The MaPUB23-mediated degradation of the MaHSFB1-MaHDA2 counteracts high temperature-inhibited chlorophyll degradation in banana

Banana fruits ripening at or above 30 °C soften but retain a green peel. Elucidating the mechanism underlying this high temperature-induced repression of chlorophyll catabolism would help to improve the marketability of bananas. Toward this end, we compared banana fruits ripened at 20 °C and 30 °C using transcriptome analysis, revealing several E3 ubiquitin ligases repressed in green fruits ripened at 30 °C. Among them, only the U-box type E3 ligase MaPUB23 promoted chlorophyll degradation when transiently overexpressed in tobacco leaves. MaPUB23 interacted with and targeted MaHSFB1 and MaHDA2 for ubiquitination and proteasomal degradation. MaHSFB1 specifically interacted with and recruited MaHDA2 to the promoters of chlorophyll catabolic genes (CCGs), transcriptionally repressed these genes via decreased histone acetylation, and inhibited chlorophyll degradation. However, high temperature attenuated the MaPUB23-mediated degradation of MaHSFB1 and MaHDA2, which enhanced the MaHSFB1-MaHDA2-mediated repression of CCGs. These results suggest that high temperature attenuates the MaPUB23-mediated proteasomal degradation of MaHSFB1-MaHDA2, leading to green ripening in banana fruit. This study thus illustrates a novel, dynamic regulatory module controlling the high temperature-induced repression of chlorophyll catabolism in banana fruits. Elucidating this mechanism provides a theoretical basis for strategies to manage temperature-induced fruit quality deterioration.