SlRCM1, which encodes tomato Lutescent1, is required for chlorophyll synthesis and chloroplast development in fruits

Genzhong Liu,Junhong Zhang,Ying Wang,Pingfei Ge,Weifang Chen,Huiyang Yu,Yuyang Zhang,Changxing Li,Lei Yuan,Jie Ye,Zhibiao Ye

doi:10.1038/s41438-021-00563-6

Abstract

In plants, chloroplasts are the sites at which photosynthesis occurs, and an increased abundance of chloroplasts increases the nutritional quality of plants and the resultant color of fruits. However, the molecular mechanisms underlying chlorophyll synthesis and chloroplast development in tomato fruits remain unknown. In this study, we isolated a chlorophyll-deficient mutant, reduced chlorophyll mutant 1 (rcm1), by ethylmethanesulfonate mutagenesis; this mutant produced yellowish fruits with altered chloroplast development. MutMap revealed that Solyc08g005010 is the causal gene underlying the rcm1 mutant phenotype. A single-nucleotide base substitution in the second exon of SlRCM1 results in premature termination of its translated protein. SlRCM1 encodes a chloroplast-targeted metalloendopeptidase that is orthologous to the BCM1 protein of Arabidopsis and the stay-green G protein of soybean (Glycine max L. Merr.). Notably, the yellowish phenotype of the lutescent1 mutant can be restored with the allele of SlRCM1 from wild-type tomato. In contrast, knockout of SlRCM1 by the CRISPR/Cas9 system in Alisa Craig yielded yellowish fruits at the mature green stage, as was the case for lutescent1. Amino acid sequence alignment and functional complementation assays showed that SlRCM1 is indeed Lutescent1. These findings provide new insights into the regulation of chloroplast development in tomato fruits.

Highlights

Fruit development is a complex and highly coordinated process that involves a series of specific physiological and biochemical changes[1,2]
Chloroplasts serve as sites of photosynthesis and carbohydrate accumulation and can be transformed into chromoplasts for carotenoid formation[3]
We discovered via BSA + DNA-Seq and MutMap that the rcm[1] mutant carries a single-nucleotide polymorphism (T → A), resulting in premature termination of the SlRCM1 protein

Summary

Introduction

Fruit development is a complex and highly coordinated process that involves a series of specific physiological and biochemical changes[1,2]. Chloroplasts serve as sites of photosynthesis and carbohydrate accumulation and can be transformed into chromoplasts for carotenoid formation[3]. Chloroplast development in tomato plants is directly proportional to fruit development and nutrient accumulation. Both the synthesis and degradation of chloroplasts in plants are in a stable state of dynamic equilibrium. Tomato fruits with full-length transcripts of GLK2 mRNA exhibit a dark green shoulder that can promote photosynthesis and accumulate more nutrients, while glk[2] mutation eliminates this green shoulder[4]. GLK2 overexpression resulted in dramatic upregulated expression of the Solyc08g005010 gene, indicating a regulatory network involving chloroplast development[5]. The TKN2 and TKN4 genes in tomato modulate the gradient of chloroplast development in fruits by regulating GLK2 expression[6]. Overexpression of the APRR2-like gene in tomato increased the content of chlorophyll in fruits[7]

Methods

Results

Conclusion