Elevated CO2 alters tissue balance of nitrogen metabolism and downregulates nitrogen assimilation and signalling gene expression in wheat seedlings receiving high nitrate supply

Calcium regulates primary nitrate response associated gene transcription in a time- and dose-dependent manner

Nitrate (NO3-) is the primary source of nitrogen preferred by most arable crops, including wheat. The pioneering experiment on primary nitrate response (PNR) was carried out three decades ago. Since then, much research has been carried out to understand the NO3- signaling. Nitrate is sensed by the dual affinity NO3- transceptor NPF6.3, which further relays the information to a master regulator NIN-like protein 7 (NLP7) through calcium-dependent protein kinases (CPK10, CPK30, CPK32), highlighting the importance of calcium ion (Ca2+) as one of the important secondary messengers in relaying the NO3- signaling in Arabidopsis. In a previous study, we found that Ca2+ regulates nitrogen starvation response in wheat. In this study, 10 days old NO3--starved wheat seedlings were exposed to various treatments. Our study on time course changes in expression of PNR sentinel genes; NPF6.1, NPF6.2, NRT2.1, NRT2.3, NR, and NIR in wheat manifest the highest level of expression at 30 min after NO3- exposure. The use of Ca2+ chelator EGTA confirmed the involvement of Ca2+ in the regulation of transcription of NPFs and NRTs as well the NO3- uptake. We also observed the NO3- dose-dependent and tissue-specific regulation of nitrate reductase activity involving Ca2+ as a mediator. The participation of Ca2+ in the PNR and NO3- signaling in wheat is confirmed by pharmacological analysis, physiological evidences, and protoplast-based Ca2+ localization.

A low red/far-red ratio restricts nitrogen assimilation by inhibiting nitrate reductase associated with downregulated TaNR1.2 and upregulated TaPIL5 in wheat (Triticum aestivum L.)

Understanding the physiological mechanism underlying nitrogen levels response to a low red/far-red ratio (R/FR) can provide new insights for optimizing wheat yield potential but has been not well documented. This study focused on the changes in nitrogen levels, nitrogen assimilation and nitrate uptake in wheat plants grown with and without additional far-red light. A low R/FR reduced wheat nitrogen accumulation and grain yield compared with the control. The levels of total nitrogen, free amino acid and ammonium were decreased in leaves but nitrate content was temporarily increased under a low R/FR. The nitrate reductase (NR) activity in leaves was more sensitive to a low R/FR than glutamine synthetase, glutamate synthase, glutamic oxalacetic transaminase and glutamic-pyruvic transaminase. Further analysis showed that a low R/FR had little effect on the NR activation state but reduced the level of NR protein and the expression of encoding gene TaNR1.2. Interestingly, a low R/FR rapidly induced TaPIL5 expression rather than TaHY5 and other members of TaPILs in wheat, suggesting that TaPIL5 was the key transcription factor response to a low R/FR in wheat and might be involved in the downregulation of TaNR1.2 expression. Besides, a low R/FR downregulated the expression of TaNR1.2 in leaves earlier than that of TaNRT1.1/1.2/1.5/1.8 in roots, which highlights the importance of NR and nitrogen assimilation in response to a low R/FR. Our results provide revelatory evidence that restricted nitrate reductase associated with downregulated TaNR1.2 and upregulated TaPIL5 mediate the suppression of nitrogen assimilation under a low R/FR in wheat.

