Remodeling of chloroplast proteome under salinity affects salt tolerance of Festuca arundinacea

Identification of a Saltol-Independent Salinity Tolerance Polymorphism in Rice Mekong Delta Landraces and Characterization of a Promising Line, Doc Phung

The Mekong Delta River in Vietnam is facing salinity intrusion caused by climate change and sea-level rise that is severely affecting rice cultivation. Here, we evaluated salinity responses of 97 rice accessions (79 landraces and 18 improved accessions) from the Mekong Delta population by adding 100 mM NaCl to the nutrient solution for up to 20 days. We observed a wide distribution in salinity tolerance/sensitivity, with two major peaks across the 97 accessions when using the standard evaluation system (SES) developed by the International Rice Research Institute. SES scores revealed strong negative correlations (ranging from - 0.68 to - 0.83) with other phenotypic indices, such as shoot elongation length, root elongation length, shoot dry weight, and root dry weight. Mineral concentrations of Na⁺ in roots, stems, and leaves and Ca²⁺ in roots and stems were positively correlated with SES scores, suggesting that tolerant accessions lower their cation exchange capacity in the root cell wall. The salinity tolerance of Mekong Delta accessions was independent from the previously described salinity tolerance-related locus Saltol, which encodes an HKT1-type transporter in the salinity-tolerant cultivars Nona Bokra and Pokkali. Indeed, genome-wide association studies using SES scores and shoot dry weight ratios of the 79 accessions as traits identified a single common peak located on chromosome 1. This SNP did not form a linkage group with other nearby SNPs and mapped to the 3' untranslated region of gene LOC_Os01g32830, over 6.5 Mb away from the Saltol locus. LOC_Os01g32830 encodes chloroplast glycolate/glycerate translocator 1 (OsPLGG1), which is responsible for photorespiration and growth. SES and shoot dry weight ratios differed significantly between the two possible haplotypes at the causal SNP. Through these analyses, we characterize Doc Phung, one of the most salinity-tolerant varieties in the Mekong Delta population and a promising new genetic resource.

Regulation of photosynthesis under salt stress and associated tolerance mechanisms

Photosynthesis is crucial for the survival of all living biota, playing a key role in plant productivity by generating the carbon skeleton that is the primary component of all biomolecules. Salinity stress is a major threat to agricultural productivity and sustainability as it can cause irreversible damage to photosynthetic apparatus at any developmental stage. However, the capacity of plants to become photosynthetically active under adverse saline conditions remains largely untapped. This study addresses this discrepancy by exploring the current knowledge on the impact of salinity on chloroplast operation, metabolism, chloroplast ultrastructure, and leaf anatomy, and highlights the dire consequences for photosynthetic machinery and stomatal conductance. We also discuss enhancing photosynthetic capacity by modifying and redistributing electron transport between photosystems and improving photosystem stability using genetic approaches, beneficial microbial inoculations, and root architecture changes to improve salt stress tolerance under field conditions. Understanding chloroplast operations and molecular engineering of photosynthetic genes under salinity stress will pave the way for developing salt-tolerant germplasm to ensure future sustainability by rehabilitating saline areas.

Influences of elevated O3 and CO2 on Cd distribution in different Festuca arundinacea tissues

It is necessary to reveal the responses of the biomass production and metal accumulation capacity of different plants to the variations of atmospheric conditions and soil metals, with the acceleration of urbanization and industrialization. In the present study, a series of experiments were designed to study the individual and interactive influences of O₃ and CO₂ fumigation on the biomass yield, variation in different leaf types, distribution of cadmium (Cd) in various tissues, and phytoremediation efficiency of Festuca arundinacea using open top chambers. The results found that an elevated O₃ content of 80 ppb, a potential O₃ content predicted for 2050, decreased the total dry mass of F. arundinacea and increased the proportion of falling leaf tissues of the species significantly. Under the same ambient CO₂ levels, O₃ fumigation increased the Cd concentrations in the roots and the fresh, mature, senescent, and dead leaf tissues by 27.8%, 133.3%, 94.4%, 125.3%, and 48.6%, respectively. An elevated CO₂ content (550 ppm) promoted the biomass yield of F. arundinacea, particularly in the falling leaf tissues. The results of the combined O₃ and CO₂ treatment showed that CO₂ fumigation alleviated the negative effects of O₃ on plant growth and increased the accumulation capacity in different plant tissues. Significantly more Cd was accumulated in senescent and dead leaves under the synergistic action of CO₂ and O₃, suggesting that the phytoremediation effect on F. arundinacea using the falling leaves harvesting method could be improved under the future atmospheric environment of high CO₂ and O₃ levels.

