Recovery of photoinactivated photosystem II in leaves: retardation due to restricted mobility of photosystem II in the thylakoid membrane

Photosynthesis on the edge: photoinhibition, desiccation and freezing tolerance of Antarctic bryophytes

In Antarctica, multiple stresses (low temperatures, drought and excessive irradiance) hamper photosynthesis even in summer. We hypothesize that controlled inactivation of PSII reaction centres, a mechanism widely studied by pioneer work of Fred Chow and co-workers, may effectively guarantee functional photosynthesis under these conditions. Thus, we analysed the energy partitioning through photosystems in response to temperature in 15 bryophyte species presenting different worldwide distributions but all growing in Livingston Island, under controlled and field conditions. We additionally tested their tolerance to desiccation and freezing and compared those with their capability for sexual reproduction in Antarctica (as a proxy to overall fitness). Under field conditions, when irradiance rules air temperature by the warming of shoots (up to 20 °C under sunny days), a predominance of sustained photoinhibition beyond dynamic heat dissipation was observed at low temperatures. Antarctic endemic and polar species showed the largest increases of photoinhibition at low temperatures. On the contrary, the variation of thermal dissipation with temperature was not linked to species distribution. Instead, maximum non-photochemical quenching at 20 °C was related (strongly and positively) with desiccation tolerance, which also correlated with fertility in Antarctica, but not with freezing tolerance. Although all the analysed species tolerated - 20 °C when dry, the tolerance to freezing in hydrated state ranged from the exceptional ability of Schistidium rivulare (that survived for 14 months at - 80 °C) to the susceptibility of Bryum pseudotriquetrum (that died after 1 day at - 20 °C unless being desiccated before freezing).

In vivo regulation of thylakoid proton motive force in immature leaves

In chloroplast, proton motive force (pmf) is critical for ATP synthesis and photoprotection. To prevent photoinhibition of photosynthetic apparatus, proton gradient (ΔpH) across the thylakoid membranes needs to be built up to minimize the production of reactive oxygen species (ROS) in thylakoid membranes. However, the regulation of thylakoid pmf in immature leaves is little known. In this study, we compared photosynthetic electron sinks, P700 redox state, non-photochemical quenching (NPQ), and electrochromic shift (ECS) signal in immature and mature leaves of a cultivar of Camellia. The immature leaves displayed lower linear electron flow and cyclic electron flow, but higher levels of NPQ and P700 oxidation ratio under high light. Meanwhile, we found that pmf and ΔpH were higher in the immature leaves. Furthermore, the immature leaves showed significantly lower thylakoid proton conductivity than mature leaves. These results strongly indicated that immature leaves can build up enough ΔpH by modulating proton efflux from the lumenal side to the stromal side of thylakoid membranes, which is essential to prevent photoinhibition via thermal energy dissipation and photosynthetic control of electron transfer. This study highlights that the activity of chloroplast ATP synthase is a key safety valve for photoprotection in immature leaves.

Multiple roles of oxygen in the photoinactivation and dynamic repair of Photosystem II in spinach leaves

Oxygen effects have long been ambiguous: exacerbating, being indifferent to, or ameliorating the net photoinactivation of Photosystem II (PS II). We scrutinized the time course of PS II photoinactivation (characterized by rate coefficient k i) in the absence of repair, or when recovery (characterized by k r) occurred simultaneously in CO2 ± O2. Oxygen exacerbated photoinactivation per se, but alleviated it by mediating the utilization of electrons. With repair permitted, the gradual net loss of functional PS II during illumination of leaves was better described phenomenologically by introducing τ, the time for an initial k r to decrease by half. At 1500 μmol photons m(-2) s(-1), oxygen decreased the initial k r but increased τ. Similarly, at even higher irradiance in air, there was a further decrease in the initial k r and increase in τ. These observations are consistent with an empirical model that (1) oxygen increased k i via oxidative stress but decreased it by mediating the utilization of electrons; and (2) reactive oxygen species stimulated the degradation of photodamaged D1 protein in PS II (characterized by k d), but inhibited the de novo synthesis of D1 (characterized by k s), and that the balance between these effects determines the net effect of O2 on PS II functionality.

