Rhizobacteria improve sugarcane growth and photosynthesis under well‐watered conditions

Morpho‐physiological changes caused by particular plant growth‐promoting rhizobacteria were evaluated in sugarcane plants under varying water availability. Under well‐watered conditions, we have found one rhizobacteria isolate (IAC‐RBcr5) able to enhance root dry matter and photosynthesis of sugarcane plants. The IAC‐RBcr5 genome was sequenced and high similarity was found with Pseudomonas putida GB‐1. Based on increased root system size of inoculated plants, we hypothesised that sugarcane plants inoculated with IAC‐RBcr5 would have improved performance under water deficit. Although IAC‐RBcr5 had improved plant leaf CO₂ assimilation under water shortage, inoculation caused reduction of biomass accumulation in sugarcane. The negative influence of water deficit on shoot growth rate and root traits such as volume, area, diameter, length and specific root area was higher in plants treated with IAC‐RBcr5 as compared to non‐inoculated ones. However, rhizobacteria‐induced improvements in leaf and root proline contents would represent a strategy for storing carbon and nitrogen during low water availability and helping both organisms to resume their metabolism after rehydration. In conclusion, we found and identified a rhizobacterium able to improve growth and photosynthesis of sugarcane plants. Such benefit for plant growth was lost under low water availability as a likely consequence of increased carbon‐energy demand by rhizobacteria and their sensitivity to drought.     

Responses of holocyclic and anholocyclic Rhopalosiphum padi populations to low‐temperature and short‐photoperiod induction

The different life cycles of aphid species make these organisms good models for studying the short‐term consequences of sex. The bird cherry‐oat aphid Rhopalosiphum padi has a wide geographic distribution and correspondingly different life cycles. In this study, the life cycles of R. padi collected from six different regions in China were characterized experimentally by comparing the responses of holocyclic and anholocyclic populations to low‐temperature and short‐photoperiod induction. Clones collected from Chuzhou, Taian, and Taigu consistently reproduced via obligate parthenogenesis, whereas clones from Hami and Baicheng were holocyclic in their response, and those from Lanzhou were both holocyclic and anholocyclic. Prolonged exposure to low temperature and a short photoperiod (LS) had negative effects on the offspring of anholocyclic aphids with regard to adult lifespan, total longevity, and fecundity compared with aphids maintained at a normal temperature and a long photoperiod (NL). Holocyclic LS R. padi had longer developmental times at all nymph stages, a shorter adult lifespan, shorter total longevity, and a lower fecundity than NL counterparts. The adult prereproduction period of gynoparae was significantly longer than that of virginoparae, and the total longevity of gynoparae was significantly shorter than that of virginoparae. Moreover, the net reproductive and gross reproduction rates, as well as the total fecundity, were roughly fivefold higher in virginoparae than in gynoparae, indicating that there is the short‐term cost of sex. When maintained on their secondary host (Triticum aestivum), gynoparae, males, and oviparae produced by holocyclic populations could survive, and gynoparae produced oviparae. However, under NL conditions, oviparae could not produce overwintering eggs on the secondary host, whereas a few overwintering eggs were generated by oviparae under LS conditions. Taken together, these results illuminate the complexity of insect responses and contribute to a complete understanding of the aphid life cycle and its evolution.     

Lack of fructose 2,6‐bisphosphate compromises photosynthesis and growth in Arabidopsis in fluctuating environments

The balance between carbon assimilation, storage and utilisation during photosynthesis is dependent on partitioning of photoassimilate between starch and sucrose, and varies in response to changes in the environment. However, the extent to which the capacity to modulate carbon partitioning rapidly through short‐term allosteric regulation may contribute to plant performance is unknown. Here we examine the physiological role of fructose 2,6‐bisphosphate (Fru‐2,6‐P₂) during photosynthesis, growth and reproduction in Arabidopsis thaliana (L.). In leaves this signal metabolite contributes to coordination of carbon assimilation and partitioning during photosynthesis by allosterically modulating the activity of cytosolic fructose‐1,6‐bisphosphatase. Three independent T‐DNA insertional mutant lines deficient in 6‐phosphofructo‐2‐kinase/fructose‐2,6‐bisphosphatase (F2KP), the bifunctional enzyme responsible for both the synthesis and degradation of Fru‐2,6‐P₂, lack Fru‐2,6‐P₂. These plants have normal steady‐state rates of photosynthesis, but exhibit increased partitioning of photoassimilate into sucrose and have delayed photosynthetic induction kinetics. The F2KP‐deficient plants grow normally in constant environments, but show reduced growth and seed yields relative to wildtype plants in fluctuating light and/or temperature. We conclude that Fru‐2,6‐P₂ is required for optimum regulation of photosynthetic carbon metabolism under variable growth conditions. These analyses suggest that the capacity of Fru‐2,6‐P₂ to modulate partitioning of photoassimilate is an important determinant of growth and fitness in natural environments.     

