L. Y. Jia

Salt stress-induced expression of rice AOX1a is mediated through an accumulation of hydrogen peroxide.

Treatment with 300 mM NaCl increased the capacity of the alternative respiratory pathway and induced the expression of AOX1a of the leaves of rice (Oryza sativa L.) seedlings. A significant increase in the content of hydrogen peroxide (H2O2) was observed in rice leaves treated with 300 mM NaCl. However, NaCl at 150 mM did not significantly affect the capacity of the alternative respiratory pathway, the content of H2O2, and the transcript level of AOX1a. Exogenous application of H2O2 enhanced the levels of the capacity of the alternative respiratory pathway and AOX1a expression. The accumulation of H2O2 in rice leaves in response to 300 mM NaCl was inhibited by the pretreatment with dimethylthiourea (DMTU, scavenger of H2O2). This treatment also suppressed the induction of AOX1a expression and the increase in the capacity of the alternative respiratory pathway under 300 mM NaCl stress. Moreover, the salt-stressed (300 mM NaCl) seedlings pretreated with 1 mM salicylhydroxamic acid (SHAM, a special inhibitor of alternative oxidase) had higher level of H2O2 production than the seedlings either subjected to 300 mM NaCl stress or SHAM treatment alone did. These observations suggest that the expression of AOX1a in response to higher salt stress is mediated through an accumulation of H2O2 and alternative oxidase could play a role in antioxidant protection under the condition of higher salt stress.

Extracellular ATP affects chlorophyll fluorescence of kidney bean (Phaseolus vulgaris) leaves through Ca2+ and H2O2-dependent mechanism

Extracellular ATP (eATP) has been considered as an important extracellular compound to mediate several physiological processes in plant cells. We investigated the effects of eATP on chlorophyll (Chl) fluorescence characteristics of kidney bean (Phaseolus vulgaris) leaves. Treatment with exogenous ATP at 1 mM showed no significant effect on the maximal photochemical efficiency of PSII. However, the treatment significantly enhanced the values of the PSII operating efficiency (ΦPSII), rate of photosynthetic electron transport through PSII (ETR), and photochemical quenching (qP), while the values of the nonphotochemical quenching (qN) and quantum yield of regulated energy dissipation of PSII (YNPQ) significantly decreased. Our observations indicated that eATP stimulated the PSII photochemistry in kidney bean leaves. Similarly, the treatment with exogenous Ca2+ or H2O2 at 1 mM caused also the significant increase in ΦPSII, qP, and ETR and the significant decrease in qN and YNPQ. LaCl3 (an inhibitor of Ca2+ channels) and dimethylthiourea (a scavenger of H2O2) abolished the effects of exogenous ATP. The results suggest that the role of eATP in enhancing the PSII photochemistry could be related to a Ca2+ or H2O2 signaling pathway.

Cyanide-resistant respiratory pathway is involved in the high-light systemic acquired acclimation of kidney bean (Phaseolus vulgaris)

After exposing one half of a low light-adapted kidney bean (Phaseolus vulgaris) leaf to high light, parameters of chlorophyll (Chl) a fluorescence, such as PSII operating efficiency, PSII maximum efficiency under light, and photochemical quenching, decreased in the opposite half of the same leaf, whereas the capacity of the cyanide-resistant respiratory pathway significantly increased. When one half of the low light-adapted leaf was exposed to low light, the opposite half pretreated with 1 mM salicylhydroxamic acid (SHAM, an inhibitor of the cyanide-resistant respiratory pathway) did not exhibit significant changes in the Chl fluorescence values compared with the without SHAM pretreatment. However, after exposing one half of the low light-adapted leaf to high light, the opposite half pretreated with 1 mM SHAM showed lower Chl fluorescence values than that without SHAM pretreatment. Our results indicate that partial exposure of the low light-adapted leaf to high light can impose a systemic stress on the PSII photochemistry. The enhanced capacity of the cyanide-resistant respiratory pathway may be involved in the maintenance of the photosynthetic performance in the leaf tissues experiencing high light-induced systemic stress.

