The yellow variegated mutant of Arabidopsis thaliana is characterized by bright-yellow true leaves that turn green- and white-sectored as leaf development proceeds. Variegation is due to the action of a nuclear recessive gene. Whereas cells in the green sectors contain morphologically normal chloroplasts, cells in the yellow and white sectors are heteroplastidic and contain plastids with rudimentary lamellar structures, as well as some normal-appearing chloroplasts, This indicates that plastids in yellow variegated are affected differently by the nuclear mutation (the mutant is "plastid autonomous"), Genetic analyses have revealed that yellow variegated is an allele of the var2 locus, and that defective plastids are not maternally inherited, The traits of plastid autonomy and lack of maternal inheritance of the plastid defect set var2 apart from other nuclear gene-induced variegations and define a novel class of variegation mutant. The primary lesion in var2 probably does not involve a blockage in the pathways of pigment biosynthesis, Under high temperatures or low light conditions, plant growth is retarded and mutant plants are nearly all-green. Considered together, our data suggest that var2 is delayed in chloroplast biogenesis, We suggest that the stochastic pattern of variegation in the mutant may be due to an interplay of factors that regulate var2 gene expression and factors that mediate rates of cell and plastid division. Plastids with a critical threshold of the partially functional var2 protein are green, while plastids containing less than the threshold of var2 activity are white.
