Phytochrome-driven changes in respiratory electron transport partitioning in soybean (Glycine max. L.) cotyledons

Light signals generated by vegetation shade facilitate acclimation to low light in shade-avoider plants

When growing in search for light, plants can experience continuous or occasional shading by other plants. Plant proximity causes a decrease in the ratio of R to far-red light (low R:FR) due to the preferential absorbance of R light and reflection of FR light by photosynthetic tissues of neighboring plants. This signal is often perceived before actual shading causes a reduction in photosynthetically active radiation (low PAR). Here, we investigated how several Brassicaceae species from different habitats respond to low R:FR and low PAR in terms of elongation, photosynthesis, and photoacclimation. Shade-tolerant plants such as hairy bittercress (Cardamine hirsuta) displayed a good adaptation to low PAR but a poor or null response to low R:FR exposure. In contrast, shade-avoider species, such as Arabidopsis (Arabidopsis thaliana), showed a weak photosynthetic performance under low PAR but they strongly elongated when exposed to low R:FR. These responses could be genetically uncoupled. Most interestingly, exposure to low R:FR of shade-avoider (but not shade-tolerant) plants improved their photoacclimation to low PAR by triggering changes in photosynthesis-related gene expression, pigment accumulation, and chloroplast ultrastructure. These results indicate that low R:FR signaling unleashes molecular, metabolic, and developmental responses that allow shade-avoider plants (including most crops) to adjust their photosynthetic capacity in anticipation of eventual shading by nearby plants.

Involvement of nitric oxide-mediated alternative pathway in tolerance of wheat to drought stress by optimizing photosynthesis

KEY MESSAGE

NO-mediated alternative pathway plays an important role in protecting wheat seedlings against drought stress through dissipating excessive reducing equivalents generated by photosynthesis. Alternative pathway (AP) has been proven to be involved in responses to various stresses. However, the mechanisms of AP in defense response to drought stress are still lacking. The aims of this work are to investigate the role of AP in drought tolerance and how AP is induced under drought stress using two wheat cultivars with different drought tolerance. Our results showed that Longchun22 cultivar is more tolerant to drought than 98SN146 cultivar. Seedlings exposed to drought led to a significant increase in AP, and it increased more in Longchun22. Furthermore, chlorophyll fluorescence parameters (Fv/Fm, ΦPSII, qP) decreased significantly in drought-treated seedlings, especially in 98SN146, indicating that photoinhibition occurred under drought stress. Pretreatment with SHAM, the malate-oxaloacetate shuttle activity and photosynthetic efficiency were further inhibited in drought-treated seedlings, resulting in more serious oxidative damage as indicated by higher levels of malondialdehyde and hydrogen peroxide. Moreover, NO modulated AP under drought stress by increasing AOX1a expression and pyruvate content. Taken together, these results indicate that NO-mediated AP is involved in optimizing photosynthesis under drought stress by avoiding the over-reduction of photosynthetic electron transport chain, thus reducing reactive oxygen species production and oxidative damage in wheat leaves.

Phytochrome-mediated regulation of plant respiration and photorespiration

The expression of genes encoding various enzymes participating in photosynthetic and respiratory metabolism is regulated by light via the phytochrome system. While many photosynthetic, photorespiratory and some respiratory enzymes, such as the rotenone-insensitive NADH and NADPH dehydrogenases and the alternative oxidase, are stimulated by light, succinate dehydrogenase, subunits of the pyruvate dehydrogenase complex, cytochrome oxidase and fumarase are inhibited via the phytochrome mechanism. The effect of light, therefore, imposes limitations on the tricarboxylic acid cycle and on the mitochondrial electron transport coupled to ATP synthesis, while the non-coupled pathways become activated. Phytochrome-mediated regulation of gene expression also creates characteristic distribution patterns of photosynthetic, photorespiratory and respiratory enzymes across the leaf generating different populations of mitochondria, either enriched by glycine decarboxylase (in the upper part) or by succinate dehydrogenase (in the bottom part of the leaf).