Extracellular ATP alleviates the salicylic acid-induced inhibition of cell viability and respiration through a Ca2+-dependent mechanism

Extracellular ATP (eATP) has been considered as signalling compound to mediate several physiological processes. Here we show that eATP played a role in alleviating the salicylic acid (SA)-induced inhibition of cell viability and respiration in tobacco (Nicotiana tabacum L. cv. Bright Yellow-2) suspension cells. Exogenous SA at higher concentrations (0.05, 0.1, 0.3, 0.5, or 0.7 mM) caused a significant reduction in respiratory O2 uptake or cell viability. The addition of exogenous ATP alleviated the SA-induced reductions in cell viability and respiration, but the effect was dependent on the concentrations of either exogenous SA or ATP. Further study shows that the alleviative effects of exogenous ATP were abolished by the addition of GdCl3 (an inhibitor of Ca2+ channels) or EGTA (a Ca2+ chelator). These results suggest that this role of extracellular ATP could be related to a Ca2+ signalling pathway.

Role of cyanide-resistant respiration during light-induced attraction of predators to herbivore-infested leaves

The present work showed that spider mite-infested leaves placed in the light were more attractive to predatory mites than the infested leaves placed in the dark; furthermore, an increase in the light intensity enhanced this attractiveness. However, the increase of the light intensity did not change the attractiveness of the uninfested leaves to predatory mites. The capacity of cyanide-resistant respiration and the photosynthetic rates of both the infested and uninfested leaves increased with increasing light intensities, whereas the photosystem (PS) II chlorophyll (Chl) fluorescence decreased. The increase of the capacity of cyanide-resistant respiration in the infested leaves was more dramatic than that in the uninfested leaves, whereas the values of photosynthetic rates and Chl fluorescence were lower in the infested leaves than those in the uninfested leaves. Treatment of the infested and uninfested leaves with 1 mM salicylhydroxamic acid (SHAM, an inhibitor of cyanide-resistant respiration) decreased photosynthetic rates and caused further reductions in PSII fluorescence, especially under a higher light intensity. In contrast, the effects of SHAM on PSII fluorescence parameters and photosynthetic rates of the infested leaves were more dramatic than on those of the uninfested leaves. The treatment with SHAM did not significantly change the attractiveness of the infested or uninfested leaves to the predatory mites under all of the light intensities tested. These results indicated that cyanide-resistant respiration was not directly associated with the light-induced attraction of predators to plants, but it could play a role in the protection of photosynthesis. Such role might become relatively more important when photosynthesis is impaired by herbivores infestation.

Extracellular ATP released by copper stress could act as diffusible signal in alleviating the copper stress-induced cell death

In the present work, by using tobacco cell suspension and wheat seedlings, we studied that eATP (extracellular ATP) released by copper (Cu) stress could act as diffusible signal in alleviating the Cu stress-induced cell death. A semipermeable membrane was fixed in the middle of a plastic box to divide the box into two equal compartments (A and B, respectively). This semipermeable membrane can prevent direct cell-to-cell (or seedling-to-seedling) contact and the diffusion of the macromolecules [such as ATPase (adenosine 5′-triphosphatase)] between these two compartments. The cell suspension directly stressed with CuCl2 was placed in compartment A and was incubated with the untreated cell suspension in compartment B. Such treatment significantly increased the levels of cell death and eATP content of the cell suspension in these two compartments. In contrast, addition of ATPase into the cell suspension directly stressed with CuCl2 decreased the eATP level in these two compartments but further increased the level of cell death in compartment B, compared to no addition of ATPase. Similar results were obtained when tobacco cell suspension was replaced by wheat seedlings. These observations indicate that when Cu stress from compartment A induced the plant cell death in compartment B, ATP transferred from compartment A could play a role in alleviating this cell death. Thus, it is suggested that eATP released by copper stress could act as diffusible signal in alleviating the Cu stress-induced cell death.

