Genetic diversity in nitrogen fertilizer responses and N gas emission in modern wheat

Crops assimilate nitrogen (N) as ammonium via the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway which is of central importance for N uptake and potentially represents a bottle neck for N fertilizer-use efficiency. The aim of this study was to assess whether genetic diversity for N-assimilation capacity exists in wheat and could be exploited for breeding. Wheat plants rapidly, within 6h, responded to N application with an increase in GS activity. This was not accompanied by an increase in GS gene transcript abundance and a comparison of GS1 and GS2 protein models revealed a high degree of sequence conservation. N responsiveness amongst ten wheat varieties was assessed by measuring GS enzyme activity, leaf tissue ammonium, and by a leaf-disc assay as a proxy for apoplastic ammonia. Based on these data, a high-GS group showing an overall positive response to N could be distinguished from an inefficient, low-GS group. Subsequent gas emission measurements confirmed plant ammonia emission in response to N application and also revealed emission of N2O when N was provided as nitrate, which is in agreement with our current understanding that N2O is a by-product of nitrate reduction. Taken together, the data suggest that there is scope for improving N assimilation capacity in wheat and that further investigations into the regulation and role of GS-GOGAT in NH3 emission is justified. Likewise, emission of the climate gas N2O needs to be reduced, and future research should focus on assessing the nitrate reductase pathway in wheat and explore fertilizer management options

Trehalose 6-phosphate regulates photosynthesis and assimilate partitioning in reproductive tissue

Transgenic maize (Zea mays) that expresses rice (Oryza sativa) TREHALOSE PHOSPHATE PHOSPHATASE1 (TPP1) from the rice MADS6 promoter, which is active over the flowering period, produces higher yields than wild type. This yield increase occurs with or without drought conditions during flowering. To understand the mechanistic basis of the increased yield, we characterised gene expression and metabolite profiles in leaves and developing female reproductive tissue – comprising florets, node, pith and shank – over the flowering period with and without drought. The MADS6 promoter was most active in the vasculature, particularly phloem companion cells in florets and pith, consistent with the largest decreases in trehalose 6-phosphate (T6P) levels (two- to threefold) being found in pith and florets. Low T6P led to decreased gene expression for primary metabolism and increased gene expression for secondary metabolism, particularly lipid-related pathways. Despite similar changes in gene expression, the pith and floret displayed opposing assimilate profiles: sugars, sugar phosphates, amino acids and lipids increased in florets, but decreased in pith. Possibly explaining this assimilate distribution, seven SWEET genes were found to be upregulated in the transgenic plants. SnRK1 activity and the, expression of the gene for the SnRK1 beta subunit, expression of SnRK1 marker genes, and endogenous trehalose pathway genes were also altered. Furthermore, leaves of the transgenic maize maintained a higher photosynthetic rate for a longer period compared to wild type. In conclusion, we found that decreasing T6P in reproductive tissues downregulates primary metabolism and upregulates secondary metabolism, resulting in different metabolite profiles in component tissues. Our data implicate T6P/SnRK1 as a major regulator of whole-plant resource allocation for crop yield improvement

Sugar sensing responses to low and high light in leaves of the C4 model grass Setaria viridis

Although sugar regulate photosynthesis, the signalling pathways underlying this process remain elusive, especially for C4 crops. To address this knowledge gap and identify potential candidate genes, we treated Setaria viridis (C4 model) plants acclimated to medium light intensity (ML, 500 µmol m-2 s-1) with low (LL, 50 µmol m-2 s-1) or high (HL, 1000 µmol m-2 s-1) light for 4 days and observed the consequences on carbon metabolism and the transcriptome of source leaves. LL impaired photosynthesis and reduced leaf content of signalling sugars (glucose, sucrose and trehalose-6-phosphate). Contrastingly, HL strongly induced sugar accumulation without repressing photosynthesis. LL more profoundly impacted leaf transcriptome, including photosynthetic genes. LL and HL contrastingly altered the expression of HXK and SnRK1 sugar sensors and trehalose pathway genes. The expression of key target genes of HXK and SnRK1 were affected by LL and sugar depletion, while surprisingly HL and strong sugar accumulation only slightly repressed the SnRK1 signalling pathway. In conclusion, we demonstrate that LL profoundly impacted photosynthesis and the transcriptome of S. viridis source leaves, while HL altered sugar levels more than LL. We also present the first evidence that sugar signalling pathways in C4 source leaves may respond to light intensity and sugar accumulation differently to C3 source leave

Current and possible approaches for improving photosynthetic efficiency

One of the most important tasks laying ahead today’s biotechnology is to improve crop productivity with the aim of meeting increased food and energy demands of humankind. Plant productivity depends on many genetic factors, including life cycle, harvest index, stress tolerance and photosynthetic activity. Many approaches were already tested or suggested to improve either. Limitations of photosynthesis have also been uncovered and efforts been taken to increase its efficiency. Examples include decreasing photosynthetic antennae size, increasing the photosynthetically available light spectrum, countering oxygenase activity of Rubisco by implementing C4 photosynthesis to C3 plants and altering source to sink transport of metabolites. A natural and effective photosynthetic adaptation, the sugar alcohol metabolism got however remarkably little attention in the last years, despite being comparably efficient as C4, and can be considered easier to introduce to new species. We also propose root to shoot carbon-dioxide transport as a means to improve photosynthetic performance and drought tolerance at the same time. Different suggestions and successful examples are covered here for improving plant photosynthesis as well as novel perspectives are presented for future research

Increasing crop yield and resilience with trehalose 6-phosphate: targeting a feast-famine mechanism in cereals for better source-sink optimisation

Food security is a pressing global issue. New approaches are required to break through a yield ceiling that has developed in recent years for the major crops. As important as increasing yield potential is the protection of yield from abiotic stresses in an increasingly variable and unpredictable climate. Current strategies to improve yield include conventional breeding, marker-assisted breeding, quantitative trait loci (QTLs), mutagenesis, creation of hybrids, genetic modification (GM), emerging genome-editing technologies, and chemical approaches. A regulatory mechanism amenable to three of these approaches has great promise for large yield improvements. Trehalose 6-phosphate (T6P) synthesized in the low-flux trehalose biosynthetic pathway signals the availability of sucrose in plant cells as part of a whole-plant sucrose homeostatic mechanism. Modifying T6P content by GM, marker-assisted selection, and novel chemistry has improved yield in three major cereals under a range of water availabilities from severe drought through to flooding. Yield improvements have been achieved by altering carbon allocation and how carbon is used. Targeting T6P both temporally and spatially offers great promise for large yield improvements in productive (up to 20%) and marginal environments (up to 120%). This opinion paper highlights this important breakthrough in fundamental science for crop improvement

Expression of the 1D×5 high molecular weight glutenin subunit protein in transgenic rice

We generated and analysed transgenic rice lines synthesizing substantial amounts of high-molecular-weight glutenin subunit (HMW-GS) from wheat. Particle bombardment has been used to transform rice cultivars ( Orysa sativa L.) with a cassette carrying the gene of 1D×5 HMW glutenin subunit. Twelve independent lines were recovered and PCR results on genomic DNS confirmed the integration of the transgene into it. Five lines set seeds. Seeds were analysed by SDS-PAGE and Western blot and we proved the presence of foreign protein in the starchy endosperm. The amount of 1D×5 HMW-GS in rice endosperm represents 0.75–3.18% of the alcohol soluble proteins. These results are the first example of significantly changing storage protein composition of rice exploiting the method of gene technology. This alteration may have considerable effect on the functional properties, including strength and stability of the dough made of transgenic rice flour