In vivo cytochrome and alternative pathway respiration in leaves of Arabidopsis thaliana plants with altered alternative oxidase under different light conditions

The in vivo activity of the alternative pathway (ν(alt)) has been studied using the oxygen isotope fractionation method in leaves of Arabidopsis thaliana modified for the expression of the AtAOX1a gene by anti-sense (AS-12) or overexpression (XX-2). Under non-stressful conditions, ν(alt) was similar in all plant lines regardless of its different alternative pathway capacities (V(alt)). Total leaf respiration (V(t)) and V(alt) were directly related to growth light conditions while electron partitioning between the cytochrome pathway (CP) and alternative pathway (AP) was unchanged by light levels. Interestingly, the AP functioned at full capacity in anti-sense plants under both growth light conditions. The role of the AP in response to a high light stress induced by short-term high light treatment (HLT) was also studied. In wild type and XX-2, both CP and AP rates increased proportionally after HLT while in AS-12, where the AP was unable to increase its rate, the CP accommodated all the increase in respiration. The results obtained under high light stress suggest that flexibility in the response of the mitochondrial electron transport chain is involved in sustaining photosynthetic rates in response to this stress while the saturated AP in AS-12 plants may contribute to the observed increase in photoinhibition.

Seasonal variation of leaf respiration and the alternative pathway in field-grown Populus × canadensis

The temperature response of plant respiration varies between species and can acclimate to changing temperatures. Mitochondrial respiration in plants has two terminal oxidases: the cytochrome c oxidase (COX) and the cyanide-resistant alternative oxidase (AOX). In Populus × canadensis var. italica, a deciduous tree species, we investigated the temperature response of leaf respiration via the alternative and cytochrome pathways, as well as seasonal changes in these pathways, using the oxygen isotope fractionation technique. The electron partitioning through the alternative pathway (τ(a) ) increased from 0 to 30-40% with measurement temperatures from 6 to 30°C at all times measured throughout the growing season. τ(a) at the growth temperature (the average temperature during 3 days prior to sampling) increased from 12 to 29% from spring until late summer and decreased thereafter. Total respiration declined throughout the growing season by 50%, concomitantly with decreases in both AOX (64%) and COX (32%) protein abundances. Our results provide new insight into the natural variability of AOX protein abundances and alternative respiration electron partitioning over immediate and seasonal timescales.

Leaf respiration and alternative oxidase in field-grown alpine grasses respond to natural changes in temperature and light

• We report the first investigation of changes in electron partitioning via the alternative respiratory pathway (AP) and alternative oxidase (AOX) protein abundance in field-grown plants and their role in seasonal acclimation of respiration. • We sampled two alpine grasses native to New Zealand, Chionochloa rubra and Chionochloa pallens, from field sites of different altitudes, over 1 yr and also intensively over a 2-wk period. • In both species, respiration acclimated to seasonal changes in temperature through changes in basal capacity (R₁₀) but not temperature sensitivity (E₀). In C. pallens, acclimation of respiration may be associated with a higher AOX : cytochrome c oxidase (COX) protein abundance ratio. Oxygen isotope discrimination (D), which reflects relative changes in AP electron partitioning, correlated positively with daily integrated photosynthetically active radiation (PAR) in both species over seasonal timescales. Respiratory parameters, the AOX : COX protein ratio and D were stable over a 2-wk period, during which significant temperature changes were experienced in the field. • We conclude that respiration in Chionochloa spp. acclimates strongly to seasonal, but not to short-term, temperature variation. Alternative oxidase appears to be involved in the plant response to both seasonal changes in temperature and daily changes in light, highlighting the complexity of the function of AOX in the field.

Changes of alternative oxidase activity, capacity and protein content in leaves of Cucumis sativus wild-type and MSC16 mutant grown under different light intensities

In vitro studies demonstrated that alternative oxidase (AOX) is biochemically regulated by a sulfhydryl-disulfide system, interaction with alpha-ketoacids, ubiquinone pool redox state and protein content among others. However, there is still scarce information about the in vivo regulation of the AOX. Cucumis sativus wild-type (WT) and MSC16 mutant plants were grown under two different light intensities and were used to analyze the relationship between the amount of leaf AOX protein and its in vivo capacity and activity at night and day periods. WT and MSC16 plants presented lower total respiration (V(t)), cytochrome oxidase pathway (COP) activity (v(cyt)) and alternative oxidase pathway (AOP) activity (v(alt)) when grown at low light (LL), although growth light intensity did not change the amount of cytochrome oxidase (COX) nor AOX protein. Changes of v(cyt) related to growing light conditions suggested a substrate availability and energy demand control. On the other hand, the total amount of AOX protein present in the tissue does not play a role in the regulation neither of the capacity nor of the activity of the AOP in vivo. Soluble carbohydrates were directly related to the activity of the AOP. However, although differences in soluble sugar contents mostly regulate the capacity of the AOP at different growth light intensities, additional regulatory mechanisms are necessary to fully explain the observed results.