H2O2-induced acclimation of photosystem II to excess light is mediated by alternative respiratory pathway and salicylic acid

Acclimation to excess light is required for optimizing plant performance under natural environment. The present work showed that the treatment of Arabidopsis leaves with exogenous H2O2 can increase the acclimation of PSII to excess light. Treatments with H2O2 also enhanced the capacity of the mitochondrial alternative respiratory pathway and salicylic acid (SA) content. Our work also showed that the lack in alternative oxidase (AOX1a) in AtAOX1a antisense line and the SA deficiency in NahG (salicylate hydroxylase gene) transgenic mutant attenuated the H2O2-induced acclimation of PSII to excess light. It indicates that the H2O2-induced acclimation of PSII to excess light could be mediated by the alternative respiratory pathway and SA.

Alternative oxidase functions in the excess excitation energy-induced resistance to pathogen infection

Although excess excitation energy (EEE) can damage the photosynthetic apparatus and deregulate many cellular processes, some studies have reported that EEE can be used by plants to optimize the resistance to pathogen infection. Here, we investigated whether the EEE-induced resistance to pathogen infection might be mediated by mitochondrial alternative oxidase (AOX). The present work showed that exposure of Arabidopsis (Arabidopsis thaliana) leaves to short-term excess light or treatment of Arabidopsis leaves with the electron transport inhibitor, DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone), restricted the growth of virulent biotrophic bacteria, Pseudomonas syringae pv tomato (Pst) DC3000. Exposure to short-term excess light or DBMIB treatment also enhanced the level of the capacity of the cyanide-resistant respiration of the infected leaves. The lack of AOX1a gene in the AtAOX1a anti-sense line attenuated the excess light- or DBMIB-induced resistance toward Pst DC3000. These results indicate that AOX may play a role in regulating the EEE-induced resistance to virulent pathogen infection.

Changes by cadmium stress in lipid peroxidation and activities of lipoxygenase and antioxidant enzymes in Arabidopsis are associated with extracellular ATP

Abstract Exposure of Arabidopsis leaves to cadmium (Cd) stress led to an increase in the level of extracellular ATP (eATP). And, Cd stress also caused significant increases in lipid peroxidation and the activities of lipoxygenase (LOX) and antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POX), and catalase (CAT). Treatment of Arabidopsis leaves with β,γ-methyleneadenosine 5’-triphosphate (AMP-PCP), which serves as a competitive inhibitor of eATP, dampened the Cd-induced increases of lipid peroxidation and the activities of LOX and antioxidant enzymes. Under Cd stress, the leaves of dorn 1-3 mutant showed lower levels of lipid peroxidation and activities of LOX and antioxidant enzymes compared with those of wild type plants. These results suggest that the changes in lipid peroxidation and activities of LOX and antioxidant enzymes under Cd stress could be associated with eATP.

Extracellular ATP is Involved in the Salicylic Acid-Induced Cell Death in Suspension-Cultured Tobacco Cells

Abstract Extracellular ATP (eATP) can function as a signaling molecule to regulate a wide range of cellular processes. We investigated the regulatory role of eATP in the cell death induced by salicylic acid (SA) in suspension-cultured tobacco (Nicotiana tabacum L.) cells. Treatment of tobacco suspension cells with SA induced cell death. The same treatment lowered the levels of eATP, accompanied by a decrease of both the respiratory O2 uptake and intracellular ATP levels in tobacco suspension cells. Treatment with β,γ-methyleneadenosine 5’-triphosphate (AMP-PCP), which is the non-hydrolysable analogue of ATP and can exclude eATP from binding sites of eATP receptors, also induced cell death in tobacco cell cultures. Treatment with exogenous ATP partially alleviated the cell death induced by SA. These observations suggest that eATP is involved in the SA-induced cell death in tobacco cell cultures.