Sodium Chloride (NaCl)-Induced Physiological Alteration and Oxidative Stress Generation in Pisum sativum (L.): A Toxicity Assessment

Salinity stress has a deleterious impact on plant development, morphology, physiology, and biochemical characteristics. Considering the NaCl-induced phytotoxicity, current investigation was done to better understand the salt-tolerant mechanisms using Pisum sativum L. (pea) as a model crop. Generally, NaCl resulted in a progressive decrease in germinative attributes and physiological and biochemical parameters of P. sativum (L.). The 400 mM NaCl level had a higher detrimental effect and reduced the germination rate, plumule, radicle length, and seedling vigor index (SVI) by 78, 89, 84, and 77%, respectively, under in vitro. Furthermore, after 400 mM NaCl exposure, physiological and enzymatic profiles like root dry biomass (71%) chl-a (66%), chl-b (54%), total chlorophyll (45%), and nitrate reductase activity (NRA) (59%) of peas were decreased. In addition, a NaCl dose-related increase in soluble protein (SP) and sugar (SS), Na⁺ and K⁺ ions, and stressor metabolites was recorded. For instance, at 400 mM NaCl, SP, SS, Na⁺ ion, K⁺ ion, root proline, and malondialdehyde (MDA) contents were significantly and maximally elevated by 65, 33, 84, 79, 85, and 89%, respectively, compared to the control (0 mM NaCl). Data analysis indicated that greater doses of pesticides dramatically increased reactive oxygen species (ROS) levels and induced membrane damage through production of thiobarbituric acid reactive substances (TBARS), as well as increased cell injury. To deal with NaCl-induced oxidative stress, plants subjected to higher salinity stress showed a considerable build-up in antioxidant levels. As an example, ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) were maximally and significantly (p ≤ 0.05) increased by 68, 80, 74, and 58%, respectively, after 400 mM NaCl exposure. The propidium iodide (PI)-stained and NaCl-treated plant roots corroborated the damaging effect of salinity-induced stress on the cell membrane, which was observed under a confocal laser microscope (CLSM). The cells exposed to 400 mM NaCl had maximum fluorescence intensity, indicating that higher level of salts can cause pronounced cell damage and reactive oxygen species (ROS) generation. The increases in superoxide ion (O₂ ^-) and hydrogen peroxide (H₂O₂) content in NaCl-treated plant tissues indicated the elevation of ROS with increasing salt levels. This finding revealed that salt stress can cause toxicity in plants by causing alteration in metabolic activity, oxidative injury, and damage to cell membrane integrity.

A tetraspanin gene regulating auxin response and affecting orchid perianth size and various plant developmental processes

The competition between L (lip) and SP (sepal/petal) complexes in P-code model determines the identity of complex perianth patterns in orchids. Orchid tetraspanin gene Auxin Activation Factor (AAF) orthologs, whose expression strongly correlated with the expansion and size of the perianth after P code established, were identified. Virus-induced gene silencing (VIGS) of OAGL6-2 in L complex resulted in smaller lips and the down-regulation of Oncidium OnAAF. VIGS of PeMADS9 in L complex resulted in the enlarged lips and up-regulation of Phalaenopsis PaAAF. Furthermore, the larger size of Phalaenopsis variety flowers was associated with higher PaAAF expression, larger and more cells in the perianth. Thus, a rule is established that whenever bigger perianth organs are made in orchids, higher OnAAF/PaAAF expression is observed after their identities are determined by P-code complexes. Ectopic expression Arabidopsis AtAAF significantly increased the size of flower organs by promoting cell expansion in transgenic Arabidopsis due to the enhancement of the efficiency of the auxin response and the subsequent suppression of the jasmonic acid (JA) biosynthesis genes (DAD1/OPR3) and BIGPETAL gene during late flower development. In addition, auxin-controlled phenotypes, such as indehiscent anthers, enhanced drought tolerance, and increased lateral root formation, were also observed in 35S::AtAAF plants. Furthermore, 35S::AtAAF root tips maintained gravitropism during auxin treatment. In contrast, the opposite phenotype was observed in palmitoylation-deficient AtAAF mutants. Our data demonstrate an interaction between the tetraspanin AAF and auxin/JA that regulates the size of flower organs and impacts various developmental processes.

Potassium and zinc increase tolerance to salt stress in wheat (Triticum aestivum L.)

Potassium and zinc are essential elements in plant growth and metabolism and plays a vital role in salt stress tolerance. To investigate the physiological mechanism of salt stress tolerance, a pot experiment was conducted. Potassium and zinc significantly minimize the oxidative stress and increase root, shoot and spike length in wheat varieties. Fresh and dry biomass were significantly increased by potassium followed by zinc as compared to control C. The photosynthetic pigment and osmolyte regulator (proline, total phenolic, and total carbohydrate) were significantly enhanced by potassium and zinc. Salt stress increases MDA content in wheat varieties while potassium and zinc counteract the adverse effect of salinity and significantly increased membrane stability index. Salt stress decreases the activities of antioxidant enzymes (superoxide dismutase, catalase and ascorbate peroxidase) while the exogenous application of potassium and zinc significantly enhanced the activities of these enzymes. A significant positive correlation was found of spike length with proline (R² = 0.966 ^∗∗∗), phenolic (R² = 0.741^∗) and chlorophyll (R² = 0.853^∗∗). The MDA content showed significant negative correlation (R² = 0.983^∗∗∗) with MSI. It is concluded that potassium and zinc reduced toxic effect of salinity while its combine application showed synergetic effect and significantly enhanced salt tolerance.