Arabidopsis plants expressing only the redox‐regulated Rca‐α isoform have constrained photosynthesis and plant growth

Rubisco activase (Rca) facilitates the release of sugar‐phosphate inhibitors from the active sites of Rubisco and thereby plays a central role in initiating and sustaining Rubisco activation. In Arabidopsis, alternative splicing of a single Rca gene results in two Rca isoforms, Rca‐α and Rca‐β. Redox modulation of Rca‐α regulates the function of Rca‐α and Rca‐β acting together to control Rubisco activation. Although Arabidopsis Rca‐α alone less effectively activates Rubisco in vitro, it is not known how CO₂ assimilation and plant growth are impacted. Here, we show that two independent transgenic Arabidopsis lines expressing Rca‐α in the absence of Rca‐β (‘Rca‐α only’ lines) grew more slowly in various light conditions, especially under low light or fluctuating light intensity, and in a short day photoperiod compared to wildtype. Photosynthetic induction was slower in the Rca‐α only lines, and they maintained a lower rate of CO₂ assimilation during both photoperiod types. Our findings suggest Rca oligomers composed of Rca‐α only are less effective in initiating and sustaining the activation of Rubisco than when Rca‐β is also present. Currently there are no examples of any plant species that naturally express Rca‐α only but numerous examples of species expressing Rca‐β only. That Rca‐α exists in most plant species, including many C3 and C4 food and bioenergy crops, implies its presence is adaptive under some circumstances.     

Night and day: short‐term variation in nitrogen chemistry and nitrous oxide emissions from streams

1. Diel variation in metabolism contributes to variation in oxygen (O₂) concentrations in streams. This variation in O₂ and other parameters (e.g. pH) can in turn affect the rates of microbial nitrogen (N) processing, concentrations of nitrogenous solutes and production of the greenhouse gas nitrous oxide (N₂O). We investigated diel variability in emissions of N₂O and the magnitude of short‐term variability in N solutes across 10 streams. 2. Nitrous oxide fluxes varied on average 2.3‐fold over diel cycles. Concentrations would be underestimated by sampling around noon, but N₂O fluxes would not show a consistent bias. Time‐weighted mean daily N₂O flux was strongly related to nitrate concentration (r² = 0.58). Diel patterns in N₂O and dissolved N species were often complex (rather than simple sinusoidal curves), probably reflecting complex underlying processes. 3. Reliance on samples obtained around noon would overestimate daily mean nitrate concentrations by 5% and underestimate ammonium by 32% (average bias across all streams and dates). 4. Dissolved organic N did not show consistent day–night variation. However, the magnitude of diel variability was similar to that observed for dissolved inorganic N. Organic and inorganic N concentrations were often similar. Both appear to be dynamic components of stream N budgets. 5. The Intergovernmental Panel on Climate Change (IPCC) relies upon an emission factor to estimate indirect agricultural N₂O emissions from streams and ground water. The measured emission factor (defined as the ratio of concentrations of N₂O‐N to ‐N) was typically below the recently revised IPCC default figure. Measured values varied on average 1.8‐fold over approximately 24‐h periods and were slightly higher at night than by day. The emission factor was actually highest in streams that were net sinks for N₂O, highlighting a conceptual problem in the current IPCC method. 6. Typical sampling programmes rely on daytime‐only sampling, which might cause bias in results. In our study streams, the bias was generally small. Diel variation in nitrate concentrations was related to mean temperature; variation in ammonium and N₂O concentrations was greatest at low concentrations of nitrite and ammonium.     

Enhanced Thermostability of Arabidopsis Rubisco activase improves photosynthesis and growth rates under moderate heat stress

Plant photosynthesis declines when the temperature exceeds its optimum range. Recent evidence indicates that the reduction in photosynthesis is linked to ribulose-1,5-bis-phosphate carboxylase/oxygenase (Rubisco) deactivation due to the inhibition of Rubisco activase (RCA) under moderately elevated temperatures. To test the hypothesis that thermostable RCA can improve photosynthesis under elevated temperatures, we used gene shuffling technology to generate several Arabidopsis thaliana RCA1 (short isoform) variants exhibiting improved thermostability. Wild-type RCA1 and selected thermostable RCA1 variants were introduced into an Arabidopsis RCA deletion (Deltarca) line. In a long-term growth test at either constant 26 degrees C or daily 4-h 30 degrees C exposure, the transgenic lines with the thermostable RCA1 variants exhibited higher photosynthetic rates, improved development patterns, higher biomass, and increased seed yields compared with the lines expressing wild-type RCA1 and a slight improvement compared with untransformed Arabidopsis plants. These results provide clear evidence that RCA is a major limiting factor in plant photosynthesis under moderately elevated temperatures and a potential target for genetic manipulation to improve crop plants productivity under heat stress conditions.     

