Control of Plant Responses to Salt Stress: Significance of Auxin and Brassinosteroids

Salinity of soils represents a significant abiotic stress factor that not only reduces productivity of most crops but also poses a threat to the global food security. Understanding the mechanisms underpinning plant stress responses as a whole is essential for enhancing crop productivity in salt-affected soils. To improve crop production on salt-affected lands, it is crucial to have a comprehensive understanding of the mechanisms underlying plant stress responses. Phytohormones are key players in these processes, regulating plant growth, development and germination. Among phytohormones, auxin and brassinosteroids (BRs) have been found to overlap to lessen salt stress in plants. In order to help plants balance growth and salt stress tolerance, auxin, BRs, and their interactions are currently known to play a number of important roles. This chapter gives a summary of these findings and discusses how molecular and genetic approaches can be used to engineer auxin, BRs, and thereby develop more salt-resistant cereal crops in the future

Genome wide identification of wheat and Brachypodium type one protein phosphatases and functional characterization of durum wheat TdPP1a

Reversible phosphorylation is an essential mechanism regulating signal transduction during development and environmental stress responses. An important number of dephosphorylation events in the cell are catalyzed by type one protein phosphatases (PP1), which catalytic activity is driven by the binding of regulatory proteins that control their substrate specificity or subcellular localization. Plants harbor several PP1 isoforms accounting for large functional redundancies. While animal PP1s were reported to play relevant roles in controlling multiple cellular processes, plant orthologs remain poorly studied. To decipher the role of plant PP1s, we compared PP1 genes from three monocot species, Brachypodium, common wheat and rice at the genomic and transcriptomic levels. To gain more insight into the wheat PP1 proteins, we identified and characterized TdPP1a, the first wheat type one protein phosphatase from a Tunisian durum wheat variety Oum Rabiaa3. TdPP1a is highly conserved in sequence and structure when compared to mammalian, yeast and other plant PP1s. We demonstrate that TdPP1a is an active, metallo-dependent phosphatase in vitro and is able to interact with AtI2, a typical regulator of PP1 functions. Also, TdPP1a is capable to complement the heat stress sensitivity of the yeast mutant indicating that TdPP1a is functional also in vivo. Moreover, transient expression of TdPP1a::GFP in tobacco leaves revealed that it is ubiquitously distributed within the cell, with a strong accumulation in the nucleus. Finally, transcriptional analyses showed similar expression levels in roots and leaves of durum wheat seedlings. Interestingly, the expression in leaves is significantly induced following salinity stress, suggesting a potential role of TdPP1a in wheat salt stress response

Plant genome modification by homologous recombination

The mechanisms and frequencies of various types of homologous recombination (HR) have been studied in plants for several years. However, the application of techniques involving HR for precise genome modification is still not routine. The low frequency of HR remains the major obstacle but recent progress in gene targeting in Arabidopsis and rice, as well as accumulating knowledge on the regulation of recombination levels, is an encouraging sign of the further development of HR-based approaches for genome engineering in plants

New Insights on plant salt tolerance mechanisms and their potential use for breeding

Soil salinization is a major threat to agriculture in arid and semi-arid regions, where water scarcity and inadequate drainage of irrigated lands severely reduce crop yield. Salt accumulation inhibits plant growth and reduces the ability to uptake water and nutrients, leading to osmotic or water-deficit stress. Salt is also causing injury of the young photosynthetic leaves and acceleration of their senescence, as the Na+ cation is toxic when accumulating in cell cytosol resulting in ionic imbalance and toxicity of transpiring leaves. To cope with salt stress, plants have evolved mainly two types of tolerance mechanisms based on either limiting the entry of salt by the roots, or controlling its concentration and distribution. Understanding the overall control of Na+ accumulation and functional studies of genes involved in transport processes, will provide a new opportunity to improve the salinity tolerance of plants relevant to food security in arid regions. A better understanding of these tolerance mechanisms can be used to breed crops with improved yield performance under salinity stress. Moreover, associations of cultures with nitrogen-fixing bactéria and arbuscular mycorrhizal fungi could serve as an alternative and sustainable strategy to increase crop yields in salt affected fields