Stability of thylakoid protein complexes and preserving photosynthetic efficiency are crucial for the successful recovery of the halophyte Cakile maritima from high salinity

Plants native to extreme habitats often face changes in environmental conditions such as salinity level and water availability. In response, plants have evolved efficient mechanisms allowing them to survive or recover. In the present work, effects of high salinity and salt-stress release were studied on the halophyte Cakile maritima. Four week-old plants were either cultivated at 0 mM NaCl or 200 mM NaCl. After one month of treatment, plants were further irrigated at either 0 mM NaCl, 200 mM NaCl, or rewatered to 0 mM NaCl (stress release). Upon salt stress, C. maritima plants exhibited reduced biomass production and shoot hydration which were associated with a decrease in the amount of chlorophyll a and b. However, under the same stressful conditions a significant increase of anthocyanin and malonyldialdehyde concentrations was noticed. Salt-stressed plants were able to maintain stable protein complexes of thylakoid membranes. Measurement of chlorophyll fluorescence and P700 redox state showed that PSI was more susceptible for damage by salinity than PSII. PSII machinery was significantly enhanced under saline conditions. All measured parameters were partially restored under salt-stress release conditions. Photoinhibition of PSI was also reversible and C. maritima was able to successfully re-establish PSI machinery indicating the high contribution of chloroplasts in salt tolerance mechanisms of C. maritima. Overall, to overcome high salinity stress, C. maritima sets a cascade of physio-biochemical and molecular pathways. Chloroplasts seem to act as metabolic centers as part of this adaptive process enabling growth restoration in this halophyte following salt stress release.

Ectopic expression of human apolipoprotein D in Arabidopsis plants lacking chloroplastic lipocalin partially rescues sensitivity to drought and oxidative stress

The chloroplastic lipocalin (LCNP) is induced in response to various abiotic stresses including high light, dehydration and low temperature. It contributes to protection against oxidative damage promoted by adverse conditions by preventing accumulation of fatty acid hydroperoxides and lipid peroxidation. In contrast to animal lipocalins, LCNP is poorly characterized and the molecular mechanism by which it exerts protective effects during oxidative stress is largely unknown. LCNP is considered the ortholog of human apolipoprotein D (APOD), a protein whose lipid antioxidant function has been characterized. Here, we investigated whether APOD could functionally replace LCNP in Arabidopsis thaliana. We introduced APOD cDNA fused to a chloroplast transit peptide encoding sequence in an Arabidopsis LCNP KO mutant line and challenged the transgenic plants with different abiotic stresses. We demonstrated that expression of human APOD in Arabidopsis can partially compensate for the lack of the plastid lipocalin. The results are consistent with a conserved function of APOD and LCNP under stressful conditions. However, if the results obtained with the drought and oxidative stresses point to the protective effect of constitutive expression of APOD in plants lacking LCNP, this effect is not as effective as that conferred by LCNP overexpression. Moreover, when investigating APOD function in thylakoids after high light stress at low temperature, it appeared that APOD could not contribute to qH, a slowly reversible form of non-photochemical chlorophyll fluorescence quenching, as described for LCNP. This work provides a base of understanding the molecular mechanism underlying LCNP protective function.

Salt induced programmed cell death in rice: evidence from chloroplast proteome signature

Soil salinity, depending on its intensity, drives a challenged plant either to death, or survival with compromised productivity. On exposure to moderate salinity, plants can often survive by sacrificing some of their cells 'in target' following a route called programmed cell death (PCD). In animals, PCD has been well characterised, and involvement of mitochondria in the execution of PCD events has been unequivocally proven. In plants, mechanistic details of the process are still in grey area. Previously, we have shown that in green tissues of rice, for salt induced PCD to occur, the presence of active chloroplasts and light are equally important. In the present work, we have characterised the chloroplast proteome in rice seedlings at 12 and 24 h after salt exposure and before the time point where the signature of PCD was observed. We identified almost 100 proteins from chloroplasts, which were divided in to 11 categories based on the biological functions in which they were involved. Our results concerning the differential expression of chloroplastic proteins revealed involvement of some novel candidates. Moreover, we observed maximum phosphorylation pattern of chloroplastic proteins at an early time point (12 h) of salt exposure.

Transcriptional reprogramming and enhanced photosynthesis drive inducible salt tolerance in sugarcane mutant line M4209

Sugarcane (Saccharum officinarum) is a globally cultivated cash crop whose yield is negatively affected by soil salinity. In this study, we investigated the molecular basis of inducible salt tolerance in M4209, a sugarcane mutant line generated through radiation-induced mutagenesis. Under salt-contaminated field conditions, M4209 exhibited 32% higher cane yield as compared with its salt-sensitive parent, Co86032. In pot experiments, post-sprouting phenotyping indicated that M4209 had significantly greater leaf biomass compared with Co86032 under treatment with 50 mM and 200 mM NaCl. This was concomitant with M4209 having 1.9-fold and 1.6-fold higher K+/Na+ ratios, and 4-fold and 40-fold higher glutathione reductase activities in 50 mM and 200 mM NaCl, respectively, which suggested that it had better ionic and redox homeostasis than Co86032. Transcriptome profiling using RNA-seq indicated an extensive reprograming of stress-responsive modules associated with photosynthesis, transmembrane transport, and metabolic processes in M4209 under 50 mM NaCl stress. Using ranking analysis, we identified Phenylalanine Ammonia Lyase (PAL), Acyl-Transferase Like (ATL), and Salt-Activated Transcriptional Activator (SATA) as the genes most associated with salt tolerance in M4209. M4209 also exhibited photosynthetic rates that were 3-4-fold higher than those of Co86032 under NaCl stress conditions. Our results highlight the significance of transcriptional reprogramming coupled with improved photosynthetic efficiency in determining salt tolerance in sugarcane.

