•Reaction products of tea catechins with acrolein were analyzed.•One molecule of tea catechins trapped up to three acrolein molecules.•The mono-acrolein-epigallocatechin gallate conjugate structure ...was determined.•Green tea powder suppressed the formation of toxic aldehydes during cake baking.•Aldehyde-detoxifying ability of green tea powder is a practical food functionality.
Lipid peroxidation-derived reactive carbonyl species (RCS) such as acrolein and 4-hydroxynonenal pose health risks. We characterized the RCS-scavenging reactions of tea catechins in an aqueous solution and in baked cake. Acrolein’s reaction with each of the major tea catechins (epicatechin, epigallocatechin, epicatechin gallate, and epigallocatechin gallate) resulted in the formation of mono-, di-, and tri-acrolein conjugates of each catechin as revealed by our LC-linear ion trap MS analysis. The formation of the acrolein-conjugates of the four catechins was confirmed in the reaction of acrolein with green tea powder (matcha) extract. The addition of matcha tea powder to cake dough significantly suppressed the accumulation of RCS during cake baking. The mono-acrolein conjugates of the four major catechins were detected in the baked cake. The RCS-scavenging capability of tea catechins offers a new functionality of matcha tea powder, and its heat stability demonstrates the usefulness of matcha as a food additive.
Almost all terrestrial plants produce green leaf volatiles (GLVs), consisting of six-carbon (C6) aldehydes, alcohols and their esters, after mechanical wounding. C6 aldehydes deter enemies, but C6 ...alcohols and esters are rather inert. In this study, we address why the ability to produce various GLVs in wounded plant tissues has been conserved in the plant kingdom. The major product in completely disrupted Arabidopsis leaf tissues was (Z)-3-hexenal, while (Z)-3-hexenol and (Z)-3-hexenyl acetate were the main products formed in the intact parts of partially wounded leaves. (13)C-labeled C6 aldehydes placed on the disrupted part of a wounded leaf diffused into neighboring intact tissues and were reduced to C6 alcohols. The reduction of the aldehydes to alcohols was catalyzed by an NADPH-dependent reductase. When NADPH was supplemented to disrupted tissues, C6 aldehydes were reduced to C6 alcohols, indicating that C6 aldehydes accumulated because of insufficient NADPH. When the leaves were exposed to higher doses of C6 aldehydes, however, a substantial fraction of C6 aldehydes persisted in the leaves and damaged them, indicating potential toxicity of C6 aldehydes to the leaf cells. Thus, the production of C6 aldehydes and their differential metabolisms in wounded leaves has dual benefits. In disrupted tissues, C6 aldehydes and their α,β-unsaturated aldehyde derivatives accumulate to deter invaders. In intact cells, the aldehydes are reduced to minimize self-toxicity and allow healthy cells to survive. The metabolism of GLVs is thus efficiently designed to meet ecophysiological requirements of the microenvironments within a wounded leaf.
Accumulation of lipid peroxide-derived aldehydes and ketones is a ubiquitous event in oxidative stress. The toxicity of these carbonyls, especially the α,β-unsaturated carbonyls (reactive carbonyls; ...RCS), in environmental-stressed plants has been demonstrated by several independent research groups, on the basis of the results that overexpression of different carbonyl-detoxifying enzymes commonly improved tolerance of the transgenic plants against environmental stresses. A positive correlation between the level of carbonyls and the stress-induced damage in these plants proves the cause–effect relationship between carbonyls and the cell injury. Comprehensive analysis of carbonyls has revealed that dozens of distinct RCS including highly toxic acrolein and 4-hydroxy-2-nonenal are contained at nmol/g fresh weight levels in the tissues of non-stressed plants. Stress treatments of plants increase the levels of these RCS, likely reaching a sub-mM order, but in the transgenic plants overproducing RCS-detoxifying enzymes, their increase is significantly suppressed. Immunological analyses have demonstrated that in non-stressed cells several proteins are modified by RCS and the extent of modification is increased on stresses. In heat-stressed leaves, the inactivation of the oxygen-evolving complex was associated with selective modification of OEC33 protein and photosystem II core proteins. RCS consume glutathione and inactivate various enzymes in chloroplasts and mitochondria, thereby accelerating oxidative stress status. Thus RCS, formed downstream of reactive oxygen species (ROS), act in a way biochemically distinct from that of ROS and play critical roles in the plant responses to oxidative stress.
► Reactive carbonyl species (RCS), products of lipid peroxides, are strong cytotoxins. ► Multiple enzymes altogether contribute to the detoxification of vast variety of RCS. ► RCS-detoxifying enzymes protects plants from various environmental stresses. ► RCS occur at μM levels in cells, and may rise to sub-mM under environmental stress. ► Limited number of proteins/enzymes are sensitively modified and inactivated by RCS.
