as a kind of research material,and study the functions of SLs in tomato shoot branching by decapitation,strigolactones synthetic analogue GR24 treatment on roots,CRISPR/Cas9 technology and plant tissue culture.In addition,roles of auxin and CKs on SLs-regulated shoot branching was studied according to the content of auxin and CKs in SLs-deficient mutant.We further study the interactions between SLs and CKs on regulating the tomato shoot branching development utilizing VIGS to get RR10-silenced plants and cytokinin synthetic analogue 6BA treatment on buds.The main results are as follows:1.We confirmed the negative regulation of SLs on tomato shoot branching.The length of the lateral shoots was significantly increased by decapitation.By contrast,simultaneous treatment of decapitation and GR24 results in the shorter lateral shoot length than the treatment with decapitation,and longer than the treatment with GR24,which preliminarily showed that SLs can partially inhibits the shoot branching caused by the removal of apical dominance.Furthermore,we utilize CRISPR/Cas9 technology to get ccd7 and max2-2 mutant,which are defective in SLs synthesis and key signaling element respectively,and found that the mutants have longer lateral shoot length and show dwarfism.Finally,most of the plant-specific transcription factor SPLs are downregulated.These results demonstrated that SLs can inhibit the development of tomato shoot branching.2.We showed that SLs regulate the shoot branching probably by regulating auxin and CKs.Expression of the synthesis and response genes of CKs were inhibited after GR24 treatment on roots;The content of auxin in the shoot apical bud and node of ccd7 mutant was increased compared with WT;the total amount of CKs was increased as compared to WT,resulting from the increase of TZ,TZR,and iPR.In the node of ccd7,the expression levels of LOG1 and LOG4 were significantly up-regulated,and an CKs response regulator RR4 was strongly induced,while CKX2,CKX7 and CKX8 genes involving in degrading CKs were down-regulated.The results showed that SLs may regulate tomato shoot branching by influencing auxin and suppressing the accumulation of CKs.3.We demonstrated the effects of strigolactones and cytokinins on the sugar metabolism in tomato lateral buds.Comparing with the WT,the expression of CWIN2,CWIN4 and VIN2 encoding sucrose invertases were significantly induced in ccd7 buds,while on the contrary,the expression of VIF,one of the sucrose invertase inhibitors,were inhibited,which showed that SLs may affect the expression of genes related to the hydrolysis progress of sucrose.Furthermore,the relative expression of CWIN2 and VIN2 were induced after 6BA treatment;the RR10-silenced plants have weak branching,and the expression of the axillary bud outgrowth inhibitor BRC1 was up-regulated.Meanwhile,the expressions of sucrose invertase encoding genes such as CWIN2,CWIN3 and VIN2 were suppressed in the lateral bud of RR10-silenced plants.Taken together,we showed that sugar may function as a downstream factor of SLs and CKs to regulate tomato shoot branching.