![]() ![]() However, low levels of ABA are maintained in vegetative tissues under non-stressful conditions, and ABA-deficient mutant plants display reduced growth phenotypes, suggesting that ABA is required for normal plant growth (Finkelstein and Rock, 2002 Lee et al., 2006). ![]() ABA synthesis is most active in embryos entering dormancy and in plants under adverse environmental conditions (Xiong and Zhu, 2003). Not surprisingly, plant cells have evolved mechanisms to control nuclear gene expression in accordance with the developmental and metabolic status of the plastid to ensure appropriate assembly of such chloroplast macromolecular complexes and the efficient regulation of metabolic pathways that cross the chloroplast membrane (Pogson et al., 2008).Ībscisic acid (ABA) is a plant hormone that is required for several aspects of plant development. Furthermore, plant cells have metabolic pathways comprising reactions that take place in the cytosol and the plastid, and such coupling of metabolic reactions necessitates the expression of plastid enzymes in coordination with cytosolic enzymes of the pathway. Multi-protein complexes in the chloroplast, such as ribosomes and Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo), consist of subunits encoded by nuclear and plastid genomes, and it has been shown that plastid defects are associated with the reduced expression of nuclear-encoded plastid proteins (Inaba and Schnell, 2008 Bang et al., 2012). Although the chloroplast/plastid is a semi-autonomous organelle with its own genome and protein synthesis machinery, more than 90% of plastid proteins are encoded by the nuclear genome and have to be imported into the chloroplast from the cytosol (Lee et al., 2009 Andres et al., 2010). Integrated activity of the chloroplast genome and the nuclear genome is required for normal development of chloroplasts. ![]() Plastids account for less than 1% of the cotyledon cell volume in the Arabidopsis seed, but they enlarge, divide and differentiate into chloroplasts within 48 h after imbibition, increasing their volume in the cell by 70-fold (Mansfield and Briarty, 1992, 1996). The eoplasts in the cotyledons are converted back into chloroplasts, and the new chloroplasts proliferate when conditions allow germination to proceed. Dicotyledonous embryos such as those of Arabidopsis have normal chloroplasts, but these de-differentiate into eoplasts when the embryo cells become dormant (Ruppel et al., 2011). However, this nutrient reserve is exhausted rapidly, and the development of chloroplasts is essential for the new seedling to become an autotroph (Rajjou et al., 2012). Storage reserves in the seed are mobilized to support growth of a newly germinated seedling. Germination is the onset of development of the plant embryo from the mature seed when environmental and internal conditions are appropriate. Taken together, these results suggest that conversion of eoplasts into chloroplasts in young seedlings is critical for the seedlings to start carbon fixation as well as for maintenance of abscisic acid levels for responding to environmental challenges. Transcriptomic analysis indicated that nuclear genes for chloroplast proteins are down-regulated, and proteins mediating chloroplast-localized steps of abscisic acid biosynthesis are expressed to a lower extent in 1-week-old rh3–4 seedlings. Interestingly, rh3–4 seedlings have lower amounts of abscisic acid prior to recovery of their chloroplasts, and were more sensitive to abiotic stresses. These chloroplast defects in rh3–4 were alleviated in 2-week-old cotyledons and true leaves. Electron tomography analysis indicated that ribosome density in the 3-day-old mutant chloroplasts is only 20% that of wild-type chloroplasts, and the ribosomes in the mutant are smaller. The most obvious abnormality in the mutant chloroplasts was their lack of normal ribosomes. Cotyledons were pale and seedling growth was retarded in the mutant. The promoter activity of RH3 is strongest in the greening tissues of 3-day and 1-week-old seedlings but reduced afterwards. To investigate the cellular and physiological roles of an Arabidopsis DEAD-box protein, RH3, we examined its expression and localization and the phenotypes of rh3–4, a T–DNA insertion mutant allele of RH3. DEAD-box proteins have been shown to play roles in multiple steps in ribosome biogenesis. The plastid has its own translation system, and its ribosomes are assembled through a complex process in which rRNA precursors are processed and ribosomal proteins are inserted into the rRNA backbone. ![]()
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