Autophagy is required for tolerance of drought and salt stress in plants

Autophagy is a protein degradation process in which cells recycle cytoplasmic contents when subjected to environmental stress conditions or during certain stages of development. Upon the induction of autophagy, a double membrane autophagosome forms around cytoplasmic components and delivers them to the vacuole or lysosome for degradation. In plants, autophagy has been shown previously to be induced during abiotic stresses including nutrient starvation and oxidative stress. In this paper, we demonstrate the induction of autophagy in high salt and osmotic stress conditions, concomitant with the upregulation of expression of an Arabidopsis thaliana autophagy-related gene AtATG18a. Autophagy-defective RNAi-AtATG18a plants are more sensitive to salt and drought conditions than wild-type plants, demonstrating a role for autophagy in the response to these stresses. NADPH oxidase inhibitors block autophagy induction upon nutrient starvation and salt stress, but not during osmotic stress, indicating that autophagy can be activated by NADPH oxidase-dependent or -independent pathways. Together our results indicate that diverse environmental stresses can induce autophagy and that autophagy is regulated by distinct signaling pathways in different conditions.     

Evidence for a trade-off strategy in stone oak (Lithocarpus) seeds between physical and chemical defense highlights fiber as an important antifeedant

Trees in the beech or oak family (Fagaceae) have a mutualistic relationship with scatter-hoarding rodents. Rodents obtain nutrients and energy by consuming seeds, while providing seed dispersal for the tree by allowing some cached seeds to germinate. Seed predation and caching behavior of rodents is primarily affected by seed size, mechanical protection, macronutrient content, and chemical antifeedants. To enhance seed dispersal, trees must optimize trade-offs in investment between macronutrients and antifeedants. Here, we examine this important chemical balance in the seeds of tropical stone oak species with two substantially different fruit morphologies. These two distinct fruit morphologies in Lithocarpus differ in the degree of mechanical protection of the seed. For 'acorn' fruit, a thin exocarp forms a shell around the seed while for 'enclosed receptacle' (ER) fruit, the seed is embedded in a woody receptacle. We compared the chemical composition of numerous macronutrient and antifeedant in seeds from several Lithocarpus species, focusing on two pairs of sympatric species with different fruit morphologies. We found that macronutrients, particularly total non-structural carbohydrate, was more concentrated in seeds of ER fruits while antifeedants, primarily fibers, were more concentrated in seeds of acorn fruits. The trade-off in these two major chemical components was more evident between the two sympatric lowland species than between two highland species. Surprisingly, no significant difference in overall tannin concentrations in the seeds was observed between the two fruit morphologies. Instead, the major trade-off between macronutrients and antifeedants involved indigestible fibers. Future studies of this complex mutualism should carefully consider the role of indigestible fibers in the foraging behavior of scatter-hoarding rodents.     

A trade-off between nitrogen uptake and use increases responsiveness to elevated CO2 in infrequently cut mixed C3 grasses

The aim of this study was to evaluate whether the responsiveness of mixed C3 grass species to elevated CO2 is related more to nitrogen uptake or to N-use efficiency. Nitrogen uptake and whole-plant N-use efficiency were investigated with two binary mixtures: Lolium perenne was mixed either with Festuca arundinacea or with Holcus lanatus. The swards were grown on sand with or without CO2 doubling, and subjected to two cutting frequencies. A C20 alcohol was used as a marker to determine species proportion in the total root mass of the mixtures. The mean residence time of N was calculated from that of 15N-labelled fertilizer. Lolium perenne took up significantly more N per unit root mass than its grass competitors, but its N-use efficiency was lower. Elevated CO2 significantly reduced the N uptake of the three grass species. A trade-off between N capture and use was found, as N-use efficiency and N-uptake rate were negatively correlated. A high N-use efficiency, and conversely low N uptake appeared to favour the responsiveness to elevated CO2 of the infrequently cut grasses.     

