Functional characterization of GPC-1 genes in hexaploid wheat

Abstract

In wheat, monocarpic senescence is a tightly regulated process during which nitrogen (N) and micronutrients stored pre-anthesis are remobilized from vegetative tissues to the developing grains. Recently, a close connection between senescence and remobilization was shown through the map-based cloning of the GPC (Grain Protein Content) gene in wheat. GPC-B1 encodes a NAC transcription factor associated with earlier senescence and increased grain protein, iron and zinc content, and is deleted or non-functional in most commercial wheat varieties. In the current research, we identified 'loss of function' ethyl methane sulphonate mutants for the two GPC-B1 homoeologous genes; GPC-A1 and GPC-D1, in a hexaploid wheat mutant population. The single gpc-a1 and gpc-d1 mutants, the double gpc-1 mutant and control lines were grown under field conditions at four locations and were characterized for senescence, GPC, micronutrients and yield parameters. Our results show a significant delay in senescence in both the gpc-a1 and gpc-d1 single mutants and an even stronger effect in the gpc-1 double mutant in all the environments tested in this study. The accumulation of total N in the developing grains showed a similar increase in the control and gpc-1 plants until 25 days after anthesis (DAA) but at 41 and 60 DAA the control plants had higher Grain N content than the gpc-1 mutants. At maturity, GPC in all mutants was significantly lower than in control plants while grain weight was unaffected. These results demonstrate that theGPC-A1 and GPC-D1 genes have a redundant function and play a major role in the regulation of monocarpic senescence and nutrient remobilization in wheat.

Detection of NAM-A1 Natural Variants in Bread Wheat Reveals Differences in Haplotype Distribution between a Worldwide Core Collection and European Elite Germplasm

“…Nevertheless, hexaploid wheats have five other NAM genes, three paralogous (on chromosomes 2A, 2B and 2D) and two homoeologous (on chromosomes 6A and 6D) of which NAM-A1 (6A) has the same role as NAM-B1 [4,8]. Most studies on NAM wheat genes used mutants [4,8], near isogenic lines [9][10][11][12] or RNAi lines [3,4] and few studies have focused on the cultivated diversity [6,13]. This paper aims to characterize natural variants of NAM-A1 in hexaploid bread wheat and to hypothesize on the biological mechanisms explaining their effects.…”

The stay-green phenotype of TaNAM-RNAi wheat plants is associated with maintenance of chloroplast structure and high enzymatic antioxidant activity

“…It has been shown that NAM transcription factors act on a specific subset of genes at early stages after anthesis, well before visible symptoms of senescence are observed (Pearce et al, 2014). Our work indicates that, at comparatively early stages of development, no differences in chloroplast ultrastructure seem to exist between control and to SE.…”

“…Evidence obtained here, which includes conservation of chloroplast size, their apparent complexity, thylakoid membrane integrity and intact grana stacking, suggest that preservation of chloroplast ultrastructure is a key component of the stay-green phenotype generated by the down-regulation of TaNAM genes. The finding that the attributes mentioned above are not 16 affected in RNAi plants provides an structural base for the observation made by Avni et al (2014) and more recently by Borrill et al (2015) showing that the stay-green phenotype generated, respectively, by the lack of TaNAM-A1 and TaNAM-D1 or RNAi-down regulation of all TaNAM genes expression in T. aestivum is actually companied by maintenance of photosynthesis. Taken together these data provide support to the idea that lack, or diminished, TaNAM activity generates a functional stay-green phenotype, and that this is accompanied by preservation of full chloroplast structure.…”

“…In this context, the preservation of chloroplast structure observed here would diminish the pool of nutrients available for export, thus contributing to reduce the export of those nutrients. Besides to the possible differences in nutrient availability derived from differential chloroplast degradation, specific transport processes could be differentially modulated in control plants and those with diminished -or absence of-NAM activity as suggested by recent transcriptome studies with GPC-A1/GPC-B1 mutant plants (Pearce et al, 2014).…”

“…A recent study showed that mutations that specifically led to lack of function of the TaNAM-B1 homeologous genes TaNAM-A1 and TaNAM-D1 also led to an stay-green phenotype; which has been characterized as a functional one (Avni et al, 2014). Further studies with TaNAM-RNAi lines led to a similar conclusion (Borrill et al, 2015).…”