Fast and furious: Early differences in growth rate drive short‐term plant dominance and exclusion under eutrophication

1. The reduction of plant diversity following eutrophication threatens many ecosystems worldwide. Yet, the mechanisms by which species are lost following nutrient enrichment are still not completely understood, nor are the details of when such mechanisms act during the growing season, which hampers understanding and the development of mitigation strategies.
2. Using a common garden competition experiment, we found that early‐season differences in growth rates among five perennial grass species measured in monoculture predicted short‐term competitive dominance in pairwise combinations and that the proportion of variance explained was particularly greater under a fertilization treatment.
3. We also examined the role of early‐season growth rate in determining the outcome of competition along an experimental nutrient gradient in an alpine meadow. Early differences in growth rate between species predicted short‐term competitive dominance under both ambient and fertilized conditions and competitive exclusion under fertilized conditions.
4. The results of these two studies suggest that plant species growing faster during the early stage of the growing season gain a competitive advantage over species that initially grow more slowly, and that this advantage is magnified under fertilization. This finding is consistent with the theory of asymmetric competition for light in which fast‐growing species can intercept incident light and hence outcompete and exclude slower‐growing (and hence shorter) species. We predict that the current chronic nutrient inputs into many terrestrial ecosystems worldwide will reduce plant diversity and maintain a low biodiversity state by continuously favoring fast‐growing species. Biodiversity management strategies should focus on controlling nutrient inputs and reducing the growth of fast‐growing species early in the season.

     

Mutation of the rice TCM12 gene encoding 2,3‐bisphosphoglycerate‐independent phosphoglycerate mutase affects chlorophyll synthesis,photosynthesis and chloroplast development at seedling stage at low temperatures

• Glycolysis is a central metabolic pathway that provides energy and products of primary metabolites. 2,3‐Biphosphoglycerate‐independent phosphoglycerate mutase (iPGAM) is a key enzyme that catalyses the reversible interconversion of 3‐phosphoglycerate (3‐PGA) to 2‐phosphoglycerate (2‐PGA) in glycolysis. Low temperature is a common abiotic stress in rice production. However, the mechanism for rice iPGAM genes is not fully understood at low temperature.
• In this study, the rice mutant tcm12, with chlorosis, malformed chloroplasts and impaired photosynthesis, was grown at a low temperature (<20 °C) to the three‐leaf stage, while the normal phenotype at 32 °C was used. Chlorophyll fluorescence analysis and transmission electron microscopy were used to examine features of the tcm12 mutant. The inheritance behaviour and function of TCM12 were then analysed thorough map‐based cloning, transgenic complementation and subcellular localisation.
• The thermo‐sensitive chlorosis phenotype was caused by a single nucleotide mutation (T→C) on the fifth exon of TCM12 (LOC_Os12g35040) encoding iPGAM, localised to both nucleus and membranes. In addition, TCM12 was constitutively expressed, and its disruption resulted in down‐regulation of some genes associated with chlorophyll biosynthesis and photosynthesis at low temperatures (20 °C).
• This is the first report of the involvement of rice iPGAM gene in chlorophyll synthesis, photosynthesis and chloroplast development, providing new insights into the mechanisms underlying early growth of rice at low temperatures.

     

Inactivation of cytosolic FUMARASE2 enhances growth and photosynthesis under simultaneous copper and iron deprivation in Arabidopsis

Copper (Cu) and iron (Fe) are essential for plant growth and are often in short supply under natural conditions. Molecular responses to simultaneous lack of both metals (-Cu-Fe) differ from those seen in the absence of either alone. Metabolome profiling of plant leaves previously revealed that fumarate levels fall under -Cu-Fe conditions. We employed lines lacking cytosolic FUMARASE2 (FUM2) activity to study the impact of constitutive suppression of cytosolic fumarate synthesis on plant growth under Cu and/or Fe deficiency. In fum2 mutants, photosynthesis and growth were less impaired under -Cu-Fe conditions than in wild-type (WT) seedlings. In particular, levels of photosynthetic proteins, chloroplast ultrastructure, amino acid profiles and redox state were less perturbed by simultaneous Cu-Fe deficiency in lines that cannot produce fumarate in the cytosol. Although cytosolic fumarate has been reported to promote acclimation of photosynthesis to low temperatures when metal supplies are adequate, the photosynthetic efficiency of fum2 lines grown under Cu-Fe deficiency in the cold was higher than in WT. Uptake and contents of Cu and Fe are similar in WT and fum2 plants under control and -Cu-Fe conditions, and lack of FUM2 does not alter the ability to sense metal deficiency, as indicated by marker gene expression. Collectively, we propose that reduced levels of cytosolic fumarate synthesis ultimately increase the availability of Fe for incorporation into metalloproteins.     

