The physiological and molecular mechanisms of tolerance to osmotic and ionic components of salinity stress are reviewed at the cellular, organ, and whole-plant level. Plant growth responds to salinity in two phases: a rapid, osmotic phase that inhibits growth of young leaves, and a slower, ionic phase that accelerates senescence of mature leaves. Plant adaptations to salinity are of three distinct types: osmotic stress tolerance, Na + or Cl − exclusion, and the tolerance of tissue to accumulated Na + or Cl − . Our understanding of the role of the HKT gene family in Na + exclusion from leaves is increasing, as is the understanding of the molecular bases for many other transport processes at the cellular level. However, we have a limited molecular understanding of the overall control of Na + accumulation and of osmotic stress tolerance at the whole-plant level. Molecular genetics and functional genomics provide a new opportunity to synthesize molecular and physiological knowledge to improve the salinity tolerance of plants relevant to food production and environmental sustainability.

https://rodas5.us.es/file/c2a700e9-8b4a-1970-d343-4a269c302970/1/estres_salino_bibliografia_scorm.zip/files/estres_salino_bibliografia.pdf

Mechanisms of salinity tolerance

http://nhjy.hzau.edu.cn/kech/ssyy/pdf/drought/4.PDF

Photosynthesis under drought and salt stress: regulation mechanisms from whole plant to cell

Numerous species of soil bacteria which flourish in the rhizosphere of plants, but which may grow in, on, or around plant tissues, stimulate plant growth by a plethora of mechanisms. These bacteria are collectively known as PGPR (plant growth promoting rhizobacteria). The search for PGPR and investigation of their modes of action are increasing at a rapid pace as efforts are made to exploit them commercially as biofertilizers. After an initial clarification of the term biofertilizers and the nature of associations between PGPR and plants (i.e., endophytic versus rhizospheric), this review focuses on the known, the putative, and the speculative modes-of-action of PGPR. These modes of action include fixing N2, increasing the availability of nutrients in the rhizosphere, positively influencing root growth and morphology, and promoting other beneficial plant–microbe symbioses. The combination of these modes of actions in PGPR is also addressed, as well as the challenges facing the more widespread utilization of PGPR as biofertilizers.

Plant growth promoting rhizobacteria as biofertilizers

Halophytes, plants that survive to reproduce in environments where the salt concentration is around 200 mM NaCl or more, constitute about 1% of the worlds flora. Some halophytes show optimal growth in saline conditions; others grow optimally in the absence of salt. However, the tolerance of all halophytes to salinity relies on controlled uptake and compartmentalization of Na+, K+ and Cl- and the synthesis of organic compatible solutes, even where salt glands are operative. Although there is evidence that different species may utilize different transporters in their accumulation of Na+, in general little is known of the proteins and regulatory networks involved. Consequently, it is not yet possible to assign molecular mechanisms to apparent differences in rates of Na+ and Cl- uptake, in root-to-shoot transport (xylem loading and retrieval), or in net selectivity for K+ over Na+. At the cellular level, H+-ATPases in the plasma membrane and tonoplast, as well as the tonoplast H+-PPiase, provide the transmembrane proton motive force used by various secondary transporters. The widespread occurrence, taxonomically, of halophytes and the general paucity of information on the molecular regulation of tolerance mechanisms persuade us that research should be concentrated on a number of model species that are representative of the various mechanisms that might be involved in tolerance.

Salinity tolerance in halophytes

The worldwide increases in both environmental damage and human population pressure have the unfortunate consequence that global food production may soon become insufficient to feed all of the world's people. It is therefore essential that agricultural productivity be significantly increased within the next few decades. To this end, agricultural practice is moving toward a more sustainable and environmentally friendly approach. This includes both the increasing use of transgenic plants and plant growth-promoting bacteria as a part of mainstream agricultural practice. Here, a number of the mechanisms utilized by plant growth-promoting bacteria are discussed and considered. It is envisioned that in the not too distant future, plant growth-promoting bacteria (PGPB) will begin to replace the use of chemicals in agriculture, horticulture, silviculture, and environmental cleanup strategies. While there may not be one simple strategy that can effectively promote the growth of all plants under all conditions, some of the strategies that are discussed already show great promise.

/pdf/plant-growth-promoting-bacteria-mechanisms-and-applications-1vmnc8frke.pdf

Plant Growth-Promoting Bacteria: Mechanisms and Applications

We have recently proposed that one way that plant growth-promoting rhizobacteria (PGPR) stimulate plant growth is through the activity of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminas...

Isolation and characterization of ACC deaminase genes from two different plant growth-promoting rhizobacteria

Both Met (methionine) and SAM ( S -adenosylmethionine), the activated form of Met, participate in a number of essential metabolic pathways in plants. The subcellular compartmentalization of Met fluxes will be discussed in the present review with respect to regulation and communication with the sulfur assimilation pathway, the network of the aspartate-derived amino acids and the demand for production of SAM. SAM enters the ethylene, nicotianamine and polyamine biosynthetic pathways and provides the methyl group for the majority of methylation reactions required for plant growth and development. The multiple essential roles of SAM require regulation of its synthesis, recycling and distribution to sustain these different pathways. A particular focus of the present review will be on the function of recently identified genes of the Met salvage cycle or Yang cycle and the importance of the Met salvage cycle in the metabolism of MTA (5′-methylthioadenosine). MTA has the potential for product inhibition of ethylene, nicotianamine and polyamine biosynthesis which provides an additional link between these pathways. Interestingly, regulation of Met cycle genes was found to differ between plant species as shown for Arabidopsis thaliana and Oryza sativa .

