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International audience; Summary The role of a subfamily of lipid globule-associated proteins, referred to as plant fibrillins (FIB1a, -1b, -2), was determined using a RNA interference (RNAi) strategy. We show that Arabidopsis plants with reduced levels of these plastid structural proteins are impaired in long-term acclimation to environmental constraint, namely photooxidative stress imposed by high light combined with cold. As a result, their photosynthetic apparatus is inefficiently protected. This leads to the prevalence of an abnormal granal and stromal membrane arrangement, as well as higher photosystem II photoinhibition under stress. The visible phenotype of FIB1-2 RNAi lines also includes retarded shoot growth and a deficit in anthocyanin accumulation under stress. All examined phenotypic effects of lower FIB levels are abolished by jasmonate (JA) treatment. An atypical expression pattern of several JA-induced genes was observed in RNAi plants. A JA-deficient mutant was found to share similar stress phenotypic characteristics with FIB RNAi plants. We conclude a new physiological role for JA, namely acclimation of chloroplasts, and that light/cold stress-related JA biosynthesis is conditioned by the accumulation of plastoglobule-associated FIB1-2 proteins. Consistent correlative data suggest that this FIB effect is mediated by plastoglobule (and triacylglycerol) accumulation as the potential site for initiating the chloroplast stress-related JA biosynthesis.

Plastid transformation technologies have developed rapidly over the last few years, reflecting their value in the study of the principal mechanisms of plastid gene expression and commercial interest in using plastids as bioreactors. Application of this technology is still limited by the difficulty of obtaining regulated, selective expression of plastid transgenes. The plastid genome is transcribed by two different types of RNA polymerase. One of them is of the eubacterial type of polymerase, and its subunits are encoded in the plastid genome [plastid-encoded RNA polymerase (PEP)]. The other one is of the phage type and nucleus-encoded [nucleus-encoded RNA polymerase (NEP)]. To obtain selective transgene expression, we have made use of the similarities and differences between the eubacterial and the plastid eubacterial type transcription systems. We created a hybrid transcription system in which the transgene is placed under the control of a eubacterial promoter which does not exist in the plastid genome and which is not recognized by the plastid endogenous transcriptional machinery. Selective transcription of the transgene is achieved by the supply of a chimeric transcription factor that interacts with PEP and directs it specifically to the foreign eubacterial-type transgene promoter. This hybrid transcription system could be used for biotechnological and fundamental research applications as well as in the characterization of the evolutionary differences between the eubacterial and the plastid eubacterial-type transcription systems.