Brassinosteroid is involved in strawberry fruit ripening

Although brassinosteroid (BR) has been suggested to play a role in strawberry fruit ripening, the defined function of this hormone remains unclear in the fruit. Here, BR content and BR receptor gene FaBRI1 expression were analysed during 'Akihime' strawberry fruit development. We found that BR levels increased during the later developmental stages, and the mRNA expression levels of FaBRI1 increased rapidly from white to initial red stages, suggesting that BR is associated with fruit ripening. This was further confirmed by exogenous application of BR and its inhibitor brassinazole (BZ) to big-green fruit, which significantly promoted and inhibited strawberry fruit ripening, respectively. More importantly, down-regulation of FaBRI1 expression in de-greening fruit markedly retarded strawberry red-colouring. In conclusion, we have provided physiological and molecular evidence to demonstrate that BR plays a role in strawberry fruit ripening. In addition, both BR content and FaBRI1 expression reached their peak levels in small-green fruit, suggesting that BR might also be involved in early strawberry fruit development. Further experiments are required to validate the role of BR in strawberry fruit cell division.