Salt tolerance and salinity effects on plants: a review.

OXIDATIVE STRESS . . . . . . . . . . . . . .. . . . . .... . . . . .. . . . . .. . . . ... .. . . . . .... . . .. .. . . . . . ... . 84 RESPONSE OF SOD TO ENVIRONMENTAL STRESS . . . . . . . . . . . . . . . .. . . . .... . . .... . . . . . . 87 Photoinhibition . . . . . . . . . . .. . ... . . .. . . . .. . . . . ..... . . . , ... ", , ... ,' , ... . ,., . . . "" . .. . ,'.' . . ,' . . . . , ., 87 Paraquat and Other Herbicides . . .. , . . .... , ..... , . . . . ... . . . .. . . . . . . . . . . . . ,.. 91 Atmospheric Pollutants . . " .... , .... , " .... " , , . . , .... , ' 94 Waterlogging and Drought .. ", . . ... , . . . . . . , .. "", . . ", . . . . . ", . . . .. ", . . .. , . . . ... """", . . " 97 The Defense Response to Pathogens . ... . . . . "" ... "" .... " ...... " ,."" .... . , 98 The Phenomenon of Cross-Tolerance . . . , , .. . . . . ...... . . , . . . . . . , . . .... , .. . . . . . . . .. .. 101 The Mechanism of SOD Regulation ........ , , ...... , " 102 GENETIC ENGINEERING OF SOD IN PLANTS .. ' ...... ' . . . . . . . . . . . . . . . . . . . . . . . .. . . . , .... ' 104 Cu/ZnSOD and MnSOD Overexpression . . . . . .. . . . . . , . . . . . . .. . . . . . .. . . . . . , .... . . . . .. . . ,.... . . 105 Prospects for Stress Tolerance through Genetic Engineering of SOD . . . . ,,, .. , . . . . . . . 106

Superoxide dismutase and stress tolerance

Molecular studies of drought stress in plants use a variety of strategies and include different species subjected to a wide range of water deficits. Initial research has by necessity been largely descriptive, and relevant genes have been identified either by reference to physiological evidence or by differential screening. A large number of genes with a potential role in drought tolerance have been described, and major themes in the molecular response have been established. Particular areas of importance are sugar metabolism and late-embryogenesis-abundant (LEA) proteins. Studies have begun to examine mechanisms that control the gene expression, and putative regulatory pathways have been established. Recent attempts to understand gene function have utilized transgenic plants. These efforts are of clear agronomic importance.

The molecular basis of dehydration tolerance in plants

Systemic acquired resistance is an important component of the disease resistance repertoire of plants. In this study, a novel synthetic chemical, benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH), was shown to induce acquired resistance in wheat. BTH protected wheat systemically against powdery mildew infection by affecting multiple steps in the life cycle of the pathogen. The onset of resistance was accompanied by the induction of a number of newly described wheat chemically induced (WCI) genes, including genes encoding a lipoxygenase and a sulfur-rich protein. With respect to both timing and effectiveness, a tight correlation existed between the onset of resistance and the induction of the WCI genes. Compared with other plant activators, such as 2,6-dichloroisonicotinic acid and salicylic acid, BTH was the most potent inducer of both resistance and gene induction. BTH is being developed commercially as a novel type of plant protection compound that works by inducing the plant's inherent disease resistance mechanisms.

Benzothiadiazole, a novel class of inducers of systemic acquired resistance, activates gene expression and disease resistance in wheat.

ions and to some extent Cl and SO 4 2 of Mg 2+ and nutrient imbalance caused by excess of Na + and Cl ions. Salinity stress response is multigenic, as a number of processes i n- volved in the tolerance mechanism are affected, such as var ious compatible solutes/osmolytes, polyamines, reactive oxygen species and antioxidant defence mecha- nism, ion transport and compartmentalization of inj u- rious ions. Various genes/cDNAs encoding proteins involved in the above-mentioned processes have been identified and isolated. The role of genes/cDNAs e n- coding proteins involved in regulating other genes/ pro- teins, signal transduction process involving hormones like ABA, JA and polyamines, and strategies to i mprove salinity stress tolerance have also been di scussed. EXCESS amount of salt in the soil adversely affects plant growth and development. Nearly 20% of the world's cul- tivated area and nearly half of the world's i rrigated lands are affected by salinity 1

Physiology and molecular biology of salinity stress tolerance in plants

Protein changes induced by salinity stress were investigated in the roots of the salt-sensitive rice cultivar Taichung native 1. We found eight proteins to be induced and obtained partial sequences of one with a molecular mass of 15 kilodaltons and an isoelectric point of 5.5. Using an oligonucleotide probe based on this information, a cDNA clone, salT, was selected and found to contain an open reading frame coding for a protein of 145 amino acid residues. salT mRNA accumulates very rapidly in sheaths and roots from mature plants and seedlings upon treatment with Murashige and Skoog salts (1%), air drying, abscisic acid (20 microM), polyethylene glycol (5%), sodium chloride (1%), and potassium chloride (1%). Generally, no induction was seen in the leaf lamina even when the stress should affect all parts of the plant uniformly. The organ-specific response of salT is correlatable with the pattern of Na+ accumulation during salt stress.

