Transcriptome analysis of the male-to-hermaphrodite sex reversal induced by low temperature in papaya

Papaya (Carica papaya L.) is a trioecious plant species, producing three sex forms, male, female, and hermaphrodite. Although the major sex types are genetically determined, the phenotypic sex expression of papaya is influenced by environmental factors. We investigated differential gene expression analysis between the 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 aiming to understanding the gene regulatory network that determinates the phenotypic sex expression in papaya. Our differential gene expression analysis revealed 1756 differentially expressed genes between normal male and male-to-hermaphrodite sex reversal flowers. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genome (KEGG) pathway enrichment analysis showed transcription factors, flower development, histone H3-K9 methylation, and plant hormone signal transduction were among the most significantly enriched GO terms and KEGG pathways. Small RNA analysis was also performed on the pistils from normal male and the male-to-hermaphrodite sex reversal flowers. Our result showed the 24 nt small RNAs were the most abundant in the pistils from both normal and sex reversal flowers, followed by 21 nt small RNAs. We detected expression of 40 plant-conserved miRNAs and 14 papaya-specific miRNAs in the pistils from one or both normal and sex reversal flowers. Sixteen miRNAs exhibited high-expression level and ten of them showed differential expression between the normal male and the male-to-hermaphrodite sex reversal flowers. Our results suggested the male-to-hermaphrodite sex reversal was likely caused by silencing the gynoecium suppression function on the sex determination pathway through epigenetic modification.