SummaryThe immutans (ira) variegation mutant of Arabidopsis thaliana contains green-and white-eactored leaves due to the action of a nuclear recessive gene. The mutation is somatically unstable, and the degree of sectoring is Influenced by light and temperature. Whereas the cells in the green sectors contain normal chloroplasts, the cells in the white sectors are heteroplasUdic and contain non-pigmented plastids that lack organized lamellar structures, as well as small pigmented plastids and/or rare normal chloroplasts. This indicates that the plastids in im white cells are not affected equally by the nuclear mutation and that the expression of irnmutans is 'plastid autonomous'. In contrast to other variegation mutants with heteroplastidic cells, the defect in im is not maternally Inherited. immutans thus represents a novel type of nuclear gene-induced variegation mutant. It has also been found that the white tissues of immutans accumulate phytoene, a non-colored C4o carotenoid intermediate. This suggests that immutans controls, either directly or indirectly, the activity of phytoene desaturaea (PDS), the enzyme that converts phytoene to zeta-carotene in higher plants. However, im is not the structural gene for PDS. A secondary effect of carotenoid deficiency, both in immutans and in wildtype plants treated with a herbicide that blocks carotenoid synthesis, is an increase in acid ribonuclease activity in white tissue. It is concluded that the novel variegation generated by the immutans mutation should offer great insight into the complex circuitry that regulates nuclear--organelle interactions.

To study the mechanisms that integrate pigment and chlorophyll a/b-binding apoprotein biosynthesis during light-harvesting complex II assembly, we have examined P-glucuronidase (CUS) enzyme activities, chlorophyll contents, and cell sizes i n fluorescenceactivated, cell-sorting-separated single cells from transgenic Arabidopsis thaliana wild-type and immufans variegation mutant plants that express an Lhcb (photosystem II chlorophyll a/b-binding polypeptide gene)/CUS promoter fusion. We found that CUS activities are positively correlated with chlorophyll content and cell size i n green cells from the control and immufans plants, indicating that Lhcb gene transcription i s coordinated with cell size in this species. Compared with the control plants, however, chlorophyll production is enhanced in the green cells of immufans; this may represent part of a strategy t o maximize photosynthesis in the green sectors t o compensate for a lack of photosynthesis in the white sectors of the mutant. Lhcb transcription is significantly higher in pure-white cells of the transgenic immufans plants than i n purewhite cells from norflurazon-treated, photooxidized A. fhaliana leaves. This suggests that immufans partially uncouples Lhcb transcription from its normal dependence on chlorophyll accumulation and chloroplast development. We conclude that immufans may play a role in regulating Lhcb transcription, and may be a key component in the signal transduction pathways that control chloroplast biogenesis.