Cytokinin-induced VvTFL1A Expression May Be Involved in the Control of Grapevine Fruitfulness

Grapevine bud fruitfulness is determined by the differentiation of uncommitted meristem (UCM) into either tendril or inflorescence. Since tendril and inflorescence differentiation have long been considered sequential steps in inflorescence development, factors that control the progression of floral meristem development may regulate the final outcome of UCM differentiation, and thus affect fruitfulness. A comparison of the expression profiles of the master regulators of floral meristem identity (FMI) during development of fruitful and non-fruitful buds along the same cane allowed associating the expression of a homolog of terminal flower 1 (TFL1, a negative regulator of FMI) to fruitful buds, and the expression of positive FMI regulators to non-fruitful buds. Combined with (a) cytokinin-induced upregulation of VvTFL1A expression in cultured tendrils, which accompanied cytokinin-derived tendril transformation into branched, inflorescence-like structures, (b) positive regulation of VvTFL1A expression by cytokinin, which was demonstrated in transgenic embryonic culture expressing GUS reporter under the control of VvTFL1A promoter, and (c) a significantly higher level of active cytokinins in fruitful positions, the data may support the assumption of cytokinin-regulated VvTFL1A activity's involvement in the control of inflorescence development. Such activity may delay acquisition of FMI and allow an extended branching period for the UCM, resulting in the differentiation of inflorescence primordia.