Methionine salvage and S-adenosylmethionine: essential links between sulfur, ethylene and polyamine biosynthesis

After cold acclimation, winter rye (Secale cereale L.) is able to withstand the formation of extracellular ice at freezing temperatures. We now show, for the first time, that cold-acclimated winter rye plants contain endogenously produced antifreeze protein. The protein was extracted from the apoplast of winter rye leaves, where ice forms during freezing. After partial purification, the protein was identified as antifreeze protein because it modified the normal growth pattern of ice crystals and depressed the freezing temperature of water noncolligatively.

Antifreeze protein produced endogenously in winter rye leaves.

Purine and pyrimidine nucleotides are major energy carriers, subunits of nucleic acids and precursors for the synthesis of nucleotide cofactors such as NAD and SAM. Despite the obvious importance of these molecules, we still have much to learn about how these nucleotides are synthesized and metabolized by plants. Moreover, of the research that has been done in this area relatively little has used genetic analysis to evaluate the function(s) of specific enzymes.

The pathways for the synthesis of nucleotides in plant cells are similar to those found in animals and microorganisms. This conclusion is based primarily on the results of studies using in vivo radiotracers, specific inhibitors of nucleotide synthesis and on analyses of the kinetic parameters of purified enzymes involved in nucleotide synthesis that are unlikely to have similar demands for purine and pyrimidine nucleotides have been used in this research. A more comprehensive understanding of the role(s) of specific nucleotide biosynthetic enzymes throughout plant development and factors that regulate their activity/expression is still lacking. Ultimately this information will explain how the requirements of different plants are met, such as those of ureide-producing legumes (Schubert and Boland, 1990) or those synthesizing caffeine (Suzuki et al., 1992; Ashihara and Crozier, 1999).

There are two principal routes for the synthesis of nucleotides: the de novo and the salvage pathways (Figures 1 and ​and2,2, Figures 3 and ​and4,4, respectively). Using 5-phosphoribosyl-1-pyrophosphate (PRPP), the de novo pathway enzymes build purine and pyrimidine nucleotides from “scratch” using simple molecules such as CO2, amino acids and tetrahydrofolate. This route of nucleotide synthesis has a high requirement for energy as compared that of the salvage pathway. For example, five of the 12 steps of de novo purine synthesis require hydrolysis of ATP or GTP but only one salvage cycle reaction uses ATP. The enzymes of both of these biosynthetic routes are classified as “housekeeping” enzymes because they perform basic, cellular activities and are assumed to be present in low, constitutive levels in all cells. Whereas the de novo pathway is thought to reside in plastids, salvage cycle enzymes may be localized in more than one compartment.




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Figure 1.


De novo biosynthetic pathway of purine nucleotides in plants. Enzymes shown are: amido phosphoribosyltransferase, (2) GAR synthetase, (3) GAR formyl transferase, (4) FGAM synthetase, (5) AIR synthetase, (6) AIR carboxylase, (7) SAICAR synthetase, (8) adenylosuccinate lyase, (9) AICAR formyl transferase, (10) IMP cyclohydrolase, (11) SAMP synthetase, (12) adenylosuccinase, (13) IMP dehydrogenase, (14) GMP synthetase.

https://bioone.org/journals/The-Arabidopsis-Book/volume-2002/issue-1/tab.0018/Purine-and-Pyrimidine-Nucleotide-Synthesis-and-Metabolism/10.1199/tab.0018.pdf

Purine and Pyrimidine Nucleotide Synthesis and Metabolism

Thellungiella, an Arabidopsis (Arabidopsis thaliana)-related halophyte, is an emerging model species for studies designed to elucidate molecular mechanisms of abiotic stress tolerance. Using a cDNA microarray containing 3,628 unique sequences derived from previously described libraries of stress-induced cDNAs of the Yukon ecotype of Thellungiella salsuginea, we obtained transcript profiles of its response to cold, salinity, simulated drought, and rewatering after simulated drought. A total of 154 transcripts were differentially regulated under the conditions studied. Only six of these genes responded to all three stresses of drought, cold, and salinity, indicating a divergence among the end responses triggered by each of these stresses. Unlike in Arabidopsis, there were relatively few transcript changes in response to high salinity in this halophyte. Furthermore, the gene products represented among drought-responsive transcripts in Thellungiella associate a down-regulation of defense-related transcripts with exposure to water deficits. This antagonistic interaction between drought and biotic stress response may demonstrate Thellungiella's ability to respond precisely to environmental stresses, thereby conserving energy and resources and maximizing its survival potential. Intriguingly, changes of transcript abundance in response to cold implicate the involvement of jasmonic acid. While transcripts associated with photosynthetic processes were repressed by cold, physiological responses in plants developed at low temperature suggest a novel mechanism for photosynthetic acclimation. Taken together, our results provide useful starting points for more in-depth analyses of Thellungiella's extreme stress tolerance.

/pdf/transcriptional-profiling-implicates-novel-interactions-5968u0ke4i.pdf

Transcriptional Profiling Implicates Novel Interactions Between Abiotic Stress and Hormonal Responses in Thellungiella, a Close Relative of Arabidopsis

Barbara A. Moffatt

Papers

Isolation and characterization of ACC deaminase genes from two different plant growth-promoting rhizobacteria

Methionine salvage and S-adenosylmethionine: essential links between sulfur, ethylene and polyamine biosynthesis

Antifreeze protein produced endogenously in winter rye leaves.

Purine and Pyrimidine Nucleotide Synthesis and Metabolism

Transcriptional Profiling Implicates Novel Interactions Between Abiotic Stress and Hormonal Responses in Thellungiella, a Close Relative of Arabidopsis