Exploring the mechanisms of grapevine single berry development and ripening

Abstract: Context and purpose of the study The strategy of single berry phenotyping is a recently rediscovered research tool that has gained great attention. The latest studies have indicated that previous physiological models based on pooling asynchronous populations of berries provided biased or blurred information on berry development key players. The possibility of monitoring and sampling single synchronized berries to study their development sequentially has opened new lines of research aimed at unraveling the genes that regulate grapevine fruit development. This study aimed to decipher the gene pathways responsible for the activation/deactivation of physiological processes involved in the green phase of growth, the onset of ripening, and the second growth phase. Material and methods The growth of single berries of V. vinifera cv. ‘Syrah’ was pictured from flowering until over-ripening through biweekly images in the experimental vineyard of Institut Agro Montpellier, France. Thirty-three single berries (11 developmental phases x 3 biological replicates) were selected for RNA Sequencing based on the relative growth curve and primary metabolite concentrations (sugars and acids) as in Savoi et al. (2021) covering key phases or changes in kinetics during the whole berry development. Specific attention was dedicated to berry softening. Here it is acknowledged as a drop in the fruit tissue firmness and a simultaneous decrease in the glucose/fructose ratio. Gene expressions were normalized and tested for time-series significance leading to twenty-five clusters representing 10,355 genes significantly modulated by development. Results Isogenes linked to the phloem sugar unloading pathway, i.e., sugar accumulation, water fluxes, and cell wall remodeling, were confirmed, together with key secondary metabolism regulators. Gene expression profiling clearly showed that these genes were suddenly activated at a specific developmental stage. Great attention was dedicated to the onset of ripening, where precisely with berry softening and, therefore, sugar entry, several genes were switched on and, as intensively, switched off at the maximum berry expansion when phloem stops. Single berry monitoring allowed distinguishing developmental, molecular events, and critical genes previously considered simultaneous in past transcriptomic studies.