Two Festuca Species-F. arundinacea and F. glaucescens-Differ in the Molecular Response to Drought, While Their Physiological Response Is Similar

Impact of photosynthetic and antioxidant capacities on drought tolerance of two closely related forage grasses, Festuca arundinacea and Festuca glaucescens, was deciphered. Within each species, two genotypes distinct in drought tolerance were subjected to a short-term drought, followed by a subsequent re-watering. The studies were focused on: (i) analysis of plant physiological performance, including: water uptake, abscisic acid (ABA) content, membrane integrity, gas exchange, and relative water content in leaf tissue; (ii) analysis of plant photosynthetic capacity (chlorophyll fluorescence; gene expression, protein accumulation, and activity of selected enzymes of the Calvin cycle); and (iii) analysis of plant antioxidant capacity (reactive oxygen species (ROS) generation; gene expression, protein accumulation and activity of selected enzymes). Though, F. arundinacea and F. glaucescens revealed different strategies in water uptake, and partially also in ABA signaling, their physiological reactions to drought and further re-watering, were similar. On the other hand, performance of the Calvin cycle and antioxidant system differed between the analyzed species under drought and re-watering periods. A stable efficiency of the Calvin cycle in F. arundinacea was crucial to maintain a balanced network of ROS/redox signaling, and consequently drought tolerance. The antioxidant capacity influenced mostly tolerance to stress in F. glaucescens.

Photochemistry and proteomics of mulberry (Morus alba L.) seedlings under NaCl and NaHCO3 stress

In order to explore the response and adaptation mechanisms of photosynthesis of the leaves of mulberry (Morus alba L.) seedlings to saline-alkali stress. Photosynthetic activity, and the response of related proteomics of M. alba seedling leaves under NaCl and NaHCO₃ stress were studied by using chlorophyll fluorescence and gas exchange technique combined with TMT proteomics. The results showed that NaCl stress had no significant effect on photosystem II (PSII) activity in M. alba seedling leaves. In addition, the expressions of proteins of the PSII oxygen-evolving complex (OEE3-1 and PPD4) and the LHCII antenna (CP24 10A, CP26, and CP29) were increased, and the photosystem I (PSI) activity in the leaves of M. alba seedlings was increased, as well as expressions of proteins, such as PsaF, PsaG, PsaH, PsaL, PsaN, and Ycf4. Under NaHCO₃ stress, the activity of PSII and PSI and the expression of their protein complexes and the electron transfer-related proteins significantly decreased. NaCl stress had little effect on RuBP regeneration during dark reaction in the leaves and the expressions of glucose synthesis related proteins and net photosynthetic rate (P_n) did not decrease significantly. The leaves could adapt to NaCl stress by reducing stomatal conductance (G_s) and increasing water use efficiency (WUE). Under NaHCO₃ stress, the expression of dark reaction-related proteins was mostly down-regulated, while G_s was reduced, which indicated that non-stomatal factors can be responsible for inhibition of carbon assimilation.

Plant Aquaporins in Infection by and Immunity Against Pathogens - A Critical Review

Plant aquaporins (AQPs) of the plasma membrane intrinsic protein (PIP) family face constant risk of hijack by pathogens aiming to infect plants. PIPs can also be involved in plant immunity against infection. This review will utilize two case studies to discuss biochemical and structural mechanisms that govern the functions of PIPs in the regulation of plant infection and immunity. The first example concerns the interaction between rice Oryza sativa and the bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo). To infect rice, Xoo uses the type III (T3) secretion system to secrete the proteic translocator Hpa1, and Hpa1 subsequently mediates the translocation of T3 effectors secreted by this system. Once shifted from bacteria into rice cells, effectors exert virulent or avirulent effects depending on the susceptibility of the rice varieties. The translocator function of Hpa1 requires cooperation with OsPIP1;3, the rice interactor of Hpa1. This role of OsPIP1;3 is related to regulatory models of effector translocation. The regulatory models have been proposed as, translocon-dependent delivery, translocon-independent pore formation, and effector endocytosis with membrane protein/lipid trafficking. The second case study includes the interaction of Hpa1 with the H2O2 transport channel AtPIP1;4, and the associated consequence for H2O2 signal transduction of immunity pathways in Arabidopsis thaliana, a non-host of Xoo. H2O2 is generated in the apoplast upon induction by a pathogen or microbial pattern. H2O2 from this source translocates quickly into Arabidopsis cells, where it interacts with pathways of intracellular immunity to confer plant resistance against diseases. To expedite H2O2 transport, AtPIP1;4 must adopt a specific conformation in a number of ways, including channel width extension through amino acid interactions and selectivity for H2O2 through amino acid protonation and tautomeric reactions. Both topics will reference relevant studies, conducted on other organisms and AQPs, to highlight possible mechanisms of T3 effector translocation currently under debate, and highlight the structural basis of AtPIP1;4 in H2O2 transport facilitated by gating and trafficking regulation.