Direct evidence that symbiotic N2 fixation in fertile grassland is an important trait for a strong response of plants to elevated atmospheric CO2

Although legumes showed a clearly superior yield response to elevated atmospheric pCO₂ compared to nonlegumes in a variety of field experiments, the extent to which this is due to symbiotic N₂ fixation per se has yet to be determined. Thus, effectively and ineffectively nodulating lucerne (Medicago sativa L.) plants with a very similar genetic background were grown in competition with each other on fertile soil in the Swiss FACE experiment in order to monitor their CO₂ response. Under elevated atmospheric pCO₂, effectively nodulating lucerne, thus capable of symbiotically fixing N₂, strongly increased the harvestable biomass and the N yield, independent of N fertilization. In contrast, the harvestable biomass and N yield of ineffectively nodulating plants were affected negatively by elevated atmospheric pCO₂ when N fertilization was low. Large amounts of N fertilizer enabled the plants to respond more favourably to elevated atmospheric pCO₂, although not as strongly as effectively nodulating plants. The CO₂‐induced increase in N yield of the effectively nodulating plants was attributed solely to an increase in symbiotic N₂ fixation of 50–175%, depending on the N fertilization treatment. N yield derived from the uptake of mineral N from the soil was, however, not affected by elevated pCO₂. This result demonstrates that, in fertile soil and under temperate climatic conditions, symbiotic N₂ fixation per se is responsible for the considerably greater amount of above‐ground biomass and the higher N yield under elevated atmospheric pCO₂. This supports the assumption that symbiotic N₂ fixation plays a key role in maintaining the C/N balance in terrestrial ecosystems in a CO₂‐rich world.     

Co-expression of genes ApGSMT2 and ApDMT2 for glycinebetaine synthesis in maize enhances the drought tolerance of plants

Glycinebetaine plays an important role in the protection mechanism of many plants under various stress conditions. In this study, genetically engineered maize plants with an enhanced ability to synthesise glycinebetaine (GB) were produced by introducing two genes, glycine sarcosine methyltransferase gene (ApGSMT2) and dimethylglycine methyltransferase gene (ApDMT2), from the bacterium Aphanothece halophytica. Southern blotting and RT-PCR analysis demonstrated that the two genes were integrated into the maize genome and expressed. The increased expression levels of ApGSMT2 and ApDMT2 under drought conditions facilitated GB accumulation in the leaves of transgenic maize plants and conferred improved drought tolerance. Under drought conditions, the transgenic plants showed an increased accumulation of sugars and free amino acids, greater chlorophyll content, a higher photosynthesis rate and biomass, and lower malondialdehyde and electrolyte leakage compared to the wild-type; these results suggest that GB provides vital protection against drought stress. Under normal conditions, the transgenic plants did not show decreased biomass and productivity, which indicated that the co-expression of ApGSMT2 and ApDMT2 in maize plays an important role in its tolerance to drought stress and does not lead to detrimental effects. It was concluded that the co-expression of ApGSMT2 and ApDMT2 in maize is an effective approach to enhancing abiotic stress tolerance in maize breeding programmes.     

The translation initiation factor eIF1A is an important determinant in the tolerance to NaCl stress in yeast and plants

Protein synthesis is very sensitive to NaCl. However, the molecular targets responsible for this sensitivity have not been described. A cDNA library of the halotolerant plant sugar beet was functionally screened in a sodium-sensitive yeast strain. We obtained a cDNA clone (BveIF1A) encoding the eukaryotic translation initiation factor eIF1A. BveIF1A was able to partially complement the yeast eIF1A-deficient strain. Overexpression of the sugar beet eIF1A specifically increased the sodium and lithium salt tolerance of yeast. This phenotype was not accompanied by changes in sodium or potassium homeostasis. Under salt stress conditions, yeast cells expressing BveIF1A presented a higher rate of amino acid incorporation into proteins than control cells. In an in vitro protein synthesis system from wheat germ, the BveIF1A recombinant protein improved translation in the presence of NaCl. Finally, transgenic Arabidopsis plants expressing BveIF1A exhibited increased tolerance to NaCl. These results suggest that the translation initiation factor eIF1A is an important determinant of sodium tolerance in yeast and plants.     

