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Using a var2-2 mutant of Arabidopsis thaliana, which lacks a homologue of the zinc-metalloprotease, FtsH, we demonstrate that this protease is required for the efficient turnover of the D1 polypeptide of photosystem II and protection against photoinhibition in vivo. We show that var2-2 leaves are much more susceptible to lightinduced photosystem II photoinhibition than wild-type leaves. Furthermore, the rate of photosystem II photoinhibition in untreated var2-2 leaves is equivalent to that of var2-2 and wild-type leaves, which have been treated with lincomycin, an inhibitor of the photosystem II repair cycle at the level of D1 synthesis. This is in contrast to untreated wild-type leaves, which show a much slower rate of photosystem II photoinhibition due to an efficient photosystem II repair cycle. The recovery of var2-2 leaves from photosystem II photoinhibition is also impaired relative to wild-type. Using Western blot analysis in the presence of lincomycin we show that the D1 polypeptide remains stable in leaves of the var2-2 mutant under photoinhibitory conditions that lead to D1 degradation in wild-type leaves and that the abundance of DegP2 is not affected by the var2-2 mutation. We conclude, therefore, that the Var2 FtsH homologue is required for the cleavage of the D1 polypeptide in vivo. In addition, we identify a conserved lumenal domain in Var2 that is unique to FtsH homologues from oxygenic phototrophs. The Photosystem II (PSII)1 complex is a large protein-pigment assembly that catalyzes the light-dependent oxidation of water to molecular oxygen in chloroplasts and cyanobacteria. At the core of PSII lies the D1/D2 heterodimer, which binds the pigments and co-factors necessary for primary photochemistry (1). The D1 polypeptide is also important because of its high rate of turnover (2). This high turnover rate is related to the vulnerability of PSII to light, with D1 being the main target for photoinactivation and subsequent damage. An efficient repair cycle for D1 is therefore of paramount importance in oxygenic phototrophs. When the rate of photoinactivation and damage of D1 exceeds the capacity for repair, photoinhibition occurs, resulting in a decrease in the maximum efficiency of PSII photochemistry.A key feature of the D1 repair cycle is the degradation of the damaged polypeptide. It is generally accepted that damaged D1 is initially cleaved at a site on the stromal loop between transmembrane helices D and E yielding a 23-kDa N-terminal fragment (3) and a 10-kDa C-terminal fragment (4). This cleavage step is believed to be initiated by structural changes within the D1 polypeptide (5), although the precise nature of the cleavage event remains unclear. One proposal is that the action of active oxygen species acts to cleave the D1 polypeptide during strong illumination (6). However, the temperature dependence of the process (7) and its sensitivity to protease inhibitors (8) indicates the involvement of enzymatic proteolysis by an unidentified protease. Following cleavage, the breakdown fragments of ...

SUMMARYExtracellular ATP regulates higher plant growth and adaptation. The signalling events may be unique to higher plants, as they lack animal purinoceptor homologues. Although it is known that plant cytosolic free Ca 2+ can be elevated by extracellular ATP, the mechanism is unknown. Here, we have studied roots of Arabidopsis thaliana to determine the events that lead to the transcriptional stress response evoked by extracellular ATP. Root cell protoplasts were used to demonstrate that signalling to elevate cytosolic free Ca 2+ is determined by ATP perception at the plasma membrane, and not at the cell wall. Imaging revealed that extracellular ATP causes the production of reactive oxygen species in intact roots, with the plasma membrane NADPH oxidase AtRBOHC being the major contributor. This resulted in the stimulation of plasma membrane Ca 2+-permeable channels (determined using patch-clamp electrophysiology), which contribute to the elevation of cytosolic free Ca 2+ . Disruption of this pathway in the AtrbohC mutant impaired the extracellular ATP-induced increase in reactive oxygen species (ROS), the activation of Ca 2+ channels, and the transcription of the MAP kinase3 gene that is known to be involved in stress responses. This study shows that higher plants, although bereft of purinoceptor homologues, could have evolved a distinct mechanism to transduce the ATP signal at the plasma membrane.

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When plants, algae, and cyanobacteria are exposed to excessive light, especially in combination with other environmental stress conditions such as extreme temperatures, their photosynthetic performance declines. A major cause of this photoinhibition is the light-induced irreversible photodamage to the photosystem II (PSII) complex responsible for photosynthetic oxygen evolution. A repair cycle operates to selectively replace a damaged D1 subunit within PSII with a newly synthesized copy followed by the light-driven reactivation of the complex. Net loss of PSII activity occurs (photoinhibition) when the rate of damage exceeds the rate of repair. The identities of the chaperones and proteases involved in the replacement of D1 in vivo remain uncertain. Here, we show that one of the four members of the FtsH family of proteases (cyanobase designation slr0228) found in the cyanobacterium Synechocystis sp PCC 6803 is important for the repair of PSII and is vital for preventing chronic photoinhibition. Therefore, the ftsH gene family is not functionally redundant with respect to the repair of PSII in this organism. Our data also indicate that FtsH binds directly to PSII, is involved in the early steps of D1 degradation, and is not restricted to the removal of D1 fragments. These results, together with the recent analysis of ftsH mutants of Arabidopsis, highlight the critical role played by FtsH proteases in the removal of damaged D1 from the membrane and the maintenance of PSII activity in vivo.