The Ubiquitin E3 Ligase MaLUL2 Is Involved in High Temperature-Induced Green Ripening in Banana Fruit.

Wei Wei,Wei Shan,Jian-Ye Chen,Ze-Xiang Zeng,Jian-Fei Kuang,Wang-Jin Lu

doi:10.3390/ijms21249386

Abstract

Harvested banana fruit ripened under warm temperatures above 24 °C remain green peel, leading to severe economic loss. E3 ubiquitin-ligases, as the major components in the ubiquitination pathway, have been implicated to play important roles in temperature-stress responses. However, the molecular mechanism underlying high temperature-triggered stay-green ripening bananas in association with E3 ubiquitin-ligases, remains largely unknown. In this study, a RING-type E3 ubiquitin ligase termed MaLUL2, was isolated and characterized from banana fruit. The MaLUL2 gene contains 1095 nucleotides and encodes a protein with 365 amino acids. The MaLUL2 protein contains a domain associated with RING2 (DAR2) and a RING domain, which are the typical characteristics of RING-type E3 ligases. MaLUL2 expression was up-regulated during high temperature-induced green ripening. Subcellular localization showed that MaLUL2 localized in the nucleus, cytoplasm, and plasma membrane. MaLUL2 displayed E3 ubiquitin ligase activity in vitro. More importantly, transient overexpression of MaLUL2 in banana fruit peel increased the level of ubiquitination in vivo and led to a stay-green phenotype, accompanying with decreased expression of chlorophyll catabolic genes. Collectively, these findings suggest that MaLUL2 might act as a negative regulator of chlorophyll degradation and provide novel insights into the regulatory mechanism of high temperature-induced green ripening bananas.

Highlights

Banana (Musa acuminata) is a tropical fruit that belong to the Musaceae family, presenting as one of mostly popular fruits worldwide [1,2]
Fv/Fm, color index, and fruit firmness in the fruit under 30 ◦C were lower, when compared with the levels in the fruit under 20 ◦C (Figure 1I–K). These results indicate that banana fruit stored at 30 ◦C inhibits chlorophyll degradation, thereby leading to stay-green ripening
We identified and characterized a RING E3 ubiquitin ligase MaLUL2 from banana fruit, and provided evidence to support the possible involvement of MaLUL2 in high temperature-induced green ripening in banana fruit

Summary

Introduction

Banana (Musa acuminata) is a tropical fruit that belong to the Musaceae family, presenting as one of mostly popular fruits worldwide [1,2]. The color of the banana peel turns rapidly from green to fully yellow, which is the most visible symptom during the fruit ripening process. Banana fruit could not exhibit a fully yellow peel and stay green when ripening under the temperatures above 24 ◦C due to the inhibition of chlorophyll degradation [4]. These green ripening fruits are regarded as poor quality by consumers and lead to a lower market price than golden yellow ripening bananas, causing a significant loss in economic value. The exploration of underlying mechanisms causing the stay-green ripening bananas will be of great practical value to maintain banana fruit quality under a high temperature

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