Effect of GA₃ Treatment on Berry Development in the Large Berry Mutant of ‘Delaware’ Grapes

Abstract

The large berry mutant (LBM) of the ‘Delaware’ (V. vinifera × (V. labrusca × V. aestivalis)) is known to produce larger berries than ‘Delaware’ when subjected to traditional double gibberellic acid-3 (GA₃) treatment. In this study, we first compared ‘Delaware’ and the LBM in terms of their berry and cluster quality to reevaluate previous findings on LBM. Next, we compared berry size without GA₃, histological characteristics of the early developmental berry, and then GA-related gene expression to reveal the characteristics of the LBM. The previous finding that LBM yielded larger berry sizes than ‘Delaware’ with double GA₃ treatment was verified, but without GA₃ treatment, LBM did not produce larger berries than ‘Delaware’. This highlights the prerequisite of exogenous GA for larger berry size in LBM. Histological analysis revealed an increase in cell numbers of the inner and outer mesocarp walls in the early developmental stage of LBM berries. Gene expression analysis of the flower buds and berries indicated an increased expression of VvGID1A, encoding the GA receptor, in LBM than in ‘Delaware’ 3 h after the first GA₃ treatment. Additionally, the expression of VvGID1A and VvGID1B was higher in LBM than that in ‘Delaware’ before and after the second GA₃ treatment. The expression of VvGID2A, which interacts with the GA receptor and promotes GA signaling, was also higher in LBM than in ‘Delaware’ before the first GA₃ treatment. However, the expression of VvSLR1, VvGAI1, and VvGAI2 that encode DELLA proteins, essential negative regulators for GA reactions, mostly showed no significant changes. These results suggest that LBM berries had higher GA sensitivity than the ‘Delaware’. Based on these results, we believe that the larger berry size in LBM may be due to increased cell numbers resulting from high GA sensitivity.