Brassinosteroid Regulates 3‑Hydroxy-3-methylglutaryl CoA Reductase to Promote Grape Fruit Development

Brassinosteroids (BRs) are known
to regulate plant growth and development.
However, only little is known about their mechanism in the regulation
of berry development in grapes. This study demonstrates that BR treatment
enhances the accumulation of fruit sugar components, reduces the content
of organic acids (e.g., tartaric acid), promotes coloration, and increases
the anthocyanin content in grape berries at the onset of the veraison,
half veraison, and full veraison stages at the rate of 0.0998, 0.0560,
and 0.0281 mg·g–1, respectively.
In addition, BR treatment was also found to accelerate the biosynthesis
of terpenoid aroma components, such as α-pinene, d-limonene,
and γ-terpinene, which influence the aromatic composition of
grapes. BRs can negatively regulate the expression of VvHMGR, a key gene involved in the mevalonate (MVA) pathway, and reduce
the activity of 3-hydroxy-3-methylglutaryl CoA reductase (HMGR). Inhibiting
the expression of HMGR promoted the accumulation
of anthocyanins and fruit coloration. Meanwhile, after the inhibition,
the contents of auxin indole-3-acetic acid (IAA), abscisic acid (ABA),
and brassinosteroid (BR) increased, while gibberellin (GA3) and zeatin riboside (ZR) decreased, and its aromatic composition
also changed. Therefore, it may be concluded that BRs inhibited HMGR
activity and cooperated with VvHMGR to regulate the
formation of color, aroma, and other quality characteristics in fruits.