Genetic and Hormonal Regulation of Chlorophyll Degradation during Maturation of Seeds with Green Embryos.

Galina Smolikova,Sergei Medvedev,Maria Vikhnina,Elena Dolgikh,Andrej Frolov

doi:10.3390/ijms18091993

Abstract

The embryos of some angiosperms (usually referred to as chloroembryos) contain chlorophylls during the whole period of embryogenesis. Developing embryos have photochemically active chloroplasts and are able to produce assimilates, further converted in reserve biopolymers, whereas at the late steps of embryogenesis, seeds undergo dehydration, degradation of chlorophylls, transformation of chloroplast in storage plastids, and enter the dormancy period. However, in some seeds, the process of chlorophyll degradation remains incomplete. These residual chlorophylls compromise the quality of seed material in terms of viability, nutritional value, and shelf life, and represent a serious challenge for breeders and farmers. The mechanisms of chlorophyll degradation during seed maturation are still not completely understood, and only during the recent decades the main pathways and corresponding enzymes could be characterized. Among the identified players, the enzymes of pheophorbide a oxygenase pathway and the proteins encoded by STAY GREEN (SGR) genes are the principle ones. On the biochemical level, abscisic acid (ABA) is the main regulator of seed chlorophyll degradation, mediating activity of corresponding catabolic enzymes on the transcriptional level. In general, a deep insight in the mechanisms of chlorophyll degradation is required to develop the approaches for production of chlorophyll-free high quality seeds.

Highlights

In green plants, photosynthesis is the principle route, supplying developing embryos with assimilates [1,2,3]
At the late stages of embryogenesis, seeds enter the period of dormancy, which is accompanied by dehydration, disintegration of photosynthetic apparatus, and chlorophyll degradation
Degradation of chlorophylls accompanies the onset of plant leaf senescence, and can be observed during maturation of seeds with green embryos

Summary

Introduction

Photosynthesis is the principle route, supplying developing embryos with assimilates [1,2,3]. At the late stages of embryogenesis, seeds enter the period of dormancy, which is accompanied by dehydration, disintegration of photosynthetic apparatus, and chlorophyll degradation The latter process often remains incomplete, and residual chlorophylls can be detected in seed tissues of many plant species [14]. The presence of residual chlorophylls in seeds dramatically reduces their tolerance to various environmental stresses [14,17,18] and results in significant losses of crop harvest yields due to the so-called “green seed problem” [15,16,19,20] It can be considered as one of the main factors negatively affecting crop productivity and quality of related products. In this review we summarize the literature data on chlorophyll degradation in maturing seeds and critically discuss the pathways of genetic and hormonal control of this process with a specific emphasis on SGR genes and their products as the key regulators of chlorophyll catabolism

Photosynthesis in Seeds with Green Embryos

Catabolism of Chlorophylls in Plants

The Role of STAY-GREEN Genes in Degradation of Seed Chlorophylls

Residual Chlorophylls in Mature Seeds

Findings

Conclusions