Functional Analysis Of Strigolactone Biosynthesis Gene MdMAX1 In The Regulation Of Axillary Bud Outgrowth In Apple

High-quality seedlings are the basis of intensive and efficient apple production.Seedlings with good branches are easy to shape.At present,the breeding of large seedlings with branches has become the main development direction of apple seedling in the world.However,the application of large seedlings with branches is less than 30% in China.The research on branching technology and related physiological and molecular mechanism is still weak.Strigolactone(SL)is the main plant hormone that inhibits branching.Most of its studies are focused on model plants,but less in apples.MAX1(MORE AXILLARY GROWTH 1)is a key gene in SL biosynthesis pathway,which is closely related to axillary bud outgrowth.Therefore,in this study,the 1-year-old ‘Nagafu No.2'(a ‘Fuji' cultivar)/T337/M.robusta Rehd.,which natural branching is poor,was as the main material.Combining exogenous hormone treatment and transgenic technology,the expression and function of SL synthesis gene Md MAX1 were identified.In addition,the transcription factors that may interact specifically with apple Md MAX1 promoter were screened by yeast one-hybrid method.The main findings are as follows:1.The treatment of decapitation could significantly induce the outgrowth of apple axillary buds,and application of GR24(SL analogue)after decapitation significantly inhibited the outgrowth of axillary buds.The expression of synthesis and signaling-related genes of SL,BRC1 gene and CK signaling-related negative regulator ARR genes in apple axillary buds were significantly induced,the content of CK and the expression of CK synthesis gene Md IPT2 were obviously inhibited after GR24 treatment.2.A total of four apple MAX1 homologous genes were identified from apple genome,and which protein structures were highly conserved.The expression of Md MAX1 homologous genes were highly in stem,axillary buds and young fruits,both in axillary buds at all determined nodes,but there was no obvious specificity.Among the four homologous genes,the expression of Md MAX1 a can up-regulate in response to exogenous GR24 treatment at 48 h.which means that Md MAX1 a may be involved in the negative regulation of apple axillary bud outgrowth.Heterologous overexpression of Md MAX1 a in tomato showed that Md MAX1 a could inhibit the outgrowth of tomato axillary buds,and overexpression of Md MAX1 a in Arabidopsis can also significantly reduce the branching level of Arabidopsis max1 mutant.3.Md MAX1 a promoter contains a variety of hormone,stress resistance and light response elements.Exogenous GR24 and Lovastatin treatment can activate the GUS activity of Md MAX1 a promoter,while light treatment can significantly inhibit its activity.The proteins interacting with Md MAX1 a promoter were screened by yeast one-hybrid technique in c DNA library,and several genes related to light response and stress resistance were obtained.The preliminary verification of yeast one-hybrid showed that there may be a weak interaction between Md BBX13 gene and Md MAX1 a promoter in apple.To sum up,there are four Md MAX1 homologous genes in apple,among which Md MAX1 a can inhibit axillary bud outgrowth,and Md MAX1 a promoter may be related to light signal,but its specific upstream regulation mechanism remains to be further explored.The function and regulatory mechanism of the other three Md MAX1 homologous genes are not clear and need to be further explored.