Transcriptome analysis of San Pedro-type fig (Ficus carica L.) parthenocarpic breba and non-parthenocarpic main crop reveals divergent phytohormone-related gene expression

Abstract

San Pedro-type fig (Ficus carica L.) produces two crops a year, parthenocarpic breba and non-parthenocarpic main crop. The mechanism underlying the two crops’ differential parthenocarpy is largely unknown. For the first time, transcriptomic analysis of young breba and main crop at four stages of developmental phase I was performed by RNA sequencing, including two pre-anthesis young fruit stages and flowers at anthesis and post-anthesis stages. KEGG enrichment analysis demonstrated that plant hormone signal transduction, zeatin biosynthesis, and diterpenoid biosynthesis pathways are differentially expressed in all main crop vs. breba groups. Further comparison demonstrated significant downregulation of IAA-amino acid hydrolase and GA20ox and upregulation of GA2ox, NCED and ACS in plant hormone synthesis and catabolism pathways in the main crop samples compared to breba, while upregulation of multiple AUX/IAA, DELLA, PP2C, and EFR transcripts characterized the main crop plant hormone signaling pathway. Plant hormone assay revealed significantly lower IAA content in very young main crop fruit and this difference was maintained until the post-anthesis flower stage, when gibberellin was also lower in main crop flowers, whereas zeatin and ABA were higher than in breba. Our results suggest that divergence of the parthenocarpic fates of breba and main crop of San Pedro-type fig occur at very early stages of syconia development where auxin may play a major role, in the absence of pollination. Main crop abscission at the end of phase I was modulated by differentially expressed phytohormone metabolism and signaling pathways.