| Summary: | <p>Plastids are an important group of plant organelles, and depending on their morphology and functions, plastids exist in different forms and have the ability to convert between these forms. Development of plastids depends on the import of preproteins from the cytosol through outer and inner envelope chloroplast membranes, TOC and TIC complexes. The TOC complex incorporates different client-specific receptors and these contact preproteins with differing specificity, controlling the proteomic composition, development fate, and functions of the plastid. The first identified chloroplast-localized E3 ligase of the plastid outer membrane, SP1, is a regulator of the TOC translocon. In Arabidopsis, SP1 functions to attach ubiquitin to TOC proteins for ubiquitin-proteasome system (UPS)-mediated degradation, controlling the proteomic composition of plastids.</p> <p>In this thesis, detail analyses of the role of SP1 were conducted in tomato and wheat plants with altered SP1 expression (both overexpression [OEX] and RNA interference-based knockdown, in each case), to further assess the extent to which its functions are conserved in different species, and to elucidate possible practical applications in crops.</p> <p>In accordance with previous findings in Arabidopsis, SP1 OEX promoted abiotic stress tolerance and leaf senescence in both tomato and wheat. SP1 was found to influence the abiotic stress tolerance where its particular effect on the plants’ responses to stress conditions was dependent on the developmental stage at which the exposure to stress occurred. SP1 RNAi delayed senescence under dark-induced and natural senescence conditions, and influenced yield in ‘stay-green’ SP1 RNAi wheat plants. SP1-mediated changes in tomato were found to alter the duration of fruit ripening, without affecting the size and firmness of the fully ripened fruit. Specifically, SP1 OEX fruit showed accelerated ripening, and SP1 RNAi fruit showed delayed ripening.</p> <p>Finally, the role of SPL2, a homologue of SP1, in tomato was investigated. The RNAi-mediated knockdown of SPL2 expression in tomato plants had similar effects on fruit ripening, leaf senescence, and abiotic stress tolerance to SP1 RNAi, even though the transcript abundance of tomato SPL2 in various plant tissues was much less than that of tomato SP1.</p>
|
|---|