Expression of Brassica napus TTG2, a regulator of trichome development, increases plant sensitivity to salt stress by suppressing the expression of auxin biosynthesis genes

WRKY transcription factors (TFs) are plant specific and play important roles in regulating diverse biological processes. To identify TFs with broad-spectrum effects on various stress responses in Brassica napus, an important oil crop grown across diverse ecological regions worldwide, we functionally characterized Bna.TTG2 genes, which are homologous to the Arabidopsis AtTTG2 (WRKY44) gene. Four Bna.TTG2 genes were capable of rescuing the trichome phenotypes of Arabidopsis ttg2 mutants. Overexpressing one Bna.TTG2 family member, BnaA.TTG2.a.1, remarkably increased trichome numbers in Arabidopsis and B. napus plants. Interestingly, the BnaA.TTG2.a.1-overexpressing plants of both species exhibited increased sensitivity to salt stress. In BnaA.TTG2.a.1-overexpressing Arabidopsis under salt stress, the endogenous indole-3-acetic acid (IAA) content was reduced, and the expression of two auxin biosynthesis genes, TRYPTOPHAN BIOSYNTHESIS 5 (TRP5) and YUCCA2 (YUC2), was downregulated. The results from yeast one-hybrid, electrophoretic mobility shift, and dual-luciferase reporter assays revealed that BnaA.TTG2.a.1 is able to bind to the promoters of TRP5 and YUC2. These data indicated that BnaA.TTG2.a.1 confers salt sensitivity to overexpressing plants by suppressing the expression of IAA synthesis genes and thus lowering IAA levels. Transgenic Arabidopsis plants with an N-terminus-deleted BnaA.TTG2.a.1 no longer showed hypersensitivity to salt stress, suggesting that the N terminus of BnaA.TTG2.a.1 plays a critical role in salt stress responses. Therefore, in addition to its classical function in trichome development, our study reveals a novel role for Bna.TTG2 genes in salt stress responses.

K(+) accumulation in the cytoplasm and nucleus of the salt gland cells of Limonium bicolor accompanies increased rates of salt secretion under NaCl treatment using NanoSIMS

Recretohalophytes with specialized salt-secreting structures (salt glands) can secrete excess salts from plant, while discriminating between Na(+) and K(+). K(+)/Na(+) ratio plays an important role in plant salt tolerance, but the distribution and role of K(+) in the salt gland cells is poorly understood. In this article, the in situ subcellular localization of K and Na in the salt gland of the recretohalophyte Limonium bicolor Kuntze is described. Samples were prepared by high-pressure freezing (HPF), freeze substitution (FS) and analyzed using NanoSIMS. The salt gland of L. bicolor consists of sixteen cells. Higher signal strength of Na(+) was located in the apoplast of salt gland cells. Compared with control, 200 mM NaCl treatment led to higher signal strength of K(+) and Na(+) in both cytoplasm and nucleus of salt gland cells although K(+)/Na(+) ratio in both cytoplasm and nucleus were slightly reduced by NaCl. Moreover, the rate of Na(+) secretion per salt gland of L. bicolor treated with 200 mM NaCl was five times that of controls. These results suggest that K(+) accumulation both in the cytoplasm and nucleus of salt gland cells under salinity may play an important role in salt secretion, although the exact mechanism is unknown.

Salinity tolerance, Na+ exclusion and allele mining of HKT1;5 in Oryza sativa and O. glaberrima: many sources, many genes, one mechanism?

BACKGROUND

Cultivated rice species (Oryza sativa L. and O. glaberrima Steud.) are generally considered among the crop species most sensitive to salt stress. A handful of lines are known to be tolerant, and a small number of these have been used extensively as donors in breeding programs. However, these donors use many of the same genes and physiological mechanisms to confer tolerance. Little information is available on the diversity of mechanisms used by these species to cope with salt stress, and there is a strong need to identify varieties displaying additional physiological and/or genetic mechanisms to confer higher tolerance.

RESULTS

Here we present data on 103 accessions from O. sativa and 12 accessions from O. glaberrima, many of which are identified as salt tolerant for the first time, showing moderate to high tolerance of high salinity. The correlation of salinity-induced senescence (as judged by the Standard Evaluation System for Rice, or SES, score) with whole-plant and leaf blade Na+ concentrations was high across nearly all accessions, and was almost identical in both O. sativa and O. glaberrima. The association of leaf Na+ concentrations with cultivar-groups was very weak, but association with the OsHKT1;5 allele was generally strong. Seven major and three minor alleles of OsHKT1;5 were identified, and their comparisons with the leaf Na+ concentration showed that the Aromatic allele conferred the highest exclusion and the Japonica allele the least. A number of exceptions to this association with the Oryza HKT1;5 allele were identified; these probably indicate the existence of additional highly effective exclusion mechanisms. In addition, two landraces were identified, one from Thailand and the other from Senegal, that show high tissue tolerance.

CONCLUSIONS

Significant variation in salinity tolerance exists within both cultivated Oryza species, and this is the first report of significant tolerance in O. glaberrima. The majority of accessions display a strong quantitative relationship between tolerance and leaf blade Na+ concentration, and thus the major tolerance mechanisms found in these species are those contributing to limiting sodium uptake and accumulation in active leaves. However, there appears to be genetic variation for several mechanisms that affect leaf Na+ concentration, and rare cases of accessions displaying different mechanisms also occur. These mechanisms show great promise for improving salt tolerance in rice over that available from current donors.