Besides major photosynthetic complexes of oxygenic photosynthesis, new electron carriers have been identified in thylakoid membranes of higher plant chloroplasts. These minor components, located in the stroma lamellae, include a plastidial NAD(P)H dehydrogenase (NDH) complex and a plastid terminal plastoquinone oxidase (PTOX). The NDH complex, by reducing plastoquinones (PQs), participates in one of the two electron transfer pathways operating around photosystem I (PSI), the other likely involving a still uncharacterized ferredoxin-plastoquinone reductase (FQR) and the newly discovered PGR5. The existence of a complex network of mechanisms regulating expression and activity of the NDH complex, and the presence of higher amounts of NDH complex and PTOX in response to environmental stress conditions the phenotype of mutants, indicate that these components likely play a role in the acclimation of photosynthesis to changing environmental conditions. Based on recently published data, we propose that the NDH-dependent cyclic pathway around PSI participates to the ATP supply in conditions of high ATP demand (such as high temperature or water limitation) and together with PTOX regulates cyclic electron transfer activity by tuning the redox state of intersystem electron carriers. In response to severe stress conditions, PTOX associated to the NDH and/or the PGR5 pathway may also limit electron pressure on PSI acceptor and prevent PSI photoinhibition.

Carbonic anhydrases (CAs) are Zn-containing metalloenzymes that catalyse the reversible hydration of CO2. We investigated the a CA and b CA families in Arabidopsis, which contain eight aCA (AtaCA1-8) and six b CA genes (AtbCA1-6). Analyses of expressed sequence tags (ESTs) from The Arabidopsis Information Resource (TAIR) database indicate that all the b CA encoding sequences, but only three of the Ata CA, are expressed. Using semiquantitative PCR experiments, functional CA genes were more strongly expressed in green tissue, but strong expression was also found in roots for b CA3, b CA6 and a CA2. Two a CA genes were shown to respond to the CO2 environment, while the others were unresponsive. Using the green fluorescent reporter protein gene fused with cDNA sequences coding for b CAs, we provided evidence that b CAs were targeted to specific subcellular compartments: b CA1 and b CA5 were targeted to the chloroplast, b CA2 and b CA3 to the cytosol, b CA4 to the plasma membrane and b CA6 to the mitochondria. The targeting and the pattern of gene expression suggest that CA isoforms play specific roles in subcellular compartments, tissues and organs. The data indicate that other CA isoforms than the well-characterized b CA1 may contribute to the CO2 transfer in the cell to the catalytic site of ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco).

In Arabidopsis the stomatal defense response, a feature of the innate immunity in plants, involves oxylipin-mediated mechanisms that are independent of the phytohormone abscisic acid.