Developmental acclimation of the thylakoid proteome to light intensity in Arabidopsis

Photosynthetic acclimation, the ability to adjust the composition of the thylakoid membrane to optimise the efficiency of electron transfer to the prevailing light conditions, is crucial to plant fitness in the field. While much is known about photosynthetic acclimation in Arabidopsis, to date there has been no study that combines both quantitative label-free proteomics and photosynthetic analysis by gas exchange, chlorophyll fluorescence and P700 absorption spectroscopy. Using these methods we investigated how the levels of 402 thylakoid proteins, including many regulatory proteins not previously quantified, varied upon long-term (weeks) acclimation of Arabidopsis to low (LL), moderate (ML) and high (HL) growth light intensity and correlated these with key photosynthetic parameters. We show that changes in the relative abundance of cytb₆f, ATP synthase, FNR2, TIC62 and PGR6 positively correlate with changes in estimated PSII electron transfer rate and CO₂ assimilation. Improved photosynthetic capacity in HL grown plants is paralleled by increased cyclic electron transport, which positively correlated with NDH, PGRL1, FNR1, FNR2 and TIC62, although not PGR5 abundance. The photoprotective acclimation strategy was also contrasting, with LL plants favouring slowly reversible non-photochemical quenching (qI), which positively correlated with LCNP, while HL plants favoured rapidly reversible quenching (qE), which positively correlated with PSBS. The long-term adjustment of thylakoid membrane grana diameter positively correlated with LHCII levels, while grana stacking negatively correlated with CURT1 and RIQ protein abundance. The data provide insights into how Arabidopsis tunes photosynthetic electron transfer and its regulation during developmental acclimation to light intensity.     

Controlled levels of salicylic acid are required for optimal photosynthesis and redox homeostasis

Sudden exposure of plants to high light (HL) leads to metabolic and physiological disruption of the photosynthetic cells. Changes in ROS content, adjustment of photosynthetic processes and the antioxidant pools and, ultimately, gene induction are essential components for a successful acclimation to the new light conditions. The influence of salicylic acid (SA) on plant growth, short-term acclimation to HL, and on the redox homeostasis of Arabidopsis thaliana leaves was assessed here. The dwarf phenotype displayed by mutants with high SA content (cpr1-1, cpr5-1, cpr6-1, and dnd1-1) was less pronounced when these plants were grown in HL, suggesting that the inhibitory effect of SA on growth was partly overcome at higher light intensities. Moreover, higher SA content affected energy conversion processes in low light, but did not impair short-term acclimation to HL. On the other hand, mutants with low foliar SA content (NahG and sid2-2) were impaired in acclimation to transient exposure to HL and thus predisposed to oxidative stress. Low and high SA levels were strictly correlated to a lower and higher foliar H(2)O(2) content, respectively. Furthermore high SA was also associated with higher GSH contents, suggesting a tight correlation between SA, H(2)O(2) and GSH contents in plants. These observations implied an essential role of SA in the acclimation processes and in regulating the redox homeostasis of the cell. Implications for the role of SA in pathogen defence signalling are also discussed.     

Distinct responses of the mitochondrial respiratory chain to long- and short-term high-light environments in Arabidopsis thaliana

In order to ensure the cooperative function with the photosynthetic system, the mitochondrial respiratory chain needs to flexibly acclimate to a fluctuating light environment. The non-phosphorylating alternative oxidase (AOX) is a notable respiratory component that may support a cellular redox homeostasis under high-light (HL) conditions. Here we report the distinct acclimatory manner of the respiratory chain to long- and short-term HL conditions and the crucial function of AOX in Arabidopsis thaliana leaves. Plants grown under HL conditions (HL plants) possessed a larger ubiquinone (UQ) pool and a higher amount of cytochrome c oxidase than plants grown under low light conditions (LL plants). These responses in HL plants may be functional for efficient ATP production and sustain the fast plant growth. When LL plants were exposed to short-term HL stress (sHL), the UQ reduction level was transiently elevated. In the wild-type plant, the UQ pool was re-oxidized concomitantly with an up-regulation of AOX. On the other hand, the UQ reduction level of the AOX-deficient aox1a mutant remained high. Furthermore, the plastoquinone pool was also more reduced in the aox1a mutant under such conditions. These results suggest that AOX plays an important role in rapid acclimation of the respiratory chain to sHL, which may support efficient photosynthetic performance.     

Synergistic effects of abiotic stresses in plants: a case study of nitrogen limitation and saturating light intensity in Arabidopsis thaliana

Under natural conditions, plants are regularly exposed to combinations of stress factors. A common example is the conjunction between nitrogen (N) deficiency and excess light. The combined effect of stress factors is often ignored in studies using controlled conditions, possibly resulting in misleading conclusions. To address this issue, the present study examined the physiological behavior of Arabidopsis thaliana under the effect of varying nitrogen levels and light intensities. The joint influence of low N and excess light had an adverse effect on plant growth, chlorophyll and anthocyanin concentrations, photochemical capacity and the abundance of proteins involved in carbon assimilation and antioxidative metabolism. In contrast, no adverse physiological responses were observed for plants under either nitrogen limitation or high light (HL) intensity conditions (i.e. single stress). The underlying mechanisms for the increased growth in conditions of HL and sufficient nitrogen were a combination of chlorophyll accumulation and an increased number of proteins involved in C3 carbon assimilation, amino acids biosynthesis and chloroplast development. In contrast, combined stress conditions shifts plants from growth to survival by displaying anthocyanin accumulation and an increased number of proteins involved in catabolism of lipids and amino acids as energy substrates. Ultimately switching plants development from growth to survival. Our results suggest that an assessment of the physiological response to the combined effect of multiple stresses cannot be directly extrapolated from the physiological response to a single stress. Specifically, the synergistic interaction between N deficiency and saturating light in Arabidopsis plants could not have been modeled via only one of the stress factors.     

<Emphasis Type="Italic">MAX4</Emphasis> gene is involved in the regulation of low inorganic phosphate stress responses in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

MAX4 gene has been shown to be involved in the regulation of shoot branching in Arabidopsis (Arabidopsis thaliana). However, little is known about the role of MAX4 gene in low inorganic phosphate (Pi) stress response in Arabidopsis. Here we showed that MAX4 gene is involved in the regulation of low Pi stress response in Arabidopsis. MAX4 gene was repressed by low Pi stress, and the max4 mutants showed lower anthocyanin content and longer primary root length. In addition, max4 mutant plants also displayed altered root architecture such as increased root-to-shoot ratio, lower lateral root number and root hair density compared with wild-type plants under low Pi stress. Higher total Pi contents were detected in shoots and roots of max4 plants than those of wild-type plants when subjected to low Pi stress, which was associated, at least in part, with increase in expression of WRKY75 as well as AtPT1 and AtPT2 genes encoding high-affinity Pi transporters. Taken together, all these results suggest that MAX4 gene mediates low Pi stress response, at least in part, by regulating the expression of WRKY75 as well as AtPT1 and AtPT2 genes.