Ascorbate peroxidase and catalase cooperate for protection against hydrogen peroxide generated in potato tubers during low-temperature storage

Coping with salinity stress: segmental group 7 chromosome introgressions from halophytic Thinopyrum species greatly enhance tolerance of recipient durum wheat

Increased soil salinization, tightly related to global warming and drought and exacerbated by intensified irrigation supply, implies highly detrimental effects on staple food crops such as wheat. The situation is particularly alarming for durum wheat (DW), better adapted to arid/semi-arid environments yet more sensitive to salt stress than bread wheat (BW). To enhance DW salinity tolerance, we resorted to chromosomally engineered materials with introgressions from allied halophytic Thinopyrum species. "Primary" recombinant lines (RLs), having portions of their 7AL arms distally replaced by 7el1L Th. ponticum segments, and "secondary" RLs, harboring Th. elongatum 7EL insertions "nested" into 7el1L segments, in addition to near-isogenic lines lacking any alien segment (CLs), cv. Om Rabia (OR) as salt tolerant control, and BW introgression lines with either most of 7el1 or the complete 7E chromosome substitution as additional CLs, were subjected to moderate (100 mM) and intense (200 mM) salt (NaCl) stress at early growth stages. The applied stress altered cell cycle progression, determining a general increase of cells in G1 and a reduction in S phase. Assessment of morpho-physiological and biochemical traits overall showed that the presence of Thinopyrum spp. segments was associated with considerably increased salinity tolerance versus its absence. For relative water content, Na+ accumulation and K+ retention in roots and leaves, oxidative stress indicators (malondialdehyde and hydrogen peroxide) and antioxidant enzyme activities, the observed differences between stressed and unstressed RLs versus CLs was of similar magnitude in "primary" and "secondary" types, suggesting that tolerance factors might reside in defined 7el1L shared portion(s). Nonetheless, the incremental contribution of 7EL segments emerged in various instances, greatly mitigating the effects of salt stress on root and leaf growth and on the quantity of photosynthetic pigments, boosting accumulation of compatible solutes and minimizing the decrease of a powerful antioxidant like ascorbate. The seemingly synergistic effect of 7el1L + 7EL segments/genes made "secondary" RLs able to often exceed cv. OR and equal or better perform than BW lines. Thus, transfer of a suite of genes from halophytic germplasm by use of fine chromosome engineering strategies may well be the way forward to enhance salinity tolerance of glycophytes, even the sensitive DW.

Dynamic Changes in the Antioxidative Defense System in the Tea Plant Reveal the Photoprotection-Mediated Temporal Accumulation of Flavonoids under Full Sunlight Exposure

To reveal the mechanisms underlying how light affects flavonoid metabolism and the potential role of flavonoids in protecting against photooxidative stress in tea leaves, tea plants adapted to low-light conditions were exposed to full sunlight over 48 h. There was an increase in the activities of catalase (CAT) and superoxide dismutase (SOD) as well as greater accumulation of reactive oxygen species, lutein, tocopherols, ascorbate and malondialdehyde, suggestive of a time-dependent response to photooxidative stress in tea leaves. Analysis of the time dependency of each element of the antioxidant system indicated that carotenoids and tocopherols exhibited the fastest response to light stress (within 3 h), followed by SOD, CAT and catechin, which peaked at 24 h. Meanwhile, flavonols, vitamin C and glutathione showed the slowest response. Subsequent identification of the main phytochemicals involved in protecting against oxidative stress using untargeted metabolomics revealed a fast and initial accumulation of nonesterified catechins that preceded the increase in flavonol glycosides and catechin esters. Gene expression analysis suggested that the light-induced accumulation of flavonoids was highly associated with the gene encoding flavonol synthase. Ultraviolet B (UV-B) irradiation further validated the time-dependent and collaborative effects of flavonoids in photoprotection in tea plants. Intriguingly, the dynamics of the metabolic response are highly distinct from those reported for Arabidopsis, suggesting that the response to light stress is not conserved across plants. This study additionally provides new insights into the functional role of flavonoids in preventing photooxidative stress and may contribute to further improving tea quality through the control of light intensity.