Differential Effect of Free-Air CO2 Enrichment (FACE) in Different Organs and Growth Stages of Two Cultivars of Durum Wheat

Wheat is a target crop within the food security context. The responses of wheat plants under elevated concentrations of CO₂ (e[CO₂]) have been previously studied; however, few of these studies have evaluated several organs at different phenological stages simultaneously under free-air CO₂ enrichment (FACE) conditions. The main objective of this study was to evaluate the effect of e[CO₂] in two cultivars of wheat (Triumph and Norin), analyzed at three phenological stages (elongation, anthesis, and maturation) and in different organs at each stage, under FACE conditions. Agronomic, biomass, physiological, and carbon (C) and nitrogen (N) dynamics were examined in both ambient CO₂ (a[CO₂]) fixed at 415 µmol mol^-1 CO₂ and e[CO₂] at 550 µmol mol^-1 CO₂. We found minimal effect of e[CO₂] compared to a[CO₂] on agronomic and biomass parameters. Also, while exposure to 550 µmol mol^-1 CO₂ increased the photosynthetic rate of CO₂ assimilation (An), the current study showed a diminishment in the maximum carboxylation (V_c,max) and maximum electron transport (J_max) under e[CO₂] conditions compared to a[CO₂] at physiological level in both cultivars. However, even if no significant differences were detected between cultivars on photosynthetic machinery, differential responses between cultivars were detected in C and N dynamics at e[CO₂]. Triumph showed starch accumulation in most organs during anthesis and maturation, but a decline in N content was observed. Contrastingly, in Norin, a decrease in starch content during the three stages and an increase in N content was observed. The amino acid content decreased in grain and shells at maturation in both cultivars, which might indicate a minimal translocation from source to sink organs. These results suggest a greater acclimation to e[CO₂] enrichment in Triumph than Norin, because both the elongation stage and e[CO₂] modified the source-sink relationship. According to the differences between cultivars, future studies should be performed to test genetic variation under FACE technology and explore the potential of cultivars to cope with projected climate scenarios.

Elevated CO2 and Water Stress in Combination in Plants: Brothers in Arms or Partners in Crime?

The changing dynamics in the climate are the primary and important determinants of agriculture productivity. The effects of this changing climate on overall productivity in agriculture can be understood when we study the effects of individual components contributing to the changing climate on plants and crops. Elevated CO₂ (eCO₂) and drought due to high variability in rainfall is one of the important manifestations of the changing climate. There is a considerable amount of literature that addresses climate effects on plant systems from molecules to ecosystems. Of particular interest is the effect of increased CO₂ on plants in relation to drought and water stress. As it is known that one of the consistent effects of increased CO₂ in the atmosphere is increased photosynthesis, especially in C₃ plants, it will be interesting to know the effect of drought in relation to elevated CO₂. The potential of elevated CO₂ ameliorating the effects of water deficit stress is evident from literature, which suggests that these two agents are brothers in arms protecting the plant from stress rather than partners in crime, specifically for water deficit when in isolation. The possible mechanisms by which this occurs will be discussed in this minireview. Interpreting the effects of short-term and long-term exposure of plants to elevated CO₂ in the context of ameliorating the negative impacts of drought will show us the possible ways by which there can be effective adaption to crops in the changing climate scenario.

Interaction of elevated CO2 and form of nitrogen nutrition alters leaf abaxial and adaxial epidermal and stomatal anatomy of wheat seedlings

Plant's stomatal physiology and anatomical features are highly plastic and are influenced by diverse environmental signals including the concentration of atmospheric CO₂ and nutrient availability. Recent reports suggest that the form of nitrogen (N) is a determinant of plant growth and nutrient nitrogen use efficiency (NUE) under elevated CO₂ (EC). Previously, we found that high nitrate availability resulted in early senescence, enhanced reactive oxygen species (ROS), and reactive nitrogen species (RNS) production and also that mixed nutrition of nitrate and ammonium ions were beneficial than sole nitrate nutrition in wheat. In this study, the interactive effects of different N forms (nitrate, ammonium, mixed nutrition of nitrate, and ammonium) and EC on epidermal and stomatal morphology were analyzed. Wheat seedlings were grown at two different CO₂ levels and supplied with media devoid of N (N₀) or with nitrate-N (NN), mixed nutrition of ammonium and nitrate (MN), or only ammonium-N (AN). The stoma length increased significantly in nitrate nutrition with a consistent reduction in stoma width. Guard cell length was higher in EC treatment as compared to AC. The guard cell width was maximum in AN-grown plants at EC. Epidermal cell density and stomatal density were lower at EC. Nitrate nutrition increased the stomatal area at EC while the reverse was true for MN and AN. Wheat plants fertilized with AN showed a higher accumulation of superoxide radical (SOR) at EC, while in NN treatment, the accumulation of hydrogen peroxide (H₂O₂) was higher at EC. Reactive oxygen species, particularly H₂O₂, can trigger mitogen-activated protein kinase (MAPK) mediated signaling and its crosstalk with abscisic acid (ABA) signaling to regulate stomatal anatomy in nitrate-fed plants. The SOR accumulation in ammonium- and ammonium nitrate-fed plants and H₂O₂ in NN-fed plants might finely regulate the sensitivity of stomata to alter water/nutrient use efficiency and productivity under EC. The data reveals that the variation in anatomical attributes viz. cell length, number of cells, etc. affected the leaf growth responses to EC and forms of N nutrition. These attributes are fine targets for effective manipulation of growth responses to EC.