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Characterization of a rice gene showing organ-specific expression in response to salt stress and drought.

β(1,3)-Glucanases are a class of hormonally and developmentally regulated plant hydrolytic enzymes, which are also induced upon pathogen infection, suggesting a role of these hydrolases in the defense response to pathogens. We have purified several β(1,3)-glucanases present in tobacco leaves from control plants and from plants treated with salicylic acid or infected with Pseudomonas syringae and studied in detail the subcellular localization of the different isoforms. Partial protein sequence analysis demonstrated that each of the different isoforms had a unique amino acid sequence and was therefore encoded by a different gene. We have also demonstrated that two of these isoforms, similar to the cytokinin/auxin-regulated isoforms previously isolated from tobacco cell suspensions, are located in the central vacuole. Upon salicylic acid treatment or P. syringae infection, three secreted isoforms are induced, belonging to the so-called pathogenesis-related proteins. These pathogenesis-related β(1,3)-glucanases are all distinct from each other and also different from the vacuolar isoforms. We demonstrate that the vacuolar isoforms are not secreted to the extracellular spaces of the plant following pathogen infection, suggesting that any function they play in the plant defense response is restricted to an intracellularly coordinated defense process.

Characterization of vacuolar and extracellular β(1,3)-glucanases of tobacco: evidence for a strictly compartmentalized plant defense system

Characterization of vacuolar and extracellular ß(1,3)-glucanases of tobacco: Evidence for a strictly compartmentalized plant defense system. Proc Natl Acad Sci USA86: 2673-2677

In the plant Nicotiana plumbaginifolia, manganese superoxide dismutase (MnSOD) is synthesized in the cytoplasm as a preprotein and is subsequently translocated to the mitochondrial matrix with corresponding cleavage of an NH2-terminal leader sequence. To determine whether the plant enzyme could replace the endogenous SOD activities of Escherichia coli and yeast, constructions have been made in appropriate vectors for expression of the preprotein and the mature MnSOD. These were introduced into SOD-deficient strains for complementation studies. In E. coli, both forms of the protein were shown to be active and able to complement SOD deficiency to different degrees. Expression of the preprotein in a yeast strain lacking a mitochondrial MnSOD resulted in a restoration of wild-type growth, only possible if the plant protein was being targeted to the mitochondria. Subsequent studies revealed that the protein was processed and that the leader sequence was cleaved at the identical position as recognized by the mitochondrial peptidase of plants. The components mediating mitochondrial import thus appear to be highly conserved between plants and yeast.

A plant manganese superoxide dismutase is efficiently imported and correctly processed by yeast mitochondria.

SummaryUpon infection of a plant by a pathogen, a series of drastic metabolic changes occur within the plant. One characteristic feature of this defence response is the synthesis of the so-called pathogenesis-related (PR) proteins. We have studied the nature, structure, and subcellular localization of the different PR proteins upon salicylic acid treatment and Pseudomonas syringae infection of Nicotiana tabacum plants. In both test systems, we could demonstrate that the PR protein fraction of tobacco consists of at least 20 to 25 different proteins, including s (1–3)-glucanases, chitinases, peroxidases, thaumatin-like proteins, the PR1 class proteins and a proteinase inhibitor-like protein. Moreover, several classes of these PR proteins segregate into specific vacuolar and secreted isoforms. Here, we present a model which could explain the role of the compartimentalized PR s (1–3)-glucanases and chitinases within the regulation of the defence response.

M. Van Den Bulcke

Papers

Characterization of a rice gene showing organ-specific expression in response to salt stress and drought.

Characterization of vacuolar and extracellular β(1,3)-glucanases of tobacco: evidence for a strictly compartmentalized plant defense system

Characterization of vacuolar and extracellular ß(1,3)-glucanases of tobacco: Evidence for a strictly compartmentalized plant defense system. Proc Natl Acad Sci USA86: 2673-2677

A plant manganese superoxide dismutase is efficiently imported and correctly processed by yeast mitochondria.

The role of vacuolar and secreted pathogenesis-related β (1-3)-glucanases and chitinases in the defence response of plants