Contrasting adaptive responses to cope with drought stress and recovery in Cenchrus ciliaris L. and their implications for tissue lignification

Cenchrus ciliaris L. is a widely used species for cattle feed in arid and semi-arid regions due to good forage value and known tolerance to drought conditions. Here, we provide insights to adaptive responses of two contrasting genotypes of C. ciliaris (drought-tolerant "RN51" and drought-sensitive "RN1") to face drought stress and recovery conditions and the implications for tissue lignification. Drought stress caused a reversible decrease in the leaf water relationship and damage to photosystem II, leading to an increased generation of reactive oxygen species and lipid peroxidation. Plants of RN51 exhibited a pronounced increase of antioxidant enzymatic activities. Unlike the drought-sensitive genotype, RN51 exhibited further development of lignified tissues and bulliform cells and had the greatest thickness of the adaxial epidermis. Drought stress led to the rapid activation of the expression of lignin biosynthesis pathway-related enzymes. The transcript level of the caffeoyl-CoA O-methyltransferase gene decreased in RN1, whereas cinnamoyl-CoA reductase transcripts were increased in RN51. After rewatering, the tolerant genotype recovered more rapidly than RN1. Even though the two genotypes survived when they were exposed to drought stress, RN1 showed the highest reduction in growth parameters, and this reduction was sustained during rewatering. The results indicated that the capacity to regulate lipid peroxidation and mitigate oxidative damage could be one of the mechanisms included in tolerance to drought stress. In addition, the development of foliar characteristics, like thickness of the adaxial epidermis, well-developed bulliform cells, and intensive lignified tissues, are considered anatomical adaptive strategies for drought tolerance in C. ciliaris.

Ameliorative Effects of Exogenous Proline on Photosynthetic Attributes, Nutrients Uptake, and Oxidative Stresses under Cadmium in Pigeon Pea (Cajanus cajan L.)

Proline plays a significant role in the plant response to stress conditions. However, its role in alleviating metal-induced stresses remains elusive. We conducted an experiment to evaluate the ameliorative role of exogenous proline on cadmium-induced inhibitory effects in pigeon pea subjected to different Cd treatments (4 and 8 mg/mL). Cadmium treatments reduced photosynthetic attributes, decreased chlorophyll contents, disturbed nutrient uptake, and affected growth traits. The elevated activity of antioxidant enzymes (superoxide dismutase, catalase, and glutathione peroxidase), in association with relatively high contents of hydrogen peroxide, thiobarbituric acid reactive substances, electrolyte leakage, and endogenous proline, was measured. Exogenous proline application (3 and 6 mM) alleviated cadmium-induced oxidative damage. Exogenous proline increased antioxidant enzyme activities and improved photosynthetic attributes, nutrient uptake (Mg²⁺, Ca²⁺, K⁺), and growth parameters in cadmium-stressed pigeon pea plants. Our results reveal that proline supplementation can comprehensively alleviate the harmful effects of cadmium on pigeon pea plants.

Boron mitigates citrus root injuries by regulating intracellular pH and reactive oxygen species to resist H+-toxicity

Boron (B)-deficiency and H⁺-toxicity are important limiting factors for plants growth in acid soils. High B supply may reduce H⁺-toxicity-induced inhibition of growth in citrus. Trifoliate orange rootstock seedlings were irrigated with nutrient solution containing either 0 μM or 10 μM H₃BO₃ at two pH levels (pH4 (H⁺-toxicity) and pH6 (normal)). The results showed that H⁺-toxicity without B severely hampered main root elongation. Simultaneously, oxidative damage caused by H⁺-toxicity led to severe damage to the apical structure of root such as root crown abscission. However, B application promoted the root length, root cell viability and reduced cell wall (CW) thickness of root tips under H⁺-toxicity. Additionally, B application reduced the H⁺-toxicity-induced reactive oxygen species (ROS) accumulation in roots as characterized by lower fluorescence intensity of H₂O₂ and O₂^- staining. Moreover, ³¹P-NMR (³¹P nuclear magnetic resonance) spectra revealed B application regulated the pH of vacuoles and cytoplasm in root tips by reducing phosphoenolpyruvate carboxykinase (PEPCase) activity while enhancing NADP malic enzyme (NADP-ME) activity during H⁺-toxicity. Collectively, our results demonstrate that B supply alleviates H⁺-toxicity and promotes root growth by reducing ROS accumulation, attenuating intracellular acidic microenvironment to ensure normal chemical reactions in root tip cells.