The secretion of the bacterial phytase PHY -US417 by Arabidopsis roots reveals its potential for increasing phosphate acquisition and biomass production during cogrowth

International audiencePhytic acid (PA) is a major source of inorganic phosphate (Pi) in the soil, however the plant lacks the capacity to utilize it for Pi nutrition and growth. Microbial phytases constitute a group of enzymes that are able to remobilize Pi from PA. Thus, the use of these phytases to increase the capacity of higher plants to remobilize Pi from PA is of agronomical interest. In the current study, we generate transgenic Arabidopsis lines (ePHY) overexpressing an extracellular form of the phytase PHY-US417 of Bacillus subtilis, which are characterized by high levels of secreted phytase activity. In presence of PA as sole source of Pi, while the wild-type plants shows hallmark of Pi deficiency phenotypes, including the induction of the expression of Pi starvation induced genes (PSI, e.g PHT1;4) and the inhibition of growth capacity, the ePHYover-expressing lines show a higher biomass production and no PSI induction. Interestingly, when co-cultived with ePHY over-expressors, wild type Arabidopsis plants (or tobacco) show repression of the PSI genes, improvement of Pi content and increases in biomass production. In line with these results, mutants in the high affinity Pi transporters, namely pht1;1 and pht1;1-1;4, both fail to accumulate Pi and to grow when co-cultured with ePHY overexpressors. Taken together, these data demonstrate the potential of secreted phytases in improving the Pi content and enhancing growth of not only the transgenic lines but also the neighbouring plants

Phytase overexpression in Arabidopsis improves plant growth under osmotic stress and in combination with phosphate deficiency

Engineering osmotolerant plants is a challenge for modern agriculture. An interaction between osmotic stress response and phosphate homeostasis has been reported in plants, but the identity of molecules involved in this interaction remains unknown. In this study we assessed the role of phytic acid (PA) in response to osmotic stress and/or phosphate deficiency in Arabidopsis thaliana. For this purpose, we used Arabidopsis lines (L7 and L9) expressing a bacterial beta-propeller phytase PHY-US417, and a mutant in inositol polyphosphate kinase 1 gene (ipk1-1), which were characterized by low PA content, 40% (L7 and L9) and 83% (ipk1-1) of the wild-type (WT) plants level. We show that the PHYoverexpressor lines have higher osmotolerance and lower sensitivity to abscisic acid than ipk1-1 and WT. Furthermore, PHY-overexpressors showed an increase by more than 50% in foliar ascorbic acid levels and antioxidant enzyme activities compared to ipk1-1 and WT plants. Finally, PHY-overexpressors are more tolerant to combined mannitol stresses and phosphate deficiency than WT plants. Overall, our results demonstrate that the modulation of PA improves plant growth under osmotic stress, likely via stimulation of enzymatic and non-enzymatic antioxidant systems, and that beside its regulatory role in phosphate homeostasis, PA may be also involved in fine tuning osmotic stress response in plants

New Insights on Plant Salt Tolerance Mechanisms and Their Potential Use for Breeding

International audienceSoil salinization is a major threat to agriculture in arid and semi-arid regions, where water scarcity and inadequate drainage of irrigated lands severely reduce crop yield. Salt accumulation inhibits plant growth and reduces the ability to uptake water and nutrients, leading to osmotic or water-deficit stress. Salt is also causing injury of the young photosynthetic leaves and acceleration of their senescence, as the Na + cation is toxic when accumulating in cell cytosol resulting in ionic imbalance and toxicity of transpiring leaves. To cope with salt stress, plants have evolved mainly two types of tolerance mechanisms based on either limiting the entry of salt by the roots, or controlling its concentration and distribution. Understanding the overall control of Na + accumulation and functional studies of genes involved in transport processes, will provide a new opportunity to improve the salinity tolerance of plants relevant to food security in arid regions. A better understanding of these tolerance mechanisms can be used to breed crops with improved yield performance under salinity stress. Moreover, associations of cultures with nitrogen-fixing bacteria and arbuscular mycorrhizal fungi could serve as an alternative and sustainable strategy to increase crop yields in salt-affected fields

