Molecular Basis Of Sex Reversal Of Papaya Male Flowers Induced By Low Temperature

Papaya(Carica papaya L.)is the 5th angiosperm plant genome that has been fully sequenced,with abundant genetic and genomic resources.Papaya has a pair of nascent sex chromosome,and is a model plant for sex chromosome evolution studies.Male and hermaphrodite are relatively unstable.Sex expression of papaya is affected by environmental factors.Hermaphrodite plants can generate infertile male flowers and "cat face-like" low-quality fruits through stamen carpellody under stress condition.Although sex determination genes of papaya haven't been cloned yet,there are at least a suppressing gene of gynoecium and a promoting gene of androecium in the sex determination region.We sequenced transcriptome and small RNA libraries of non-functional rudimentary pistils from normal male flowers and developed and functional pistils from the male-to-hermaphrodite sex reversal flowers induced by low temperature and conducted differential gene expression as the 1st step to decipher the mechanism underlying the sex reversal phenomenonOur differential gene expression analysis revealed 1756 significantly differentially expressed genes between normal male and male-to-hermaphrodite sex reversal flowers,including 1189 up-regulated and 567 down-regulated genes.Gene ontology and Kyoto Encyclopedia of Genes and Genome pathway enrichment analysis showed transcription factor,flower development,histone H3-K9 methylation,and plant hormone signal transduction were among the most significantly enriched GO terms and KEGG pathways.Among the 1756 differentially expressed genes,there are 182 transcription factors and 88 hormone-related genes.Surprisingly,all the 57 genes involved in histone H3 lysine 9 methylation were up-regulated in the male-to-hermaphrodite sex reversal flowers.Small RNA-seq analysis was also performed.Besides 28 plant-conserved and 23 papaya-specific miRNAs reported by previous studies,we identified 16 additional plant plant-conserved and 9 additional papaya-specific miRNAs using miRDeep-P packages.We found 9 miRNAs exhibited significantly differentially expressed expression between normal male and male-to-hermaphrodite sex reversal flowers.Our results suggested that the male-to-hermaphrodite papaya sex reversal was likely caused by silencing the gynoecium suppression function on the sex determination pathway through epigenetic modification.Hormone signal transduction,transcription factors,and histone methylation played important roles during the process.