Enhanced Chlorophyll Degradation Triggers the Pod Degreening of "Golden Hook," a Special Ecotype in Common Bean (Phaseolus vulgaris L.).

Bo Hu,Jiayin Yang,Hongyan Wu,Jinlong Zhu,Ning Guo,Zhengjun Xia,Kun Xu,Yi Gao,Danhua Zhu,Hong Zhai

doi:10.3389/fgene.2020.570816

Abstract

To reveal genetic factors or pathways involved in the pod degreening, we performed transcriptome and metabolome analyses using a yellow pod cultivar of the common bean “golden hook” ecotype and its green pod mutants yielded via gamma radiation. Transcriptional profiling showed that expression levels of red chlorophyll catabolite reductase (RCCR, Phvul.008G280300) involved in chlorophyll degradation was strongly enhanced at an early stage (2 cm long) in wild type but not in green pod mutants. The expression levels of genes involved in cellulose synthesis was inhibited by the pod degreening. Metabolomic profiling showed that the content of most flavonoid, flavones, and isoflavonoid was decreased during pod development, but the content of afzelechin, taxifolin, dihydrokaempferol, and cyanidin 3-O-rutinoside was remarkably increased in both wild type and green pod mutant. This study revealed that the pod degreening of the golden hook resulting from chlorophyll degradation could trigger changes in cellulose and flavonoids biosynthesis pathway, offering this cultivar a special color appearance and good flavor.

Highlights

The common bean (Phaseolus vulgaris L.), as one of the most important legume crops worldwide, provides a major source of dietary protein, complex carbohydrates, dietary fiber, numerous vitamins, and trace minerals (Tharanathan and Mahadevamma, 2003)
It is well known that the color conversion is generally accompanied by chlorophyll degradation; the chlorophyll content constantly decreased during the green-peel fruit ripening (Gambi et al, 2018)
Previous researches had shown that pheophorbide a oxygenase (PAO) and red chlorophyll catabolite reductase (RCCR) participated in the chlorophyll catabolic pathway

Summary

Introduction

The common bean (Phaseolus vulgaris L.), as one of the most important legume crops worldwide, provides a major source of dietary protein, complex carbohydrates, dietary fiber, numerous vitamins, and trace minerals (Tharanathan and Mahadevamma, 2003). Plant pigments, including carotenoid, anthocyanin, and chlorophyll, are in charge of fruit peel or flesh color (Tanaka et al, 2008). Many fruit flesh or peel and leaves with green color are caused by the high chlorophyll content (Xu et al, 2020). When carotenoids are masked by excessive chlorophyll, the fruit flesh or peel and leaves appear green. The yellow color of carotenoids is unmasked upon chlorophyll degradation during ripening or senescence. One turned yellow during ripening due to loss of chlorophylls a/b and carotenoids, while the other stayed green until fully ripen, which could be accounted for the lower the expression levels of the genes for chlorophyll degradation, including CLH, PAO, and NYC1-like (Charoenchongsuk et al, 2015). Synthesis and degradation of chlorophyll are under control of the coordinate regulatory cascade, in which the malfunction of key steps or genes would affect total chlorophyll content and the color appearance

Methods

Results

Conclusion