Functional Analysis Of CsMYB Transcription Factors In Regulating Lignification Of Postharvest Citrus Fruit Juice Sac

Citrus is an important fruit worldwide.It is valued for being highly nutritious and having an appealing flavor.Lignification of citrus fruit juice sacs is a serious physiological disorder which occurs during postharvest storage.However,their transcriptional regulation mechanisms remains unclear.Lignin is one of the most important components of the plant cell wall,and the expression and transcriptional regulation of lignin biosynthesis-related genes have been studied widely in Arabidopsis and other plants.In this study,we isolated and identified CsMYB330 and CsMYB308 transcription factors,and found that their expression levels are significantly altered during the lignification of citrus fruits juice sacs.We recently found that CsMYB330 and CsMYB308 can recognize and bind AC elements in the Cs4CLl promoter and finely regulate expression of the Cs4CLl gene.In this regulatory process,CsMYB330 was identified as a transcriptional activator,whereas CsMYB308 appears to be a transcriptional repressor.In addition,we demonstrated that expression of the Cs4CL1 gene was significantly altered in fruits juice sacs overexpressing these two transcription factors by transient assay.Furthermore,the potential involvement of a MYB transcription factor,CsMYB85,was identified and isolated and found that it has homologs in Arabidopsis and other plants.All of these regulate lignin biosynthesis.During juice sac lignification,CsMYB85 expression levels significantly increase.Therefore,this gene may control lignin biosynthesis.The results indicated that CsMYB85 binds CsMYB330 promoter,regulates its expression,and interacts with CsMYB308 in yeast and tobacco.A transient assay indicated that Cs4CLl expression levels and lignin content were significantly increased in fruit juice sacs overexpressing CsMYB85.At4CL1 expression levels and lignin content were also significantly increased in Arabidopsis overexpressing CsMYB85.Therefore,the CsMYB85 transcription factors participate in fruit juice sac lignification.These results indicate that the transcription factors CsMYB330,CsMYB308 and CsMYB85 play an important role in the lignification of citrus fruits juice sacs and provide novel insights into the transcriptional regulation associated with fruits juice sac lignification.