Interactive Effects of Elevated CO2 and Growth Temperature on the Tolerance of Photosynthesis to Acute Heat Stress in C3 and C4 Species

Determining effects of elevated CO2 on the tolerance of photosynthesis to acute heat-stress (heat wave) is necessary for predicting plant responses to global warming, as photosynthesis is thermolabile and acute heat-stress and atmospheric CO2 will increase in the future. Few studies have examined this, and past results are variable, which may be due to methodological variation. To address this, we grew two C3 and two C4 species at current or elevated CO2 and three different growth temperatures (GT). We assessed photosynthetic thermotolerance in both unacclimated (basal tolerance) and preheat-stressed (preHS = acclimated) plants. In C3 species, basal thermotolerance of net photosynthesis (Pn) was increased In high CO2, but in C4 species, Pn thermotlerance was decreased by high CO2 (except Zea maya at low GT); CO2 effects in preHS plants were mostly small or absent, though high CO2 was detrimental in one C3 and one C4 species at warmer GT. Though high CO2 generally decreased stomatal conductance, decreases in Pn during heat stress were mostly due to non-stomatal effects. Photosystem II (PSII) efficiency was often decreased by high CO2 during heat stress, especially at high GT; CO2 effects on post-PSll electron transport were variable. Thus, high CO2 often affected photosynthetic theromotolerance, and the effects varied with photosynthetic pathway, growth temperature, and acclimation state. Most importantly, in heat-stressed plants at normal or warmer growth temperatures, high CO2 may often decrease, or not benefit as expected, tolerance of photosynthesis to acute heat stress. Therefore, interactive effects of elevated CO2 and warmer growth temperatures on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.     

Expression of AtSAP5 in cotton up-regulates putative stress-responsive genes and improves the tolerance to rapidly developing water deficit and moderate heat stress

The regulation of gene expression is a key factor in plant acclimation to stress, and it is thought that manipulation of the expression of critical stress-responsive genes should ultimately provide increased protection against abiotic stress. The aim of this study was to test the hypothesis that the ectopic expression of the AtSAP5 (AT3G12630) gene in transgenic cotton (Gossypium hirsutum, cv. Coker 312) will improve tolerance to drought and heat stress by up-regulating the expression of endogenous stress-responsive genes. The SAP5 gene is a member of the stress-associated family of genes that encode proteins containing A20/AN1 zinc finger domains. Under non-stressful conditions, cotton plants that expressed the AtSAP5 gene showed elevated expression of at least four genes normally induced during water deficit or heat stress. The rate of net CO(2) assimilation A for three of four transgenic lines tested was less sensitive to rapidly developing water deficit over 4d than untransformed wild-type plants, but the recovery of A following drought was not significantly affected. The enhanced protection of photosynthesis during drought was determined to be primarily at the biochemical level, since the extent of stomatal closure was not significantly different for all genotypes. Expression of AtSAP5 resulted in the complete protection of photosystem (PS) II complexes from photodamage at mid-day after 4d of drought, whereas wild-type plants experienced a 20% decline in active photosystem II (PSII) complexes. In addition, enhanced protection of seedling growth and leaf viability was associated with the expression of AtSAP5. Since A for the transgenic plants was significantly more heat tolerant than A for wild-type plants, we conclude that ectopic expression of SAP genes is a potentially viable approach to improving carbon gain and productivity for cotton grown in semi-arid regions with severe drought and heat stress.     

Bacteria and archaea as the sources of traits for enhanced plant phenotypes

Rising global demand for food and population increases are driving the need for improved crop productivity over the next 30 years. Plants have inherent metabolic limitations on productivity such as inefficiencies in carbon fixation and sensitivity to environmental conditions. Bacteria and archaea inhabit some of the most inhospitable environments on the planet and possess unique metabolic pathways and genes to cope with these conditions. Microbial genes involved in carbon fixation, abiotic stress tolerance, and nutrient acquisition have been utilized in plants to enhance plant phenotypes by increasing yield, photosynthesis, and abiotic stress tolerance. Transgenic plants expressing bacterial and archaeal genes will be discussed along with emerging strategies and tools to increase plant growth and yield.     

