NUCLEAR ORGANELLE INTERACTIONS - THE IMMUTANS VARIEGATION MUTANT OF ARABIDOPSIS IS PLASTID AUTONOMOUS AND IMPAIRED IN CAROTENOID BIOSYNTHESIS

The immutans (im) variegation mutant of Arabidopsis thaliana contains green- and white-sectored leaves due to the action of a nuclear recess ive gene. The mutation is somatically unstable, and the degree of sect oring is influenced by light and temperature. Whereas the cells in the green sectors contain normal chloroplasts, the cells in the white sec tors are hetero-plastidic and contain non-pigmented plastids that lack organized lamellar structures, as well as small pigmented plastids an d/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 immutans is 'plastid autonomous'. In contrast to oth er variegation mutants with hetero-plastidic cells, the defect in im i s not maternally inherited. immutans thus represents a novel type of n uclear gene-induced variegation mutant. It has also been found that th e white tissues of immutans accumulate phytoene, a non-colored C-40 ca rotenoid intermediate. This suggests that immutans controls, either di rectly or indirectly, the activity of phytoene desaturase (PDS), the e nzyme that converts phytoene to zeta-carotene in higher plants. Howeve r, im is not the structural gene for PDS. A secondary effect of carote noid deficiency, both in immutans and in wildtype plants treated with a herbicide that blocks carotenoid synthesis, is an increase in acid r ibonuclease activity in white tissue. It is concluded that the novel v ariegation generated by the immutans mutation should offer great insig ht into the complex circuitry that regulates nuclear-organelle interac tions.