The Mechanisms Responsible for N Deficiency in Well-Watered Wheat Under Elevated CO2

Elevated CO₂ concentration [e(CO₂)] often promotes plant growth with a decrease in tissue N concentration. In this study, three experiments, two under hydroponic and one in well-watered soil, including various levels or patterns of CO₂, humidity, and N supply were conducted on wheat (Triticum aestivum L.) to explore the mechanisms of e[CO₂]-induced N deficiency (ECIND). Under hydroponic conditions, N uptake remained constant even as transpiration was limited 40% by raising air relative humidity and only was reduced about 20% by supplying N during nighttime rather than daytime with a reduction of 85% in transpiration. Compared to ambient CO₂ concentration, whether under hydroponic or well-watered soil conditions, and whether transpiration was kept stable or decreased to 12%, e[CO₂] consistently led to more N uptake and higher biomass, while lower N concentration was observed in aboveground organs, especially leaves, as long as N supply was insufficient. These results show that, due to compensation caused by active uptake, N uptake can be uncoupled from water uptake under well-watered conditions, and changes in transpiration therefore do not account for ECIND. Similar or lower tissue NO 3 - -N concentration under e[CO₂] indicated that NO 3 - assimilation was not limited and could therefore also be eliminated as a major cause of ECIND under our conditions. Active uptake has the potential to bridge the gap between N taken up passively and plant demand, but is limited by the energy required to drive it. Compared to ambient CO₂ concentration, the increase in N uptake under e[CO₂] failed to match the increase of carbohydrates, leading to N dilution in plant tissues, the apparent dominant mechanism explaining ECIND. Lower N concentration in leaves rather than roots under e[CO₂] validated that ECIND was at least partially also related to changes in resource allocation, apparently to maintain root uptake activity and prevent more serious N deficiency.

Growth and Nutritional Quality of Lemnaceae Viewed Comparatively in an Ecological and Evolutionary Context

This review focuses on recently characterized traits of the aquatic floating plant Lemna with an emphasis on its capacity to combine rapid growth with the accumulation of high levels of the essential human micronutrient zeaxanthin due to an unusual pigment composition not seen in other fast-growing plants. In addition, Lemna's response to elevated CO₂ was evaluated in the context of the source-sink balance between plant sugar production and consumption. These and other traits of Lemnaceae are compared with those of other floating aquatic plants as well as terrestrial plants adapted to different environments. It was concluded that the unique features of aquatic plants reflect adaptations to the freshwater environment, including rapid growth, high productivity, and exceptionally strong accumulation of high-quality vegetative storage protein and human antioxidant micronutrients. It was further concluded that the insensitivity of growth rate to environmental conditions and plant source-sink imbalance may allow duckweeds to take advantage of elevated atmospheric CO₂ levels via particularly strong stimulation of biomass production and only minor declines in the growth of new tissue. It is proposed that declines in nutritional quality under elevated CO₂ (due to regulatory adjustments in photosynthetic metabolism) may be mitigated by plant-microbe interaction, for which duckweeds have a high propensity.