SlGRAS4 accelerates fruit ripening by regulating ethylene biosynthesis genes and SlMADS1 in tomato

Yudong Liu,Wang Lu,Zhengguo Li,Yuan Shi,Deding Su

doi:10.1038/s41438-020-00431-9

Abstract

GRAS proteins are plant-specific transcription factors that play crucial roles in plant development and stress responses. However, their involvement in the ripening of economically important fruits and their transcriptional regulatory mechanisms remain largely unclear. Here, we demonstrated that SlGRAS4, encoding a transcription factor of the GRAS family, was induced by the tomato ripening process and regulated by ethylene. Overexpression of SlGRAS4 accelerated fruit ripening, increased the total carotenoid content and increased PSY1 expression in SlGRAS4-OE fruit compared to wild-type fruit. The expression levels of key ethylene biosynthesis genes (SlACS2, SlACS4, SlACO1, and SlACO3) and crucial ripening regulators (RIN and NOR) were increased in SlGRAS4-OE fruit. The negative regulator of tomato fruit ripening, SlMADS1, was repressed in OE fruit. Exogenous ethylene and 1-MCP treatment revealed that more endogenous ethylene was derived in SlGRAS4-OE fruit. More obvious phenotypes were observed in OE seedlings after ACC treatment. Yeast one-hybrid and dual-luciferase assays confirmed that SlGRAS4 can directly bind SlACO1 and SlACO3 promoters to activate their transcription, and SlGRAS4 can also directly repress SlMADS1 expression. Our study identified that SlGRAS4 acts as a new regulator of fruit ripening by regulating ethylene biosynthesis genes in a direct manner. This provides new knowledge of GRAS transcription factors involved in regulating fruit ripening.

Highlights

Fruit ripening can be classified as climacteric or nonclimacteric, depending on the presence or absence, respectively, of massive ethylene production during ripening[1]
The expression level of SlGRAS4 gradually increased from breaker to red ripe fruit, which accompanied the fruit ripening process (Fig. 1a), suggesting that SlGRAS4 may play a role during fruit ripening in tomato
The SlGRAS4-OE fruit exhibited earlier ripening than the WT fruit, and the OE fruit showed an orange color at 37 DPA, whereas the WT was still at the mature green stage; when the WT fruit reached the orange stage at 42 DPA, the OE fruit was red ripe (Fig. 2a)

Summary

Introduction

Fruit ripening can be classified as climacteric or nonclimacteric, depending on the presence or absence, respectively, of massive ethylene production during ripening[1]. Many biological changes, including color conversion, softening, and sugar/ acidity alteration, occur during the fruit ripening process, and the ethylene burst is closely related to the rise in climacteric respiration. Ethylene is an important phytohormone for fruit ripening, and ethylene biosynthesis is strictly regulated during fruit ripening. Fourteen ACS genes and 6 ACO genes have been identified in tomato, and the expression of ACS2, ACS4, ACO1 and ACO2 was significantly induced by fruit ripening initiation, suggesting that they may act as the main genes for ethylene biosynthesis during tomato fruit ripening[4]. Ethylene production exhibited a massive increase associated with fruit ripening in climacteric fruits and was considered system 2. Ethylene production shifted from system 1 to system

Methods

Results

Conclusion