The dominance of salt water across the surface of the earth has lead to the widespread occurrence of salt-affected soils. Salt-tolerant plants (halophytes) have evolved to grow on these soils, with halophytes and less tolerant plants showing a wide range of adaptations. However, most of our crops are salt-sensitive. Consequently, salinity is an ever-present threat to agriculture, especially in areas where secondary salinisation has developed through irrigation or deforestation. Attempts to improve the salt tolerance of crops have met with very limited success, due to the complexity of the trait, both genetically and physiologically. Tolerance shows all the characteristics of a multigenic trait, with quantitative trait loci (QTLs) identified in barley, citrus, rice and tomato. Attempts to produce salt-tolerant crops have involved both the domestication of halophytes and the manipulation of existing crop species through conventional breeding programmes, the use of in vitro selection, pooling physiological traits, interspecific hybridisation, the use of marker-aided selection and the development of transgenic plants. After 10 years of research, the value of using transgenic plants to alter salt tolerance has yet to be tested in the field. The use of physiological traits in breeding programmes and the domestication of halophytes currently offer viable alternatives to the development of tolerance through the use of transgenic technologies.

Why does salinity pose such a difficult problem for plant breeders

Effect of salinity stress on finger millet (Eleusine coracana (L.) Gaertn): Histochemical and morphological analysis of coleoptile and coleorhizae

Differential effects of NaCl and Na2SO4 on the halophyte Prosopis strombulifera are explained by different responses of photosynthesis and metabolism.

In the wake of changing climatic conditions, plants are frequently exposed to a wide range of biotic and abiotic stresses at various stages of their development, all of which negatively affect their growth, development, and productivity. Drought is one of the most devastating abiotic stresses for most cultivated crops, particularly in arid and semiarid environments. Conventional breeding and biotechnological approaches are used to generate drought-tolerant crop plants. However, these techniques are costly and time-consuming. Plant-colonizing microbes, notably, endophytic fungi, have received increasing attention in recent years since they can boost plant growth and yield and can strengthen plant responses to abiotic stress. In this review, we describe these microorganisms and their relationship with host plants, summarize the current knowledge on how they “reprogram” the plants to promote their growth, productivity, and drought tolerance, and explain why they are promising agents in modern agriculture.

https://www.mdpi.com/1422-0067/23/16/9194/pdf?version=1660645193

Is Endophytic Colonization of Host Plants a Method of Alleviating Drought Stress? Conceptualizing the Hidden World of Endophytes

Since specific metabolites may be associated with salinity tolerance, this study aimed to decipher the salinity tolerance mechanism in date palm based on the information encoded by the metabolomic profiles of the salt-tolerant “Umsila” and salt-susceptible “Zabad” cultivars when grown under salinity conditions. Changes in the metabolomic profiles of the leaf and root tissues were determined using hydrophilic interaction liquid chromatography (HILIC) and reverse-phase liquid chromatography (RPLC) mass spectrometry. The global untargeted metabolomic analysis showed the presence of 4878 metabolites accumulated in leaf and root tissues of the date palm seedlings. Principal component analysis (PCA) revealed the presence of unique groups of metabolites for each treatment and tissue type. Pathway analysis showed the involvement of some of these metabolites in the biosynthesis of several types of membranous lipids and glycolipids molecules such as 18:0-lyso-phosphatidylethanolamine (lysoPE), and also the synthesis of cell-wall components such as 16-hydroxy hexadecanoic acid, which is an intermediate metabolite in cutin, suberin, and wax biosynthesis. Moreover, antioxidant flavonoids such as (+)-catechin and epicatechin, vitamins such as B9, phytohormone-associated compounds such as dihydrozeatin-9-N-glucoside-O-glucoside, and osmolytes such as the sulfonic amino acid taurine were all significantly (p ≤ 0.05, FWER ≤ 0.05) altered in response to salinity in both cultivars. These results indicate that salinity tolerance in date palm involved a multi-dimensional mechanism, which includes the modification of the cell-wall and cellular membranes, the production of oxygen species scavengers, the adjustment of cellular osmotic pressure, and probably the alteration of the hormonal balance. The intracultivar metabolomic profile comparison strategy performed in this study represents an approach that may pave the road toward the identification of salinity tolerance mechanisms in date palm based on the final protein products.

Comparative Metabolic Profiling of Two Contrasting Date Palm Genotypes Under Salinity

Salt mixtures induce anatomical modifications in the halophyte Prosopis strombulifera (Fabaceae: Mimosoideae)

Epichloid endophytic fungi, vertically transmitted symbionts of grasses, can increase plant tolerance to biotic and abiotic stress. Our aim was to identify ecophysiological mechanisms by which the endophyte Epichloë occultans confers drought tolerance to the annual grass Lolium multiflorum Lam. Endophyte-associated or endophyte-free plants were either well-watered or subjected to water deficit. We evaluated plant biomass, root length and nitrogen concentration, and we assessed intrinsic water use efficiency (iWUE) and its components net photosynthesis and stomatal conductance, by carbon and oxygen isotope analysis of shoot tissues. Endophyte-free plants produced more biomass than endophyte-associated ones at field capacity, while water deficit strongly reduced endophyte-free plants biomass. As a result, both types of plants produced similar biomass under water restriction. Based on oxygen isotope composition of plant cellulose, stomatal conductance decreased with water deficit in both endophyte-associated and endophyte-free plants. Meanwhile, carbon isotope composition indicated that iWUE increased with water deficit only in endophyte-associated plants. Thus, the isotope data indicated that net photosynthesis decreased more strongly in endophyte-free plants under water deficit. Additionally, endophyte presence reduced root length but increased its hydraulic conductivity. In conclusion, endophytic fungi confer drought tolerance to the host grass by adjusting shoot and root physiology.

Endophytic fungi and drought tolerance: ecophysiological adjustment in shoot and root of an annual mesophytic host grass.

Prosopis genus is an important member of semiarid, arid and saline environments around the world. This genus includes shrubs and trees that exhibit a high economic and ecological potential in different American regions. These plants are considered to be unique terrestrial species due to their combined ability to fix nitrogen and grow under high-salinity conditions. The South American halophyte, Prosopis strombulifera (Lam) Benth, is distributed from the Arizona desert (U.S.A.) to Patagonia (Argentina) and is especially abundant in the salinized areas of central Argentina. The soil of these areas is characterized by similar proportions of NaCl and Na2SO4. P. strombulifera species showed a halophytic response to NaCl surviving up to 1 M NaCl in in-vitro experiments, but in contrast, a strong growth inhibition at lower Na2SO4 concentrations was observed. These differential responses to the most abundant salts present in salinized soils of Argentina make this species an excellent model to study salt-tolerance mechanisms in halophytic plants. This chapter provides an overview of different salt tolerance mechanisms in the American halophyte Prosopis strombulifera, especially phytohormone pattern, oxidative responses and production of biomolecules. This halophyte may be considered as a new useful genetic source to improve crop salt tolerance and a promising plant as source of natural products for pharmaceutical industry.

Fabián Garello

Papers

Salt mixtures induce anatomical modifications in the halophyte Prosopis strombulifera (Fabaceae: Mimosoideae)

Endophytic fungi and drought tolerance: ecophysiological adjustment in shoot and root of an annual mesophytic host grass.

Morphophysiology and Biochemistry of Prosopis strombulifera Under Salinity. Are Halophytes Tolerant to All Salts