Boron inhibits aluminum-induced toxicity to citrus by stimulating antioxidant enzyme activity

Aluminum (Al) toxicity is a major factor limiting plant productivity. The objective of the present study was to develop the mechanisms of boron (B) alleviating aluminum toxicity in citrus. The results showed that aluminum toxicity severely hampered root elongation. Interestingly, under aluminum exposure, boron supply improved superoxide dismutase activity while reducing peroxidase, catalase and polyphenol oxidase activities. Likewise, the contents of H₂O₂, lipid peroxidation, protein and proline in roots were markedly decreased by boron application under aluminum exposure. Our results demonstrated that boron could alleviate aluminum toxicity by regulating antioxidant enzyme activities in the roots.

Co-expression of monodehydroascorbate reductase and dehydroascorbate reductase from Brassica rapa effectively confers tolerance to freezing-induced oxidative stress

Plants are exposed to various environmental stresses and have therefore developed antioxidant enzymes and molecules to protect their cellular components against toxicity derived from reactive oxygen species (ROS). Ascorbate is a very important antioxidant molecule in plants, and monodehydroascorbate reductase (MDHAR; EC 1.6.5.4) and dehydroascorbate reductase (DHAR; EC 1.8.5.1) are essential to regeneration of ascorbate for maintenance of ROS scavenging ability. The MDHAR and DHAR genes from Brassica rapa were cloned, transgenic plants overexpressing either BrMDHAR and BrDHAR were established, and then, each transgenic plant was hybridized to examine the effects of co-expression of both genes conferring tolerance to freezing. Transgenic plants co-overexpressing BrMDHAR and BrDHAR showed activated expression of relative antioxidant enzymes, and enhanced levels of glutathione and phenolics under freezing condition. Then, these alteration caused by co-expression led to alleviated redox status and lipid peroxidation and consequently conferred improved tolerance against severe freezing stress compared to transgenic plants overexpressing single gene. The results of this study suggested that although each expression of BrMDHAR or BrDHAR was available to according tolerance to freezing, the simultaneous expression of two genes generated synergistic effects conferring improved tolerance more effectively even severe freezing.

The inactivation of horseradish peroxidase isoenzyme A2 by hydrogen peroxide: an example of partial resistance due to the formation of a stable enzyme intermediate

The inactivation of horseradish peroxidase A2 (HRP-A2) with H2O2 as the sole substrate has been studied. In incubation experiments it was found that the fall in HRP-A2 activity was non-linearly dependent on H2O2 concentrations and that a maximum level of inactivation of approximately 80% (i.e. approximately 20% residual activity) was obtained with 2,000 or more equivalents of H2O2. Further inactivation was only induced at much higher H2O2 concentrations. Spectral changes during incubations of up to 5 days showed the presence of a compound III-like species whose abundance was correlated to the level of resistance observed. Inactivation was pH dependent, the enzyme being much more sensitive under acid conditions. A partition ratio (r1 approximately equals 1,140 at pH 6.5) between inactivation and catalysis was calculated from the data. The kinetics of inactivation followed single exponential time curves and were H2O2 concentration dependent. The apparent maximum rate constant of inactivation was lambdamax=3.56+/-0.07x10(-4)s(-1) and the H2O2 concentration required to give lambdamax/2 was K2=9.94+/-0.52 mM. The relationship lambdamax<ki has been shown to apply and thus the rate constant of inactivation has been calculated as ki=1.9x10(-3)s(-1). HRP-A2 possessed catalase-like oxygen gas-releasing activity, the catalytic constant being k3=2.2 s(-1), and the affinity for H2O2 as K2=23 mM. Catalase-like activity was pH dependent and favoured under more basic conditions. A mechanistic model has been developed and used to explain the behaviour of HRP-A2. The model suggests that, in common with HRP-C, mechanism-based (suicide) inactivation is being observed but that a fraction of the HRP-A2 is protected from inactivation in the form of a modified compound III species.