Comparative genomic analysis of transgenic poplar dwarf mutant reveals numerous differentially expressed genes involved in energy flow

In our previous research, the Tamarix androssowii LEA gene (Tamarix androssowii late embryogenesis abundant protein Mrna, GenBank ID: DQ663481) was transferred into Populus simonii × Populus nigra. Among the eleven transgenic lines, one exhibited a dwarf phenotype compared to the wild type and other transgenic lines, named dwf1. To uncover the mechanisms underlying this phenotype, digital gene expression libraries were produced from dwf1, wild-type, and other normal transgenic lines, XL-5 and XL-6. Gene expression profile analysis indicated that dwf1 had a unique gene expression pattern in comparison to the other two transgenic lines. Finally, a total of 1246 dwf1-unique differentially expressed genes were identified. These genes were further subjected to gene ontology and pathway analysis. Results indicated that photosynthesis and carbohydrate metabolism related genes were significantly affected. In addition, many transcription factors genes were also differentially expressed in dwf1. These various differentially expressed genes may be critical for dwarf mutant formation; thus, the findings presented here might provide insight for our understanding of the mechanisms of tree growth and development.

Mobility of photosynthetic proteins

The mobility of photosynthetic proteins represents an important factor that affects light-energy conversion in photosynthesis. The specific feature of photosynthetic proteins mobility can be currently measured in vivo using advanced microscopic methods, such as fluorescence recovery after photobleaching which allows the direct observation of photosynthetic proteins mobility on a single cell level. The heterogeneous organization of thylakoid membrane proteins results in heterogeneity in protein mobility. The thylakoid membrane contains both, protein-crowded compartments with immobile proteins and fluid areas (less crowded by proteins), allowing restricted diffusion of proteins. This heterogeneity represents an optimal balance as protein crowding is necessary for efficient light-energy conversion, and protein mobility plays an important role in the regulation of photosynthesis. The mobility is required for an optimal light-harvesting process (e.g., during state transitions), and also for transport of proteins during their synthesis or repair. Protein crowding is then a key limiting factor of thylakoid membrane protein mobility; the less thylakoid membranes are crowded by proteins, the higher protein mobility is observed. Mobility of photosynthetic proteins outside the thylakoid membrane (lumen and stroma/cytosol) is less understood. Cyanobacterial phycobilisomes attached to the stromal side of the thylakoid can move relatively fast. Therefore, it seems that stroma with their active enzymes of the Calvin-Benson cycle, are a more fluid compartment in comparison to the rather rigid thylakoid lumen. In conclusion, photosynthetic protein diffusion is generally slower in comparison to similarly sized proteins from other eukaryotic membranes or organelles. Mobility of photosynthetic proteins resembles restricted protein diffusion in bacteria, and has been rationalized by high protein crowding similar to that of thylakoids.

Dynamic flexibility in the structure and function of photosystem II in higher plant thylakoid membranes: the grana enigma

Grana are not essential for photosynthesis, yet they are ubiquitous in higher plants and in the recently evolved Charaphyta algae; hence grana role and its need is still an intriguing enigma. This article discusses how the grana provide integrated and multifaceted functional advantages, by facilitating mechanisms that fine-tune the dynamics of the photosynthetic apparatus, with particular implications for photosystem II (PSII). This dynamic flexibility of photosynthetic membranes is advantageous in plants responding to ever-changing environmental conditions, from darkness or limiting light to saturating light and sustained or intermittent high light. The thylakoid dynamics are brought about by structural and organizational changes at the level of the overall height and number of granal stacks per chloroplast, molecular dynamics within the membrane itself, the partition gap between appressed membranes within stacks, the aqueous lumen encased by the continuous thylakoid membrane network, and even the stroma bathing the thylakoids. The structural and organizational changes of grana stacks in turn are driven by physicochemical forces, including entropy, at work in the chloroplast. In response to light, attractive van der Waals interactions and screening of electrostatic repulsion between appressed grana thylakoids across the partition gap and most probably direct protein interactions across the granal lumen (PSII extrinsic proteins OEEp-OEEp, particularly PsbQ-PsbQ) contribute to the integrity of grana stacks. We propose that both the light-induced contraction of the partition gap and the granal lumen elicit maximisation of entropy in the chloroplast stroma, thereby enhancing carbon fixation and chloroplast protein synthesizing capacity. This spatiotemporal dynamic flexibility in the structure and function of active and inactive PSIIs within grana stacks in higher plant chloroplasts is vital for the optimization of photosynthesis under a wide range of environmental and developmental conditions.