Transcriptomic analysis reveals differences in fruit maturation between two kiwifruit cultivars

Abstract

The commonly accepted kiwifruit harvest index based solely on soluble solids content (SSC) has lost its original significance owing to the increased sophistication of marketing coupled with the commercialisation of new cultivars, many with yellow flesh when ripe. The precision of novel harvest indices may be improved by a molecular understanding of fruit maturation changes in commonly monitored fruit attributes, including SSC, flesh colour and firmness. Transcriptional changes in the early-maturing Actinidia chinensis var. chinensis ‘Zesy002’ and later maturing A. chinensis var. deliciosa ‘Hayward’, grown at a single site, have been quantified weekly in the period leading up to and past the commercial harvest period. Transcriptomic data highlighted numerous maturation related changes in the fruit, and differences between the two cultivars. Examples of gene changes of relevance for non-structural carbohydrates included the large sub-unit 4 of ADP-glucose pyrophosphorylase (APL4) indicative of starch synthesis, BETA AMYLASE 3.2 (BAM3.2) for starch breakdown and the sugar transporter (SWEET9a). The association between STAYGREEN2 (SGR2) with flesh degreening was strong in ‘Zesy002’, although a later increase in ‘Hayward’ to levels similar to those in ‘Zesy002’ was not accompanied by an equivalent flesh colour change. In ‘Zesy002’ there were numerous individual cell wall gene changes associated with the change to rapid softening, including EXPANSIN7 (EXP7), POLYGALACTURONASE1 (PG1), PECTATE LYASE (PL), PECTIN METHYL ESTERASE (PME) and XYLOGLUCAN TRANSGLYCOSYLASE/ HYDROLASE (XTH). However, among these genes, it is possible to see similar changes in ‘Hayward’ which were not associated with a marked change in softening rate, including for EXP7 and PG1. The most obvious start points for changes in transcription of these genes were the seed coat colour change, the cessation of growth, and the change to ripening (rapid softening and starch breakdown). The findings are discussed with respect to fruit maturation and the possible use of gene markers as harvest indices.