A critical review on the improvement of photosynthetic carbon assimilation in C3 plants using genetic engineering

Global warming is one of the most serious challenges facing us today. It may be linked to the increase in atmospheric CO2 and other greenhouse gases (GHGs), leading to a rise in sea level, notable shifts in ecosystems, and in the frequency and intensity of wild fires. There is a strong interest in stabilizing the atmospheric concentration of CO2 and other GHGs by decreasing carbon emission and/or increasing carbon sequestration. Biotic sequestration is an important and effective strategy to mitigate the effects of rising atmospheric CO2 concentrations by increasing carbon sequestration and storage capacity of ecosystems using plant photosynthesis and by decreasing carbon emission using biofuel rather than fossil fuel. Improvement of photosynthetic carbon assimilation, using transgenic engineering, potentially provides a set of available and effective tools for enhancing plant carbon sequestration. In this review, firstly different biological methods of CO2 assimilation in C3, C4 and CAM plants are introduced and three types of C4 pathways which have high photosynthetic performance and have evolved as CO2 pumps are briefly summarized. Then (i) the improvement of photosynthetic carbon assimilation of C3 plants by transgenic engineering using non-C4 genes, and (ii) the overexpression of individual or multiple C4 cycle photosynthetic genes (PEPC, PPDK, PCK, NADP-ME and NADP-MDH) in transgenic C3 plants (e.g. tobacco, potato, rice and Arabidopsis) are highlighted. Some transgenic C3 plants (e.g. tobacco, rice and Arabidopsis) overexpressing the FBP/SBPase, ictB and cytochrome c6 genes showed positive effects on photosynthetic efficiency and growth characteristics. However, over the last 28 years, efforts to overexpress individual, double or multiple C4 enzymes in C3 plants like tobacco, potato, rice, and Arabidopsis have produced mixed results that do not confirm or eliminate the possibility of improving photosynthesis of C3 plants by this approach. Finally, a prospect is provided on the challenges of enhancing carbon assimilation of C3 plants using transgenic engineering in the face of global warming, and the trends of the most promising approaches to improving the photosynthetic performance of C3 plants.     

Rubisco regulation: a role for inhibitors

In photosynthesis Rubisco catalyses the assimilation of CO(2) by the carboxylation of ribulose-1,5-bisphosphate. However, the catalytic properties of Rubisco are not optimal for current or projected environments and limit the efficiency of photosynthesis. Rubisco activity is highly regulated in response to short-term fluctuations in the environment, although such regulation may not be optimally poised for crop productivity. The regulation of Rubisco activity in higher plants is reviewed here, including the role of Rubisco activase, tight binding inhibitors, and the impact of abiotic stress upon them.     

Enhancing drought tolerance in C(4) crops

Adaptation to abiotic stresses is a quantitative trait controlled by many different genes. Enhancing the tolerance of crop plants to abiotic stresses such as drought has therefore proved to be somewhat elusive in terms of plant breeding. While many C(4) species have significant agronomic importance, most of the research effort on improving drought tolerance has focused on maize. Ideally, drought tolerance has to be achieved without penalties in yield potential. Possibilities for success in this regard are highlighted by studies on maize hybrids performed over the last 70 years that have demonstrated that yield potential and enhanced stress tolerance are associated traits. However, while our understanding of the molecular mechanisms that enable plants to tolerate drought has increased considerably in recent years, there have been relatively few applications of DNA marker technologies in practical C(4) breeding programmes for improved stress tolerance. Moreover, until recently, targeted approaches to drought tolerance have concentrated largely on shoot parameters, particularly those associated with photosynthesis and stay green phenotypes, rather than on root traits such as soil moisture capture for transpiration, root architecture, and improvement of effective use of water. These root traits are now increasingly considered as important targets for yield improvement in C(4) plants under drought stress. Similarly, the molecular mechanisms underpinning heterosis have considerable potential for exploitation in enhancing drought stress tolerance. While current evidence points to the crucial importance of root traits in drought tolerance in C(4) plants, shoot traits may also be important in maintaining high yields during drought.     

