From Model to Crop: Functional Analysis of a STAY-GREEN Gene in the Model Legume Medicago truncatula and Effective Use of the Gene for Alfalfa Improvement    

Abstract

Medicago truncatula has been developed into a model legume. Its close relative alfalfa (Medicago sativa) is the most widely grown forage legume crop in the United States. By screening a large population of M. truncatula mutants tagged with the transposable element of tobacco (Nicotiana tabacum) cell type1 (Tnt1), we identified a mutant line (NF2089) that maintained green leaves and showed green anthers, central carpels, mature pods, and seeds during senescence. Genetic and molecular analyses revealed that the mutation was caused by Tnt1 insertion in a STAY-GREEN (MtSGR) gene. Transcript profiling analysis of the mutant showed that loss of the MtSGR function affected the expression of a large number of genes involved in different biological processes. Further analyses revealed that SGR is implicated in nodule development and senescence. MtSGR expression was detected across all nodule developmental zones and was higher in the senescence zone. The number of young nodules on the mutant roots was higher than in the wild type. Expression levels of several nodule senescence markers were reduced in the sgr mutant. Based on the MtSGR sequence, an alfalfa SGR gene (MsSGR) was cloned, and transgenic alfalfa lines were produced by RNA interference. Silencing of MsSGR led to the production of stay-green transgenic alfalfa. This beneficial trait offers the opportunity to produce premium alfalfa hay with a more greenish appearance. In addition, most of the transgenic alfalfa lines retained more than 50% of chlorophylls during senescence and had increased crude protein content. This study illustrates the effective use of knowledge gained from a model system for the genetic improvement of an important commercial crop.

Functional characterization of GPC-1 genes in hexaploid wheat

“…A functional stay-green trait enables the plant to synthesize and store greater levels of assimilates, which can lead to increased crop yields under certain environments (Richards 2000;Christopher et al 2008;Zhou et al 2011;Derkx et al 2012). Our photosynthesis measurements showed that the GPC-1 mutants maintain photosynthetic activity for longer than the control lines; hence we can classify the GPC-1 mutants as functional stay-green mutants.…”

Update on the biochemistry of chlorophyll breakdown

“…This points to a crucial structural rather than direct biochemical role for SGR in Chl breakdown. Plant Mol Biol (2013) 82:505-517 511 Besides exhibiting a leaf stay-green phenotype, a Medicago truncatula SGR mutant was also affected in nodule senescence (Zhou et al 2011). This surprising finding indicates that the role of SGR may go beyond its requirement for Chl breakdown; it possibly could have a more general role in nitrogen remobilization, maybe by recruiting proteases for the degradation of protein (complexes) during senescence.…”

Molecular cloning and characterization of a chlorophyll degradation regulatory gene (ZjSGR) from Zoysia japonica

“…Transient overexpression of OsSGR by agro-infiltration in tobacco leaves induced subsequent senescence (Park et al, 2007). MtSGR was possibly involved in alfalfa nodule development and senescence (Zhou et al, 2011). However, the role of SGRs in regulating chlorophyll degradation and senescence is still largely unknown, particularly in non-model plants.…”

“…The identification of stay-green mutants in several plant species has made it possible to further investigate the underlying mechanism (Armstead et al, 2006;Park et al, 2007;Zhou et al, 2011). Stay-green (SGR) proteins play important roles in the regulation of plant chlorophyll degradation and senescence, and they have been characterized in Arabidopsis thaliana (Ren et al, 2007), Medicago truncatula (Zhou et al, 2011), and Solanum lycopersicum (Luo et al, 2013).…”