Arabidopsis cue mutants with defective plastids are impaired primarily in the photocontrol of expression of photosynthesis-associated nuclear genes

Plant photoreceptors detect light cues and initiate responses ranging from chloroplast differentiation to the control of morphogenesis and flowering. The photocontrol of photosynthesis-related nuclear genes appears closely related to retrograde plastid signals by which the status of the organelle controls the expression of nuclear genes. However, what specific role, if any, plastid-originated signals play in light responses is poorly understood: it has in the past been proposed that plastid signals play a role in all responses to high fluence far-red light perceived by the light-labile phytochrome A, irrespective of whether they involve photosynthesis-related genes. To explore this further, we have re-examined the phenotype of three cue (cab-underexpressed) Arabidopsis mutants, defective in chloroplast development. The mutants have underdeveloped etioplasts, with increasing impairments in cue6, cue8 and cue3. The mutants show only small defects in photocontrol of hypocotyl elongation and cotyledon opening under prolonged far-red or red light, and normal photocontrol under blue. On the other hand, the expression of photosynthesis-associated nuclear genes is much more impaired in the mutants in the dark and following red or far-red light short treatments or continuous light, than that of those phytochrome-dependent genes tested which are not associated with photosynthesis. Furthermore, red/far-red photoreversible responses involving photosynthesis-related genes (induction of Lhcb1–cab promoter activity, and photoreversible extent of greening) mediated by phytochrome B and other photo-stable phytochromes, both show a reduction in the cue mutants, which correlates with the etioplast defect. Our evidence demonstrates that plastid-derived signals need to be operational in order for the phytochrome control of photosynthetic nuclear genes to occur.     

The Arabidopsis nuclear DAL gene encodes a chloroplast protein which is required for the maturation of the plastid ribosomal RNAs and is essential for chloroplast differentiation

Altered pigmentation is an easily scored and sensitive monitor of plastid function. We analyzed in detail a yellow colored transposon-tagged mutant (dal1-2) that is allelic to the dal mutant previously identified (Babiychuk et al., 1997). Mesophyll cells of mutant plants possess abnormal nucleoids and more but smaller plastids than wild type cells. Plastid development in dal1-2 is not altered in the dark but is arrested at the early steps of thylakoid assembly. The amino acid sequence of the protein deduced from our cDNA clone is 21 amino acids longer than the previously published DAL sequence (Babiychuk et al., 1997) and allowed us to show that DAL codes for a chloroplast protein. The dal1-2 mutation has a global negative effect on plastid RNA accumulation and on expression of nuclear encoded photosynthetic genes. We show that the plastid RNA polymerases, the nuclear-encoded NEP and the plastid-encoded PEP, are functional in the mutant. Precursor 16S and 23S rRNA species specifically accumulate at a high level in the mutant but the 5-end and the long 3-end trailer are not modified. We suggest that the dal mutation is involved in plastid rRNA processing and consequently in translation and early chloroplast differentiation.     

Evidence that the plastid signal and light operate via the samecis-acting elements in the promoters of nuclear genes for plastid proteins

Nuclear-encoded genes for proteins of the photosynthetic maschinery represent a particular subset of genes. Their expression is cooperatively stimulated by discrete factors including the developmental stage of plastids and light. We have analyzed in transgenic tobacco the plastid- and light-dependent expression of a series of 5 promoter deletions of various nuclear genes from spinach, of fusions of defined promoter segments with the 90-bp 35S RNA CaMV minimal promoter, as well as with mutations in sequences with homologies to characterizedcis-elements, to address the question of whether the plastid signal and light operate via the same or differentcis-acting elements. In none of the 160 different transgenic lines (representing 32 promoter constructs from seven genes) analyzed, could significant differences be identified in the responses to the two regulatory pathways. The data are compatible with the idea that both signals control the expression of nuclear genes for plastid proteins via the samecis-acting elements.     

Acclimation of Arabidopsis thaliana to the light environment: Changes in composition of the photosynthetic apparatus

Acclimation to changes in the light environment was investigated in Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta. Plants grown under four light regimes showed differences in their development, morphology, photosynthetic performance and in the composition of the photosynthetic apparatus. Plants grown under high light showed higher maximum rates of oxygen evolution and lower levels of light-harvesting complexes than their low light-grown counterparts; plants transferred to low light showed rapid changes in maximum photosynthetic rate and chlorophyll-a/b ratio as they became acclimated to the new environment. In contrast, plants grown under lights of differing spectral quality showed significant differences in the ratio of photosystem II to photosystem I. These changes are consistent with a model in which photosynthetic metabolism provides signals which regulate the composition of the thylakoid membrane.Abbreviations Aac1 gene encoding actin - Chl chlorophyll - F far-red-enriched light (R:FR = 0.72) - FR far-red light - H high light (400 mol · m^–2 · s^–1) - L low light (100 ml · m^–2 · s^–1) - LHCII light-harvesting complex of PSII - Lhcb genes encoding the proteins of LHCII - R red light - Rbcs genes encoding the small subunit of Rubisco - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - W white light (R:FR = 1.40) This work was supported by Natural Environment Research Council Grant No. GR3/7571A. We would like to thank H. Smith (Botany Department, University of Leicester) and E. Murchie (University of Sheffield) for helpful discussions.