Photosynthesis and antioxidative defense mechanisms in deciphering drought stress tolerance of crop plants

Crop plants are regularly exposed to an array of abiotic and biotic stresses, among them drought stress is a major environmental factor that shows adverse effects on plant growth and productivity. Because of this these factors are considered as hazardous for crop production. Drought stress elicits a plethora of responses in plants resulting in strict amendments in physiological, biochemical, and molecular processes. Photosynthesis is the most fundamental physiological process affected by drought due to a reduction in the CO₂ assimilation rate and disruption of primary photosynthetic reactions and pigments. Drought also expedites the generation of reactive oxygen species (ROS), triggering a cascade of antioxidative defense mechanisms, and affects many other metabolic processes as well as affecting gene expression. Details of the drought stress-induced changes, particularly in crop plants, are discussed in this review, with the major points: 1) leaf water potentials and water use efficiency in plants under drought stress; 2) increased production of ROS under drought leading to oxidative stress in plants and the role of ROS as signaling molecules; 3) molecular responses that lead to the enhanced expression of stress-inducible genes; 4) the decrease in photosynthesis leading to the decreased amount of assimilates, growth, and yield; 5) the antioxidant defense mechanisms comprising of enzymatic and non-enzymatic antioxidants and the other protective mechanisms; 6) progress made in identifying the drought stress tolerance mechanisms; 7) the production of transgenic crop plants with enhanced tolerance to drought stress.     

Effects of elevated CO2 on the tolerance of photosynthesis to acute heat stress in C3, C4, and CAM species

Determining the effect of elevated CO(2) on the tolerance of photosynthesis to acute heat stress (AHS) is necessary for predicting plant responses to global warming because photosynthesis is heat sensitive and AHS and atmospheric CO(2) will increase in the future. Few studies have examined this effect, and past results were variable, which may be related to methodological variation among studies. In this study, we grew 11 species that included cool and warm season and C(3), C(4), and CAM species at current or elevated (370 or 700 ppm) CO(2) and at species-specific optimal growth temperatures and at 30°C (if optimal ≠ 30°C). We then assessed thermotolerance of net photosynthesis (P(n)), stomatal conductance (g(st)), leaf internal [CO(2)], and photosystem II (PSII) and post-PSII electron transport during AHS. Thermotolerance of P(n) in elevated (vs. ambient) CO(2) increased in C(3), but decreased in C(4) (especially) and CAM (high growth temperature only), species. In contrast, elevated CO(2) decreased electron transport in 10 of 11 species. High CO(2) decreased g(st) in five of nine species, but stomatal limitations to P(n) increased during AHS in only two cool-season C(3) species. Thus, benefits of elevated CO(2) to photosynthesis at normal temperatures may be partly offset by negative effects during AHS, especially for C(4) species, so effects of elevated CO(2) on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.     

Plant abiotic stress: deciphering remedial strategies for emerging problem

Growing in their natural environment, plants often encounter unfavorable environmental conditions that interrupt normal plant growth and productivity. Drought, high/low temperature and saline soils are the most common abiotic stresses that plants encounter in their natural environments. Molecular and genomic analyses have facilitated gene discovery and enabled genetic engineering using several functional or regulatory genes that are known to be involved in stress response and preliminary tolerance, to activate specific or broad pathways related to abiotic stress tolerance in plants. Through the use of transgenic technology, goals such as production of plants with desired traits that were unattainable with traditional selection programs are achieved. This review deals with recent advancement in understanding the role of various stress responsive genes and their critical importance for explaining the control mechanism of abiotic stress tolerance and engineering stress tolerant crops based on the expression of specific stress related genes.     