Effect of different leaf age on the relationship between the CO2 uptake and water vapour efflux in tobacco plants

CO₂ absorption (PAT) and transpiration (E) rates, and leaf diffusion resistance (r_i) were individually studied in all leaves of tobacco plants (Nicotiana tabacum L. cv. Wisconsin 38) before flowering. Differences between old, middle age and young leaves were in all characteristics studied and found statistically significant. In all three leaf age groups E was closely correlated to r_i. No similar correlation was discovered between P_N and r_i. The highest ratiosP _N /E in young and middle age leaves indicate that the increase of the internal resistance to photosynthesis with leaf age was more rapid than that of r_i.     

Biomass allocation is an important determinant of the tannin concentration in growing plants

BACKGROUND AND AIMS: Condensed tannins (CTs) in the diet affect consumers in a concentration-dependent manner. Because of their importance in plant defence against herbivores and pathogens as well as their potential application against gastrointestinal parasites of ruminants in agronomy, an understanding of the seasonal dynamics of CT concentrations during plant growth is essential. METHODS: Over a vegetation period, CT concentrations in leaves, stems and roots and the biomass proportions between these organs were investigated in Onobrychis viciifolia, Lotus corniculatus and Cichorium intybus. Based on the experimental data, a model has been suggested to predict CT concentrations in harvestable biomass of these species. KEY RESULTS: During the experiment, leaf mass fractions of plants decreased from 85, 64, 85 to 30, 18, 39 % d. wt in Onobrychis, Lotus and Cichorium, respectively, and proportions of stems and roots increased accordingly. While CT concentrations almost doubled in leaves in Onobrychis (from 52 to 86 mg g(-1) d. wt, P<0.001) and Lotus (from 25 to 54 mg g(-1) d. wt, P<0.001), they were stable at low levels in expanding leaves of Cichorium (5 mg g(-1) d. wt) and in stems and roots of all investigated species. Due to an inverse effect of the increasing CT concentrations in leaves and simultaneous dilution from increasing proportions of 'CT-poor' stems, CT concentrations in harvestable biomass were stable over time in all investigated species: 62, 26 and 5 mg g(-1) d. wt for Onobrychis, Lotus and Cichorium, respectively. CONCLUSIONS: As a consequence of the unequal distribution of tannins in different plant parts and due to the changing biomass proportions between them, various herbivores (e.g. a leaf-eating insect and a grazing ruminant) may find not only different concentrations of CT in their diets but also different CT dynamics during the season. For the prediction of seasonal variations of CT concentrations, biomass allocation and accumulation of none-CT plant material are likely to be as important predictors as the knowledge of CT synthesis and its regulation.     

A precise spacing between the NPA domains of aquaporins is essential for silicon permeability in plants

The controversy surrounding silicon (Si) benefits and essentiality in plants is exacerbated by the differential ability of species to absorb this element. This property is seemingly enhanced in species carrying specific nodulin 26‐like intrinsic proteins (NIPs), a subclass of aquaporins. In this work, our aim was to characterize plant aquaporins to define the features that confer Si permeability. Through comparative analysis of 985 aquaporins in 25 species with differing abilities to absorb Si, we were able to predict 30 Si transporters and discovered that Si absorption is exclusively confined to species that possess NIP‐III aquaporins with a GSGR selectivity filter and a precise distance of 108 amino acids (AA) between the asparagine–proline–alanine (NPA) domains. The latter feature is of particular significance since it had never been reported to be essential for Si selectivity. Functionality assessed in the Xenopus oocyte expression system showed that NIPs with 108 AA spacing exhibited Si permeability, while proteins differing in that distance did not. In subsequent functional studies, a Si transporter from poplar mutated into variants with 109‐ or 107‐AA spacing failed to import, and a tomato NIP gene mutated from 109 to 108 AA exhibited a rare gain of function. These results provide a precise molecular basis to classify higher plants into Si accumulators or excluders.     

