Site-specific photosynthesis in wheat (Triticum aestivum L.)

Site-specific photosynthesis in wheat (Triticum aestivum L.)

Global food security is threatened by increasing population growth and the adverse effects of climate change. To sustain the expected global population of 9.5 billion by 2050, food production must increase by at least 60%. Exploitation of the more efficient C4 mode of photosynthesis of important food crops has been suggested as a resolution for the foreseen risk as most of the cereal crops perform C3 photosynthesis which is less efficient. Wheat (Triticum aestivum L.) is one of the most widely produced cereals globally and accounts for 21% of the world’s daily dietary protein intake. Spike photosynthesis is believed to play a vital role in grain filling in wheat, with the contribution from ear photosynthesis to the total yield being 10–44%, depending on genetic and environmental factors. That is, it seems possible that different photosynthetic traits to the main photosynthetic organs may be expressed in specific sites of the same plant, although it is less prominent. Here we refer to this phenomenon as site-specific photosynthesis. It is also hypothesized that, along with photosynthesis, other vital metabolic processes such as sucrose metabolism and nitrogen (N) assimilation vary site-specifically and may play significant roles in grain quality and quantity. The magnitude of this variation in metabolic processes varies even within the same species, indicating a significant genetic difference. In addition, the site-specific variation of metabolic processes in wheat probably depends on ontogeny, being more pronounced during grain filling. Numerous studies have been conducted to elucidate photosynthesis and other metabolic pathways in the flag leaves of wheat. However, there are few site-specific studies of plant metabolic processes, and such information is essential to improve crop yield potential. Therefore, this project aimed to dissect the site-specific, genotypic, and temporal variation of key metabolic processes associated with photosynthesis using pericarps and flag leaves of three wheat genotypes (Huandoy, Amurskaja 75 and Greece25) at different growth stages.

Most gas exchange protocols are designed for leaves. Therefore, to acquire gas exchange properties of wheat spikes, I developed and optimized a new protocol using a LI-COR 6400XT portable photosynthesis system. The newly developed method was used to estimate the biological capacity of carbon (C) assimilation (Vcmax: maximum rate of Rubisco carboxylation and Jmax: maximum rate of photosynthetic electron transport) of wheat spikes at different ontogenies based on carbon dioxide response curves (A-Ci curves). This method could be applied to the irregularly shaped organs of other species, such as the panicles of rice. The measurements of Vcmax, Jmax, and triose phosphate utilization (TPU) of wheat spikes and flag leaves facilitated by using the novel gas exchange protocol, together with expression of genes involved in photosynthesis and sucrose metabolism, demonstrated site-specific variation in the biological capacity of C assimilation at early ontogeny that dissipated by late grainfilling. Transcript abundance of genes related to large and small sub-units of Rubisco (rbcS and rbcL) in flag leaves was significantly higher than in the pericarps. At early ontogeny, wheat pericarps exhibited a strong, positive correlation between the biological capacity of C assimilation and expression of key genes related to sucrose metabolism. The strong correlation between spike dry weight and the biological capacity of spike photosynthesis along with other observations suggested that metabolic processes in wheat spikes may play a major role in grain filling and thus total yield production. Site-specific, genotypic and temporal variation in the expression pattern of key genes of the C4 pathway in wheat revealed no evidence for grain-specific C4 photosynthesis, despite evidence of gene expression related to C4 pathway enzymes in pericarps and flag leaves. This inference is supported by δ13C observations. Site-specific, genotypic and temporal patterns of N metabolism suggested that wheat pericarps may play an important role in grain quality (protein concentration) and crop yield potential specifically after leaf senescence. The metabolic responses of wheat spikes in response to elevated temperature showed genotype-dependent differences in target gene expression and proteomics. The proteomic studies revealed that along with photosynthesis, other metabolic processes such as the antioxidant defense system, protein synthesis, and energy production were significantly affected at elevated temperatures. Of the differentially expressed proteins, those related to antioxidant defense systems were more significant in Huandoy than Amurskaja 75, indicating that Huandoy was under greater heat stress than Amurskaja 75. This inter-varietal difference was confirmed by the 60% yield decrease in Huandoy compared with a 5% yield increase in Amursksja 75 at elevated temperatures. That is, my results show that Amursksja 75 is relatively heat tolerant making it a possible candidate for crop improvement programs for future climates. However, the picture about spike metabolic processes in response to elevated temperatures remains incomplete as in both genotypes, there were numerous uncharacterized proteins which may play significant roles in wheat metabolism at a higher temperature.

Findings of this study point to the importance of exploring the site-specific, genotypic, and temporal variation of photosynthesis and other metabolic processes of vital food crops to develop higher-yielding crop varieties in the future.