Regulatory mechanisms after short‐ and long‐term perturbed lysine biosynthesis in the aspartate pathway: the need for isogenes in Arabidopsis thaliana

The aspartate‐derived amino acid pathway in plants is an intensively studied metabolic pathway, because of the biosynthesis of the four essential amino acids lysine, threonine, isoleucine and methionine. The pathway is mainly controlled by the key regulatory enzymes aspartate kinase (AK; EC 2.7.2.4), homoserine dehydrogenase (HSDH; EC 1.1.1.3) and 4‐hydroxy‐tetrahydrodipicolinate synthase (EC 4.3.3.7), formerly referred to as dihydrodipicolinate synthase (DHDPS). They are encoded by isoenzyme families and it is not known why such families are evolutionarily maintained. To gain more insight into the specific roles and regulation of the isoenzymes, we inhibited DHDPS in Arabidopsis thaliana with the chemical compound (N,N‐dimethylglycinatoboranyloxycarbonylmethyl)‐dimethylamine‐borane (DDAB) and compared the short‐term effects on the biochemical and biomolecular level to the long‐term adaptations in dhdps knockout mutants. We found that DHDPS2 plays a crucial role in controlling lysine biosynthesis, thereby stabilizing flux through the whole aspartate pathway. Moreover, DHDPS2 was also shown to influence the threonine level to a large extent. In addition, the lysine‐sensitive AKs, AKLYS1 and AKLYS3 control the short‐ and long‐term responses to perturbed lysine biosynthesis in Arabidopsis thaliana.     

Ozone effects on wheat in relation to CO2: modelling short‐term and long‐term responses of leaf photosynthesis and leaf duration

A combined stomatal–photosynthesis model was extended to simulate the effects of ozone exposure on leaf photosynthesis and leaf duration in relation to CO₂. We assume that ozone has a short‐term and a long‐term effect on the Rubisco‐limited rate of photosynthesis, A_c. Elevated CO₂ counteracts ozone damage via stomatal closure. Ozone is detoxified at uptake rates below a threshold value above which A_c decreases linearly with the rate of ozone uptake. Reduction in A_c is transient and depends on leaf age. Leaf duration decreases depending on accumulated ozone uptake. This approach is introduced into the mechanistic crop simulation model AFRCWHEAT2. The derived model, AFRCWHEAT2‐O3, is used to test the capability of these assumptions to explain responses at the plant and crop level. Simulations of short‐term and long‐term responses of leaf photosynthesis, leaf duration and plant and crop growth to ozone exposure in response to CO₂ are analysed and compared with experimental data derived from the literature. The model successfully reproduced published responses of leaf photosynthesis, leaf duration, radiation use efficiency and final biomass of wheat to elevated ozone and CO₂. However, simulations were unsatisfactory for cumulative radiation interception which had some impact on the accuracy of predictions of final biomass. There were responses of leaf‐area index to CO₂ and ozone as a result of effects on tillering which were not accounted for in the present model. We suggest that some model assumptions need to be tested, or analysed further to improve the mechanistic understanding of the combined effects of changes in ozone and CO₂ concentrations on leaf photosynthesis and senescence. We conclude that research is particularly needed to improve the understanding of leaf‐area dynamics in response to ozone exposure and elevated CO₂.     

Optimizing photosynthesis under fluctuating light: The role of the Arabidopsis STN7 kinase

Optimal photosynthetic performance requires that equal amounts of light are absorbed by photosystem ii (PSii) and photosystem i (PSi), which are functionally linked through the photosynthetic electron transport chain. However, photosynthetic organisms must cope with light conditions that lead to the preferential stimulation of one or the other of the photosystems. Plants react to such imbalances by mounting acclimation responses that redistribute excitation energy between photosystems and restore the photosynthetic redox poise. in the short term, this involves the so-called state transition process, which, over periods of minutes, alters the antennal crosssections of the photosystems through the reversible association of a mobile fraction of light-harvesting complex ii (LHCii) with PSi or PSii. Longer-lasting changes in light quality initiate a long-term response (LTr), occurring on a timescale of hours to days, that redresses imbalances in excitation energy by changing the relative amounts of the two photosystems. Despite the differences in their timescales of action, state transitions and LTr are both triggered by the redox state of the plastoquinone (PQ) pool, via the activation of the thylakoid kinase STN7, which appears to act as a common redox sensor and/or signal transducer for both responses. This review highlights recent findings concerning the role of STN7 in coordinating short- and long-term photosynthetic acclimation responses.Key words: state transitions, long-term acclimation, photosynthesis, STN7, Arabidopsis