Exogenous gibberellic acid application induces the overexpression of key genes for pedicel lignification and an increase in berry drop in table grape

Most table grape (Vitis vinifera L.) varieties require gibberellic acid (GA3) applications to obtain an adequate berry size in order to satisfy market requirements. However, GA3 treatments also produce severe berry drop in some cultivars, which occurs mainly after a cold storage period during post-harvest. Berry drop in bunches treated with GA3 has been related to the hardening and thickening of the pedicel produced by the over-accumulation of cellulose and its lignification. The main goal of this study was to compare the morphology and gene expression in pedicel samples of genotypes contrasting for berry drop susceptibility. These genotypes are Thompson Seedless, which exhibits a low incidence of berry drop, and a genetic line (Line #23) of INIA's breeding program that is very susceptible to berry drop at harvest and after storage in bunches sprayed with GA3. The parameters measured to study this phenomenon during fruit growth and post-harvest storage included fruit detachment force (FDF), hardness and thickness of the pedicel and berry drop frequency. Histological analyses of pedicel structures at harvest showed an increase in cell size and deposition of lignin in the cortex zone in both contrasting genotypes treated with GA3. The expression profile in both genotypes of the key lignin biosynthesis genes Vv4CL4, VvCCR1L and VvCAD1 analyzed by quantitative real time PCR (qPCR) revealed evident changes in response to GA3 treatments. In particular, gene VvCAD1 is overexpressed (100X) in pedicels of line #23 treated with GA3 after 30 and 45 days in cold storage compared to control. Moreover, the frequency of berry drop was higher for Line #23 treated with GA3 than for the control (23% vs. 1%). Our results suggest that gibberellic acid regulates the expression of the biosynthesis of lignin genes, generating changes in cell wall composition and pedicel structure that result in an increase in berry drop.