Elinor Thompson

FtsH Is Involved in the Early Stages of Repair of Photosystem II in Synechocystis sp PCC 6803

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.

Plant extracellular ATP signalling by plasma membrane NADPH oxidase and Ca2+ channels

Extracellular 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 Ca2+ 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 Ca2+ 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 Ca2+-permeable channels (determined using patch-clamp electrophysiology), which contribute to the elevation of cytosolic free Ca2+. Disruption of this pathway in the AtrbohC mutant impaired the extracellular ATP-induced increase in reactive oxygen species (ROS), the activation of Ca2+ 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.

An Arabidopsis flavonoid transporter is required for anther dehiscence and pollen development

FLOWER FLAVONOID TRANSPORTER (FFT) encodes a multidrug and toxin efflux family transporter in Arabidopsis thaliana. FFT (AtDTX35) is highly transcribed in floral tissues, the transcript being localized to epidermal guard cells, including those of the anthers, stigma, siliques and nectaries. Mutant analysis demonstrates that the absence of FFT transcript affects flavonoid levels in the plant and that the altered flavonoid metabolism has wide-ranging consequences. Root growth, seed development and germination, and pollen development, release and viability are all affected. Spectrometry of mutant versus wild-type flowers shows altered levels of a glycosylated flavonol whereas anthocyanin seems unlikely to be the substrate as previously speculated. Thus, as well as adding FFT to the incompletely described flavonoid transport network, it is found that correct reproductive development in Arabidopsis is perturbed when this particular transporter is missing.

Identifying the transporters of different flavonoids in plants

We recently identified a new component of flavonoid transport pathways in Arabidopsis. The MATE protein FFT (Flower Flavonoid Transporter) is primarily found in guard cells and seedling roots, and mutation of the transporter results in floral and growth phenotypes. The nature of FFT’s substrate requires further exploration but our data suggest that it is a kaempferol diglucoside. Here we discuss potential partner H+-ATPases and possible redundancy among the close homologues within the large Arabidopsis MATE family.

A Fertile Field: The Mutual Influence and Parallel Histories of Auxin and Flavonoids

Auxin is notable for its influence on almost every aspect of plant growth and development. Its effects are so familiar but they continue to pose questions, features that are mirrored by the flavonoids. Although these secondary metabolites’ biosynthesis is one of the best studied of all pathways, their functions are so broad as to be perplexing. The ubiquity of flavonoids in fruit and flowers and the appearance of anthocyanins in the plant stress response often make these pigments’ presence obvious. Their effects, however, extend beyond these visible roles to communication with other organisms and to auxin transport inhibition. Decades of study have shown that they are capable of altering auxin flow in the root, and inflorescence phenotypes of flavonoid mutants suggest a perturbation of development that could also be related to altered auxin distribution in aerial tissues. A compound that is able to regulate such an important hormone as auxin might be expected to have an enormous impact in the plant, and now quite a body of evidence has accumulated to support this hypothesis—but there remain, nevertheless, questions over its relevance in “real-life” plant growth.

The role of FtsH proteins in photosystem biosynthesis and turnover in Arabidopsis and Synechocystis

Both Arabidopsis thaliana and Synechocystis PCC 6803 genomes encode several homologues of Escherichia coli FtsH. These proteins are AAA-family proteases (ATPases associated with a variety of cellular activities), which are widely distributed in prokaryotes and eukaryotes. Among their diverse roles, FtsH proteins are important in photosynthesis. Of four found in Synechocystis, one (encoded by open-reading frame slr0228) appears to be involved in photosystem I (PSI) biosynthesis, and it has been suggested that it may also participate in photosystem II (PSII) D1-protein turnover (see S8 poster by P. Silva et al.). In Arabidopsis, mutation of the `Var2?-ftsH leads to severe leaf-variegation [1]. Whereas fluorescence-emission spectra of Synechocystis show a higher PSI:PSII ratio in wild-type (WT) than in slr0228? cells, in Arabidopsis, WT and Var2 have similar PSI:PSII peaks under normal growth conditions. High-light treatment, however, results in greater photoinhibition of Var2-FtsH? plants, measured by Fv/Fm. Fluorescence emission spectra also show reduced recovery from photoinhibitory high light in Var2 compared with WT. These experiments and Western blots indicate that the D1 repair cycle is impeded in the Arabidopsis FtsH mutant as in the cyanobacterium. Sequence similarities in the slr0228 and Var2 ftsH genes indeed suggest they may be closely-related proteins. To study the role of slr0228 in reaction centre biosynthesis, site-directed mutants have been made in PSII? and chlorophyll-synthesis-deficient Synechocystis strains. Further work characterising the role of plant and cyanobacterial FtsH proteins is presented. 1. Chen M, Choi Y, Voytas DF, Rodermel S: Mutations in the Arabidopsis VAR2 locus cause leaf variegation due to the loss of a chloroplast FtsH protease. Plant J 2000, 22:303.