Lipid peroxide‐derived reactive carbonyl species (RCS), generated downstream of reactive oxygen species (ROS), are critical damage‐inducing species in plant aluminum (Al) toxicity. In mammals, RCS ...are scavenged primarily by glutathione (reduced form of glutathione, GSH), but in plant Al stress, contribution of GSH to RCS detoxification has not been evaluated. In this study, Arabidopsis plants overexpressing the gene AtGR1 (accession code At3g24170), encoding glutathione reductase (GR), were generated, and their performance under Al stress was examined. These transgenic plants (GR‐OE plants) showed higher GSH levels and GSH/GSSG (oxidized form of GSH) ratio, and an improved Al tolerance as they suffered less inhibition of root growth than wild‐type under Al stress. Exogenous application of 4‐hydroxy‐2‐nonenal, an RCS responsible for Al toxicity in roots, markedly inhibited root growth in wild‐type plants. GR‐OE plants suffered significantly smaller inhibition, indicating that the enhanced GSH level increased the capacity of RCS detoxification. The generation of H2O2 due to Al stress in GR‐OE plants was lower by 26% than in wild‐type. Levels of various RCS, such as malondialdehyde, butyraldehyde, phenylacetaldehyde, (E)‐2‐heptenal and n‐octanal, were suppressed by more than 50%. These results indicate that high levels of GSH and GSH/GSSG ratio by GR overexpression contributed to the suppression of not only ROS, but also RCS. Thus, the maintenance of GSH level by overexpressing GR reinforces dual detoxification functions in plants and is an efficient approach to enhance Al tolerance.
Summary
In auxin‐stimulated roots, production of reactive oxygen species (ROS) via the hormone‐induced activation of respiratory burst oxidase homologous NADPH oxidases facilitates lateral root (LR) ...formation. In this study, in order to verify that ROS can modulate auxin signaling, we examined the involvement of the lipid peroxide‐derived agents known as reactive carbonyl species (RCS) in LR formation. When auxin was added to Arabidopsis thaliana roots, the levels of RCS, for example acrolein, 4‐hydroxynonenal and crotonaldehyde, were increased prior to LR formation. Addition of the carbonyl scavenger carnosine suppressed auxin‐induced LR formation. Addition of RCS to the roots induced the expression of the auxin‐responsive DR5 promoter and the TIR1, IAA14, ARF7, LBD16 and PUCHI genes and facilitated LR formation without increasing the endogenous auxin level. DR5 and LBD16 were activated in the LR primordia. The auxin signaling‐deficient mutants arf7 arf19 and slr‐1 did not respond – and tir1 afb2 appeared to show a poor response – to RCS. When given to the roots RCS promoted the disappearance of the AXR3NT–GUS fusion protein, i.e. the degradation of the auxin/indole‐3‐acetic acid protein, as did auxin. These results indicate that the auxin‐induced production of ROS and their downstream products RCS modulate the auxin signaling pathway in a feed‐forward manner. RCS are key agents that connect the ROS signaling and the auxin signaling pathways.
Significance Statement
Auxin induces the formation of reactive oxygen species (ROS), which promote lateral root (LR) formation. We show that ROS and lipid peroxide‐derived reactive carbonyl species (RCS), signal mediators downstream of ROS, facilitate the degradation of the auxin/indole‐3‐acetic acid repressor, and thereby enforce the auxin signal for LR formation. Specifically, ROS and RCS constitute a feed‐forward pathway to modulate the auxin signaling for LR formation. The RCS provide a connection between the ROS signal and auxin signaling pathways.
Oxidative injury of the root elongation zone is a primary event in aluminum (Al) toxicity in plants, but the injuring species remain unidentified. We verified the hypothesis that lipid ...peroxide-derived aldehydes, especially highly electrophilic α,β-unsaturated aldehydes (2-alkenals), participate in Al toxicity. Transgenic tobacco (Nicotiana tabacum) overexpressing Arabidopsis (Arabidopsis thaliana) 2-alkenal reductase (AER-OE plants), wild-type SR1, and an empty vector-transformed control line (SR-Vec) were exposed to AlCl₃ on their roots. Compared with the two controls, AER-OE plants suffered less retardation of root elongation under AlCl₃ treatment and showed more rapid regrowth of roots upon Al removal. Under AlCl₃ treatment, the roots of AER-OE plants accumulated Al and H₂O₂ to the same levels as did the sensitive controls, while they accumulated lower levels of aldehydes and suffered less cell death than SR1 and SR-Vec roots. In SR1 roots, AlCl₃ treatment markedly increased the contents of the highly reactive 2-alkenals acrolein, 4-hydroxy-(E)-2-hexenal, and 4-hydroxy-(E)-2-nonenal and other aldehydes such as malondialdehyde and formaldehyde. In AER-OE roots, accumulation of these aldehydes was significantly less. Growth of the roots exposed to 4-hydroxy-(E)-2-nonenal and (E)-2-hexenal were retarded more in SR1 than in AER-OE plants. Thus, the lipid peroxide-derived aldehydes, formed downstream of reactive oxygen species, injured root cells directly. Their suppression by AER provides a new defense mechanism against Al toxicity.
