Abstract
Loquat (Eriobotrya japonica) is a subtropical tree that bears fruit that ripens during late spring. Fruit size is one of the dominant factors inhibiting the large-scale production of this fruit crop. To date, little is known about fruit size regulation. In this study, we first discovered that cell size is more important to fruit size than cell number in loquat and that the expression of the EjBZR1 gene is negatively correlated with cell and fruit size. Virus-induced gene silencing (VIGS) of EjBZR1 led to larger cells and fruits in loquat, while its overexpression reduced cell and plant size in Arabidopsis. Moreover, both the suppression and overexpression of EjBZR1 inhibited the expression of brassinosteroid (BR) biosynthesis genes, especially that of EjCYP90A. Further experiments indicated that EjCYP90A, a cytochrome P450 gene, is a fruit growth activator, while EjBZR1 binds to the BRRE (CGTGTG) motif of the EjCYP90A promoter to repress its expression and fruit cell enlargement. Overall, our results demonstrate a possible pathway by which EjBZR1 directly targets EjCYP90A and thereby affects BR biosynthesis, which influences cell expansion and, consequently, fruit size. These findings help to elucidate the molecular functions of BZR1 in fruit growth and thus highlight a useful genetic improvement that can lead to increased crop yields by repressing gene expression.
Highlights
Fruits provide humans with various nutrients, are enjoyable to eat, and are essential to our daily lives[1]
The results showed that 615-bp products were obtained in both the Virus-induced gene silencing (VIGS) and control fruits, while no amplification product was obtained in the mock fruit with the specific primer pair (Fig. 3E)
Together with the results from the gene expression experiment in VIGS loquat fruit, these results suggest that the repression of BR biosynthesis gene expression is a crucial means by which EjBZR1 regulates fruit size
Summary
Fruits provide humans with various nutrients, are enjoyable to eat, and are essential to our daily lives[1]. Though the molecular controls of the size of organs, such as leaves and flowers, are well known[3], knowledge of fruit size regulation is unclear. Organ Size 1) are believed to modulate fruit size by regulating cell expansion[6,7]. Little is known about fruit size regulation in perennial fruit trees. Loquat (Eriobotrya Lindl.) is a subtropical evergreen fruit tree belonging to the apple subfamily in Rosaceae that bears nutritious and succulent fruits. The cultivated species, Eriobotrya japonica, initiates bud differentiation in inflorescences in late summer, and the fruits ripen during late spring and early summer[9,10]. Under rigorous management in semiarid subtropical regions with abundant sunlight, such as the Miyi area (Panzhihua, Sichuan, China), the trees can produce fruits from December to March of the year.
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