SlBEL11 regulates flavonoid biosynthesis, thus fine-tuning auxin efflux to prevent premature fruit drop in tomato

Xiufen Dong, Xianfeng Liu, Lina Cheng, Ruizhen Li, Siqi Ge, Sai Wang, Yue Cai, Yang Liu, Sida Meng, Cai-Zhong Jiang, Chun-Lin Shi, Tianlai Li, Daqi Fu, Mingfang Qi, Tao Xu

Journal of Integrative Plant Biology; 29 February 2024; https://doi.org/10.1111/jipb.13627

ABSTRACT

Auxin regulates flower and fruit abscission, but how developmental signals mediate auxin transport in abscission remains unclear. Here, we reveal the role of the transcription factor BEL1-LIKE HOMEODOMAIN11 (SlBEL11) in regulating auxin transport during abscission in tomato (Solanum lycopersicum). SlBEL11 is highly expressed in the fruit abscission zone, and its expression increases during fruit development. Knockdown of SlBEL11 expression by RNA interference (RNAi) caused premature fruit drop at the breaker (Br) and 3 d post-breaker (Br+3) stages of fruit development. Transcriptome and metabolome analysis of SlBEL11-RNAi lines revealed impaired flavonoid biosynthesis and decreased levels of most flavonoids, especially quercetin, which functions as an auxin transport inhibitor. This suggested that SlBEL11 prevents premature fruit abscission by modulating auxin efflux from fruits, which is crucial for the formation of an auxin response gradient. Indeed, quercetin treatment suppressed premature fruit drop in SlBEL11-RNAi plants. DNA affinity purification sequencing (DAP-seq) analysis indicated that SlBEL11 induced expression of the transcription factor gene SlMYB111 by directly binding to its promoter. Chromatin immunoprecipitation-quantitative polymerase chain reaction and electrophoretic mobility shift assay showed that S. lycopersicum MYELOBLASTOSIS VIRAL ONCOGENE HOMOLOG111 (SlMYB111) induces the expression of the core flavonoid biosynthesis genes SlCHS1, SlCHI, SlF3H, and SlFLS by directly binding to their promoters. Our findings suggest that the SlBEL11–SlMYB111 module modulates flavonoid biosynthesis to fine-tune auxin efflux from fruits and thus maintain an auxin response gradient in the pedicel, thereby preventing premature fruit drop.

See https://onlinelibrary.wiley.com/doi/10.1111/jipb.13627

Figure 1: Premature fruit drop in SlBEL11-RNAi (RNA interference) plants

(A) Phenotype of wild-type (WT) and three SlBEL11-RNAi lines. Scale bars, 2 cm. (B) Relative SlBEL11 transcript levels in WT and three different SlBEL11-RNAi lines during fruit development. Values represent the mean ±  SD from three biological replicates. Statistical significance was assessed using a two-way analysis of variance (ANOVA) with Tukey's multiple comparisons test. ****P < 0.0001. (C) Frequency of fruit drop in WT and SlBEL11-RNAi plants. The values represent the mean ± SD from three biological replicates. Statistical significance was assessed using a two-way ANOVA with Tukey's multiple comparisons test. ns, no significant difference; ****P < 0.0001.