Necessity of Photosynthesis Research

Photosynthesis is essential to life on earth as it converts sunlight into biochemical energy used by nearly all life forms. It is the primary process for generating plant biomass. As a result of photosynthesis, carbon dioxide, as well as inorganic nitrogen and sulfur, are converted into reduced molecules (e.g. sugars, lipids, amino acids and other cell building blocks), and oxygen is produced through photosynthetic water oxidation. Thus, plant and algal photosynthesis affects global geochemical processes, in particular carbon cycling (36), and is an important factor to be considered in global climate change models. Aside from these fundamental aspects of photosynthesis, agricultural production of food, feed, fiber, natural chemicals, and biofuel feedstocks is directly affected by limitations in the rate and yield of photosynthesis and the capacity to utilize fixed carbon (128, 143). Some of the greatest challenges humankind is currently facing – feeding an exponentially growing global population, supplying sufficient energy to sustain this global population, and averting negative environmental impacts due to human activities – can or need to be addressed using photosynthetic organisms (i.e. agricultural crops) (35). Therefore, the need for state-of-the-art photosynthesis research to improve the efficiency of this process in traditional crops or for the development of novel crops and products has never been more urgent. 

Collaborators participating in this regional project will place considerable focus on understanding and improving the response of photosynthesis to genetic, developmental and environmental factors that limit productivity. The research spans all levels of organization from the molecular and cellular through the leaf, whole plant and canopy.  Particular emphasis will be placed on abiotic stresses (i.e., heat, cold, drought and salinity), nitrogen- and water-use efficiency, carbon flux pathways, and the signal transduction mechanisms that initiate the plant response. Factors that enhance or limit agricultural productivity generally do so by impacting photosynthesis.  The gains in yield of the major crops over the past half-century have come about primarily from breeding for greater harvest index (HI) and through management, particularly the application of fertilizer (47).  For well-fertilized crops, HI has approached the maximum achievable for many crops, and future yield gains will depend on increasing total production (123).  Greater productivity, which is necessary to meet the food, feed, fuel and fiber needs of a growing world population (104), requires improving the basic efficiency of the photosynthetic process for light energy capture and the utilization of this energy for the synthesis and utilization of organic molecules (123).  For maximum benefit, the efficiency of the process must be increased under both optimal and sub-optimal conditions, consonant with the changes in temperature, CO₂, and precipitation anticipated from climate change. 

Importance and Consequences

Global climatic trends are negatively impacting the yields of our major crops used for the production of food, feed, fiber, and biofuels (80). Sustaining the productivity of traditional crops will require improvements in the efficiency of photosynthesis that go beyond traditional breeding and selection (93). Similarly, the development of novel feedstocks for biofuel/chemicals production in a way that is sustainable, that is not in competition with food, and that reduces greenhouse gas production requires novel approaches and non-traditional crops including possibly algae (116, 134). Only a concerted and vertically integrated effort encompassing all aspects of photosynthesis will ensure that appropriate solutions will be found for some of these most pressing problems currently facing society. Innovative thinking will be required that does not stop at traditional agricultural systems and crops, but may enable transitioning to new crops dedicated to the production of biofuels and chemicals instead of food. As photosynthesis is at the basis of biomass production, we need to find innovative ways to overcome its limitations. Failure to do so now will limit our future ability to produce sufficient food, feed, fiber, and fuel in a rapidly changing climate. 

Technical Feasibility

Genomic resources and next generation sequencing technologies have advanced to the point that a wealth of information can be generated for any species of plant or alga (e.g., date and oil palm) (1, 20), in a relative short time span and at reasonable costs. Thus, the raw material for genetic engineering of novel crops or algal strains is readily available. Large-scale phenomics focused on chloroplast proteins in model plants such as Arabidopsis has become possible (81). Moreover, using comparative genomics, reconstruction of metabolism from gene expression and genomic data has become readily feasible for any organism. One example is the recent metabolic reconstruction of the alga Chlamydomonas reinhardtii (23). In addition, our knowledge and analysis of primary metabolism of plants and metabolic networks has advanced to levels (76) that can enable the rational design of novel crops, which will meet our needs. Despite this progress, the task of genetically transforming crop plants and analyzing the consequences (phenotypes) is still tedious and time consuming. Synthetic biology efforts with plants, involving stacking or replacing multiple genes, are lagging behind those for bacterial systems (98). Moreover, photosynthesis is one of the most complex processes found in nature, requiring hundreds of genes and proteins, and multiple and overlapping levels of regulation. Thus, to enable rapid progress in the basic discovery process and to devise strategies for the improvement of photosynthesis in crops, facile model organisms such as Arabidopsis (119) or model microalgae and cyanobacteria such as Chlamydomonas (99) and Synechocystis (25) will have to be employed to quickly identify the most promising directions for photosynthetic pathway and crop improvement. Following this guidance, collaboration with geneticists and breeders, associating traits with genomic regions of mapping populations can support marker-assisted selection (50). Such an integrated approach requires multifaceted expertise and, thus, will benefit from synergy derived from a multi-investigator effort. 

Multi State Effort

Providing a conceptual framework through the current project, this North Central Regional Group of scientists and others located throughout the US has already successfully made progress on understanding diverse aspects of photosynthesis bringing together a complementary set of expertise. While global issues as laid out above are addressed, practical solutions to these problems often have local solutions (e.g., by taking into account climatic zones to which specific crop species or algae are adapted). Continued effort by the current group will contribute towards these main goals while also enabling local solutions of particular value to the North Central Region and other participating states. Participating partners are listed for each focus area below. 

