Abstract

Abscisic acid (ABA) is a major regulator of non-climacteric fruit ripening; however, the role of ABA in the ripening of climacteric fruit is not clear. Here, as a typical climacteric fruit, apricots were used to investigate the role of ABA in fruit ripening.

Based on weighted gene co-expression network analysis (WGCNA) of our previous transcriptome data, we treated 'Danxing' fruit with exogenous ABA and obtained ABA receptor genes, genes related to ABA biosynthesis and signal transduction, and analyzed the response of these candidate genes to exogenous ABA during fruit ripening. Subsequently, the full length of candidate PYLs genes were cloned, and their putative function were analyzed by phylogenetic analysis and protein structure domain analysis. And then the function of one candidate gene PaPYL9 was verified by using transgenic tomato. Furthermore, the response genes in transgenic tomato were screened by transcriptome sequencing, and ultimately the related regulatory network was proposed.

The results showed that the injection of exogenous 1.89 mM ABA remarkably promoted fruit coloration, and increased the color index for red grapes (CIRG) and the total soluble solids (TSS) content, but significantly decreased the firmness and titratable acid (TA) content (P < 0.01). Nordihydroguaiaretic acid (NDGA), the inhibitor of ABA, appeared to have the converse role in TA, TSS, CIRG and firmness, during the ripening process. One NCED (9-cis-epoxycarotenoiddioxygenase) and five ABA receptor genes related to signal transduction were mined from the transcriptome data of apricot fruit through WGCNA. Compared with the control, the expression levels of NCED1, PYL9 (PYR/PYL/RCAR), SnRK2 (SUCROSE NON-FERMENTING1 (SNF1)-RELATED PROTEIN KINASE 2S), and ABF2 (ABRE-binding bZIP transcription) were induced dramatically by ABA treatment (P < 0.01), while NDGA treatment significantly inhibited their expression.

Based on gene expression and protein domain analysis, we inferred that PaPYL9 is putatively involved in apricot fruit ripening. Overexpression of PaPYL9 in Micro-TOM tomatoes resulted in the promotion of early ripening. Simultaneously, the expression levels of genes related ethylene biosynthesis, chlorophyll degradation, fruit softening, flavor formation, pigment synthesis, and metabolism were all significantly induced in overexpression of PaPYL9 tomatoes. This indicates the central role of ABA in climacteric fruit ripening. A regulatory network was tentatively proposed, laying the foundation to unveil the molecular mechanism of the regulatory role of PaPYL9 in fruit ripening.