SUMMARY
The Arabidopsis thaliana aldehyde oxidase 3 (AAO3) catalyzes the oxidation of abscisic aldehyde (ABal) to abscisic acid (ABA). Besides ABal, plants generate other aldehydes that can be toxic ...above a certain threshold. AAO3 knockout mutants (aao3) exhibited earlier senescence but equivalent relative water content compared with wild‐type (WT) during normal growth or upon application of UV‐C irradiation. Aldehyde profiling in leaves of 24‐day‐old plants revealed higher accumulation of acrolein, crotonaldehyde, 3Z‐hexenal, hexanal and acetaldehyde in aao3 mutants compared with WT leaves. Similarly, higher levels of acrolein, benzaldehyde, crotonaldehyde, propionaldehyde, trans‐2‐hexenal and acetaldehyde were accumulated in aao3 mutants upon UV‐C irradiation. Aldehydes application to plants hastened profuse senescence symptoms and higher accumulation of aldehydes, such as acrolein, benzaldehyde and 4‐hydroxy‐2‐nonenal, in aao3 mutant leaves as compared with WT. The senescence symptoms included greater decrease in chlorophyll content and increase in transcript expression of the early senescence marker genes, Senescence‐Related‐Gene1, Stay‐Green‐Protein2 as well as NAC‐LIKE, ACTIVATED‐BY AP3/P1. Notably, although aao3 had lower ABA content than WT, members of the ABA‐responding genes SnRKs were expressed at similar levels in aao3 and WT. Moreover, the other ABA‐deficient mutants aba2 and 9‐cis‐poxycarotenoid dioxygenase3‐2 (nced3‐2), that has functional AAO3 exhibited similar aldehydes accumulation and chlorophyll content like WT under normal growth conditions or UV‐C irradiation. These results indicate that the absence of AAO3 oxidation activity and not the lower ABA and its associated function is responsible for the earlier senescence symptoms in aao3 mutant.
Significance Statement
The current study demonstrates that AAO3, known to catalyze the last step of ABA biosynthesis by oxidizing ABal, is essential in preventing leaf damage by oxidizing toxic levels of aldehydes in rosette leaves. We provided credible evidences that premature senescence in this study was not because of low ABA content and its consequential function in aao3, but rather due to higher aldehyde accumulation, which is caused by AAO3 deficiency.
In plants, the galactolipids monogalactosyldiacylglycerol (MGDG) and digalactodiacylglycerol (DGDG) are major constituents of photosynthetic membranes in chloroplasts. One of the key enzymes for the ...biosynthesis of these galactolipids is MGDG synthase (MGD). To investigate the role of MGD in the plant's response to salt stress, we cloned an MGD gene from rice (Oryza sativa) and generated tobacco (Nicotiana tabacum) plants overexpressing OsMGD. The MGD activity in OsMGD transgenic plants was confirmed to be higher than that in the wild-type tobacco cultivar SRI. Immunoblot analysis indicated that OsMGD was enriched in the outer envelope membrane of the tobacco chloroplast. Under salt stress, the transgenic plants exhibited rapid shoot growth and high photosynthetic rate as compared with the wild type. Transmission electron microscopy observation showed that the chloroplasts from salt-stressed transgenic plants had well-developed thylakoid membranes and properly stacked grana lamellae, whereas the chloroplasts from salt-stressed wild-type plants were fairly disorganized and had large membrane-free areas. Under salt stress, the transgenic plants also maintained higher chlorophyll levels. Lipid composition analysis showed that leaves of transgenic plants consistently contained significantly higher MGDG (including 18: 3-16: 3 and 18: 3-18: 3 species) and DGDG (including 18:3-16:3, 18:3-16:0, and 18:3-18:3 species) contents and higher DGDG-MGDG ratios than the wild type did under both control and salt stress conditions. These results show that overexpression of OsMGD improves salt tolerance in tobacco and that the galactolipids MGDG and DGDG play an important role in the regulation of chloroplast structure and function in the plant salt stress response.
We have demonstrated that reactive carbonyl species (RCS) function as an intermediate downstream of hydrogen peroxide (H2O2) production during abscisic acid (ABA) signaling in guard cells using ...transgenic tobacco plants overexpressing alkenal reductase. We investigated the conversion of the RCS production into downstream signaling events in the guard cells. Both ABA and H2O2 induced production of the RCS, such as acrolein and 4-hydroxy-(E)-2-nonenal, in epidermal tissues of wild-type Arabidopsis thaliana plants. Application of the RCS scavengers, carnosine and pyridoxamine, did not affect the ABA-induced H2O2 production but inhibited the ABA- and H2O2-induced stomatal closure. Both acrolein and 4-hydroxy-(E)-2-nonenal induced stomatal closure in a plasma membrane NAD(P)H oxidase mutant atrbohD atrbohF as well as in the wild type, but not in a calcium-dependent kinase mutant cpk6. Acrolein activated plasma membrane Ca2+-permeable cation (ICa) channels, triggered cytosolic free Ca2+ concentration (Ca2+cyt) elevation, and induced stomatal closure accompanied by depletion of glutathione in the guard cells. These results suggest that RCS production is a signaling event between the ROS production and Ca2+cyt elevation during guard cell ABA signaling.
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