Likely Impacts

Efforts by the group are organized into four themes (Objectives). While the details and outcomes will be discussed below in the main body of the proposal, likely impacts falling under these themes can be briefly summarized as follows: 

1. Identify strategies to optimize the assembly and function of the photosynthetic membrane. Chloroplasts are the organelles that perform photosynthesis in both plants and algae. Chloroplasts also contain a large number of enzymes, highlighting the role of this organelle as a biochemical production factory. As semi-autonomous organelles, chloroplasts do not function by themselves, but rely on extensive communication with other organelles within the cell, and with the whole plant. The import of nuclear-encoded proteins or membrane lipids assembled at the endoplasmic reticulum provide two examples (16, 73). As primary photosynthate and many other metabolites (e.g., fatty acids), are only synthesized in chloroplasts, they have to be exported to be available to other cellular compartments. The integration of chloroplast biogenesis into overall cell development requires intricate signaling processes as does the adjustment of the photosynthetic electron transport chain to changing conditions. Within the chloroplast photosynthesis occurs on a specialized structure called the thylakoid membrane, which is itself dynamically remodeled in response to development and stress cues. The architectural dynamics of the grana thylakoids are involved in regulating and maintaining photosynthetic Studies will focus on how thylakoid membranes change their shape, the functional consequences of structural alterations, and effects of whole-plant stresses and developmental cues on the thylakoid membrane and chloroplast lipid changes. Likely impacts are a better understanding of photosynthetic energy transformation, the development of the thylakoid membranes under developmental and stress regimens, and the development of tools that can be used to assess thylakoid and inner envelope connectivity (MI-ABR, NE-AES, WA-AES). 

2. Photosynthetic Capture and Photorespiratory Release of CO₂. Photosynthetic carbon fixation and photorespiratory release of CO₂ have long been recognized as limitations to crop productivity (46, 94, 100, 121). Considerable focus will be placed on engineering improvements in the organization of the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and the heat-sensitive Rubisco activase (RCA) that regulates its function. Microbial CO₂-concentrating mechanisms will be investigated for potential improvements in the photosynthesis of algae and crop plants. Studies will examine the mechanisms of RCA regulation in soybean including redox regulation, protein phosphorylation and possible ‘cross-talk’ between the two regulatory processes. Emphasis will also be placed on the mechanisms that control carbon flux through primary and secondary metabolic pathways. Likely impacts will be insights into the regulation of carbon fixation and the generation of modified enzymes that improve primary and secondary carbon metabolism in plants. (IL-ARS, MI-ABR, MO-AES, NV-AES, WA-AES). 

3. Mechanisms Regulating Photosynthate Partitioning. Manipulation of carbon partitioning and understanding its regulation is central to advances in yield formation in cereal and oilseed crops, to the design of new crops for biofuel or commodity chemicals production that, e.g., divert carbon from carbohydrate synthesis into useful triacylglycerols (34), or terpenoid products (85). This work will lead to the development and commercial application of novel organisms that are endowed with the heterologous expression of terpene synthases (from higher plants) in either production-type plants or microalgae. The work will further seek to divert carbon flux and to limit catabolic reactions resulting in respiration that reduces the fraction of photosynthate available for yield formation. Transport phenomena at the cell, tissue and whole plant level will be considered, sugar-sensing mechanisms will be explored, as well as partitioning between storage carbohydrates, lipids, terpenoids and stress-protective compounds. Likely impacts will be defining the mechanisms that regulate photosynthate partitioning into the biosynthetic pathways for sucrose, starch, sugar alcohols, terpenoids, and lipids. Strategies will be developed to alter carbon partitioning by engineering bypass pathways to overcome rate and yield barriers to commercial application. (CA-AES, FL-AES, IL-ARS, KS-AES, MI-ABR, MO-ARS, MT-AES, OH-AES, VA-AES, WA-AES). 

4. Developmental and Environmental Limitations to Photosynthetic Productivity. Factors such as leaf anatomy (122) or environmental stress conditions such as high light (90), excess salinity, nitrogen deficiency, drought or heat stress (111) greatly affect photosynthesis. Leaf stomata, regulating photosynthetic productivity as a ‘gateway’ for CO₂ influx, are subject to complex active regulation. In the case of drought, the plant hormone abscisic acid (ABA) inhibits stomatal opening and promotes stomatal closure, supporting water conservation. A dynamic model of ABA-induced stomatal closure includes more than 40 identified network components (75); additional plant water status regulation is responsive to factors including light intensity, temperature, vapor pressure, and leaf CO₂ partial pressure. Feedback regulation of photosynthesis by sugar sensing and signal transduction mechanisms are known, but incompletely understood. Frequently, these regulatory pathways result from a cascading sequence of processes. Particular emphasis will be placed on identifying processes driving responses to abiotic stresses (e.g., temperature, water, and salinity), nitrogen use, and global atmospheric change. Likely impacts will be identifying genomic regions associated with developmental and environmental factors that influence photosynthetic productivity at the whole plant and canopy levels. (IL-AES, IL-ARS, KS-AES, MI-ABR, MO-AES, MS-AES, NV-AES, OH-AES).