Growth temperature can alter the temperature dependent stimulation of photosynthesis by elevated carbon dioxide in Albutilon theophrasti

Stimulation of photosynthesis in response to elevated carbon dioxide concentration [CO2] in the short-term (min) should be highly temperature dependent at high photon flux. However, it is unclear if long-term (days, weeks) adaptation to a given growth temperature alters the temperature-dependent stimulation of photosynthesis to [CO2]. In velveltleaf (Albutilon theophrasti), the response of photosynthesis, determined as CO2 assimilation, was measured over a range of internal CO2 concentrations at 7 short-term measurement (12, 16, 20, 24, 28, 32, 36 degrees C) temperatures for each of 4 long-term growth (16, 20, 28 and 32 degrees C) temperatures. In vivo estimates of VCmax, the maximum RuBP saturated rate of carboxylation, and Jmax, the light-saturated rate of potential electron transport, were determined from gas exchange measurements for each temperature combination. Overall, previous exposure to a given growth temperature adjusted the optimal temperatures of Jmax and VCmax with subsequently greater enhancement of photosynthesis at elevated [CO2] (i.e., a greater enhancement of photosynthesis at elevated [CO2] was observed at low measurement temperatures for A. theophrasti grown at low growth temperatures compared with higher growth temperatures, and vice versa for plants grown and measured at high temperatures). Previous biochemical based models used to predict the interaction between rising [CO2] and temperature on photosynthesis have generally assumed no growth temperature effect on carboxylation kinetics or no limitation by Jmax. In the current study, these models over predicted the temperature dependence of the photosynthetic response to elevated [CO2] at temperatures above 24 degrees C. If these models are modified to include long-term adjustments of Jmax and VCmax to growth temperature, then greater agreement between observed and predicted values was obtained.     

The response of photosynthesis and stomatal conductance to rising [CO2]: mechanisms and environmental interactions

This review summarizes current understanding of the mechanisms that underlie the response of photosynthesis and stomatal conductance to elevated carbon dioxide concentration ([CO2]), and examines how downstream processes and environmental constraints modulate these two fundamental responses. The results from free-air CO2 enrichment (FACE) experiments were summarized via meta-analysis to quantify the mean responses of stomatal and photosynthetic parameters to elevated [CO2]. Elevation of [CO2] in FACE experiments reduced stomatal conductance by 22%, yet, this reduction was not associated with a similar change in stomatal density. Elevated [CO2] stimulated light-saturated photosynthesis (Asat) in C3 plants grown in FACE by an average of 31%. However, the magnitude of the increase in Asat varied with functional group and environment. Functional groups with ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)-limited photosynthesis at elevated [CO2] had greater potential for increases in Asat than those where photosynthesis became ribulose-1,5-bisphosphate (RubP)-limited at elevated [CO2]. Both nitrogen supply and sink capacity modulated the response of photosynthesis to elevated [CO2] through their impact on the acclimation of carboxylation capacity. Increased understanding of the molecular and biochemical mechanisms by which plants respond to elevated [CO2], and the feedback of environmental factors upon them, will improve our ability to predict ecosystem responses to rising [CO2] and increase our potential to adapt crops and managed ecosystems to future atmospheric [CO2].     

Photosynthetic characteristics of non‐foliar organs in main C3 cereals

Photosynthesis in non‐foliar organs plays an important role in crop growth and productivity, and it has received considerable research attention in recent years. However, compared with the capability of photosynthetic CO₂ fixation in leaves, the distinct attributes of photosynthesis in the non‐foliar organs of wheat (a C₃ species) are unclear. This review presents a comprehensive examination of the photosynthetic characteristics of non‐foliar organs in wheat. Compared with leaves, non‐foliar organs had a higher capacity to refix respired CO₂, higher tolerance to environmental stresses and slower terminal senescence after anthesis. Additionally, whether C₄ photosynthetic metabolism exists in the non‐foliar organs of wheat is discussed, as is the advantage of photosynthesis in non‐foliar organs during times of abiotic stress. Introducing the photosynthesis‐related genes of C₄ plants into wheat, which are specifically expressed in non‐foliar organs, can be a promising approach for improving wheat productivity.     

转基因改良植物的胁迫耐性

干旱、盐碱和低温等逆境是严重影响栽培植物生产的非生物胁迫因素。导入外源目的的基因已发展成为改良作物对逆境胁迫耐性的新途径。现今已应用于植物胁迫改良的基因包括编码活性氧清除酶类、膜修饰酶类、胁迫诱导蛋白和渗调物质合成酶等基因。