Functional characterization of DHN-5, a dehydrin showing a differential phosphorylation pattern in two Tunisian durum wheat (Triticum durum Desf.) varieties with marked differences in salt and drought tolerance

Water-deficit stress caused by drought and soil salinization adversely affects plant growth and crop productivity. Dehydrins are involved in the adaptation to water and osmotic stress. We have identified a wheat dehydrin named DHN-5 that is closely related to the maize RAB17. The full-length cDNA of Dhn-5 gene encodes a putative protein of 227 amino acids and contains 2 conserved lysine-rich-K-segment (EKKGIMDKIKEKLPG) repeats preceded by a stretch of eight serine residues, characteristic of group 2 LEA family. The Northern blot analyses showed a strong accumulation of Dhn-5 transcript in mature wheat embryos and to a lesser extent in ABA and salt-treated seedlings. Interestingly, DHN-5 protein accumulated differentially in two Tunisian durum wheat (Triticum durum Desf.) varieties with marked differences in salt and drought tolerance. By using specific dehydrin antibodies and 2D immunoblot analysis on proteins extracted from mature embryos in these two varieties, a differential phosphorylation pattern of DHN-5 was observed. In the resistant variety (R), beside a basic protein spot, a series of acidic spots were detected whereas in the sensitive variety (S) the acidic spots were weakly detectable. These acidic forms correspond to highly phosphorylated forms of DHN-5, which can be removed by alkaline phosphatase treatment. Accumulation of phosphorylated DHN-5 mainly in the R variety suggests a role of P-DHN-5 in preservation of cell integrity during late embryogenesis and desiccation. Subcellular localization of the DHN-5:GFP fusion protein indicated that DHN-5 would be primarily nuclear, suggesting a nuclear role in wheat osmotic stress response. The observed differential phosphorylation pattern of DHN-5 in the resistant and sensitive wheat varieties could be used as a basis for a molecular screen of tolerance/sensitivity to drought and salt stresses in wheat germplasm.This work was supported jointly by grants from the Ministry of Scientific Research, Technology and Development of Competencies, Tunisia and the Agence Espagnole de cooperation Internationale (AECI) Officina Técnica de Cooperación, Spain.Peer reviewe

The Activity of the Durum Wheat (<i>Triticum durum</i> L.) Catalase 1 (TdCAT1) Is Modulated by Calmodulin

Plant catalases (CAT) are involved in the cellular scavenging of the reactive oxygen species during developmental processes and in response to abiotic and biotic stresses. However, little is known about the regulation of the CAT activity to ensure efficient antioxidant function. Using bioinformatic analyses, we showed that durum wheat catalase 1 (TdCAT1) harbors highly conserved cation-binding and calmodulin binding (CaMBD) domains which are localized at different positions of the protein. As a result, the catalytic activity of TdCAT1 is enhanced in vitro by the divalent cations Mn2+ and Fe2+ and to a lesser extent by Cu2+, Zn2+, and Mg2+. Moreover, the GST-pull down assays performed here revealed that TdCAT1 bind to the wheat CaM (TdCaM1.3) in a Ca2+-independent manner. Furthermore, the TdCaM1.3/Ca2+ complex is stimulated in a CaM-dose-dependent manner by the catalytic activity of TdCAT1, which is further increased in the presence of Mn2+ cations. The catalase activity of TdCAT1 is enhanced by various divalent cations and TdCaM1.3 in a Ca-dependent manner. Such effects are not reported so far and raise a possible role of CaM and cations in the function of CATs during cellular response to oxidative stress