Chloroplastic ATP synthase optimizes the trade-off between photosynthetic CO2 assimilation and photoprotection during leaf maturation

In the present study, we studied the role of chloroplastic ATP synthase in photosynthetic regulation during leaf maturation. We measured gas exchange, chlorophyll fluorescence, P700 redox state, and the electrochromic shift signal in mature and immature leaves. Under high light, the immature leaves displayed high levels of non-photochemical quenching (NPQ) and P700 oxidation ratio, and higher values for proton motive force (pmf) and proton gradient (ΔpH) across the thylakoid membranes but lower values for the activity of chloroplastic ATP synthase (g_H⁺) than the mature leaves. Furthermore, g_H⁺ was significantly and positively correlated with CO₂ assimilation rate and linear electron flow (LEF), but negatively correlated with pmf and ΔpH. ΔpH was significantly correlated with LEF and the P700 oxidation ratio. These results indicated that g_H⁺ was regulated to match photosynthetic capacity during leaf maturation, and the formation of pmf and ΔpH was predominantly regulated by the alterations in g_H⁺. In the immature leaves, the high steady-state ΔpH increased lumen acidification, which, in turn, stimulated photoprotection for the photosynthetic apparatus via NPQ induction and photosynthetic control. Our results highlighted the importance of chloroplastic ATP synthase in optimizing the trade-off between CO₂ assimilation and photoprotection during leaf maturation.     

Phosphorus application and elevated CO2 enhance drought tolerance in field pea grown in a phosphorus-deficient vertisol

Background and Aims Benefits to crop productivity arising from increasing CO₂ fertilization may be offset by detrimental effects of global climate change, such as an increasing frequency of drought. Phosphorus (P) nutrition plays an important role in crop responses to water stress, but how elevated CO₂ (eCO₂) and P nutrition interact, especially in legumes, is unclear. This study aimed to elucidate whether P supply improves plant drought tolerance under eCO₂.Methods A soil-column experiment was conducted in a free air CO₂ enrichment (SoilFACE) system. Field pea (Pisum sativum) was grown in a P-deficient vertisol, supplied with 15 mg P kg⁻¹ (deficient) or 60 mg P kg⁻¹ (adequate for crop growth) and exposed to ambient CO₂ (aCO₂; 380–400 ppm) or eCO₂ (550–580 ppm). Drought treatments commenced at flowering. Measurements were taken of soil and leaf water content, photosynthesis, stomatal conductance, total soluble sugars and inorganic P content (Pi).Key Results Water-use efficiency was greatest under eCO₂ when the plants were supplied with adequate P compared with other treatments irrespective of drought treatment. Elevated CO₂ decreased stomatal conductance and transpiration rate, and increased the concentration of soluble sugars and relative water contents in leaves. Adequate P supply increased concentrations of soluble sugars and Pi in drought-stressed plants. Adequate P supply but not eCO₂ increased root length distribution in deeper soil layers.Conclusions Phosphorus application and eCO₂ interactively enhanced periodic drought tolerance in field pea as a result of decreased stomatal conductance, deeper rooting and high Pi availability for carbon assimilation in leaves.     

Respiratory oxygen uptake is not decreased by an instantaneous elevation of [CO2], but is increased with long-term growth in the field at elevated [CO2

Averaged across many previous investigations, doubling the CO2 concentration ([CO2]) has frequently been reported to cause an instantaneous reduction of leaf dark respiration measured as CO2 efflux. No known mechanism accounts for this effect, and four recent studies have shown that the measurement of respiratory CO2 efflux is prone to experimental artifacts that could account for the reported response. Here, these artifacts are avoided by use of a high-resolution dual channel oxygen analyzer within an open gas exchange system to measure respiratory O2 uptake in normal air. Leaf O2 uptake was determined in response to instantaneous elevation of [CO2] in nine contrasting species and to long-term elevation in seven species from four field experiments. Over six hundred separate measurements of respiration failed to reveal any decrease in respiratory O2 uptake with an instantaneous increase in [CO2]. Respiration was found insensitive not only to doubling [CO2], but also to a 5-fold increase and to decrease to zero. Using a wide range of species and conditions, we confirm earlier reports that inhibition of respiration by instantaneous elevation of [CO2] is likely an experimental artifact. Instead of the expected decrease in respiration per unit leaf area in response to long-term growth in the field at elevated [CO2], there was a significant increase of 11% and 7% on an area and mass basis, respectively, averaged across all experiments. The findings suggest that leaf dark respiration will increase not decrease as atmospheric [CO2] rises.