The Arabidopsis immutans mutation affects plastid differentiation and the morphogenesis of white and green sectors in variegated plants

The immutans (im) variegation mutant of Arabidopsis has green and white lea f sectors due to the action of a nuclear recessive gene, IMMUTANS (IM). Thi s gene encodes the IM protein, which is a chloroplast homolog of the mitoch ondrial alternative oxidase. Because the white sectors of im accumulate the noncolored carotenoid, phytoene, IM likely serves as a redox component in phytoene desaturation. In this paper, we show that IM has a global impact o n plant growth and development and is required for the differentiation of m ultiple plastid types, including chloroplasts, amyloplasts, and etioplasts. IM promoter activity and IM mRNAs are also expressed ubiquitously in Arabi dopsis. IM transcript levels correlate with carotenoid accumulation in some , but not all, tissues. This suggests that IM function is not limited to ca rotenogenesis. Leaf anatomy is radically altered in the green and white sec tors of im: Mesophyll cell sizes are dramatically enlarged in the green sec tors and palisade cells fail to expand in the white sectors. The green im s ectors also have significantly higher than normal rates of O-2 evolution an d elevated chlorophyll alb ratios, typical of those found in "sun" leaves. We conclude that the chan-es in structure and photosynthetic function of th e green leaf sectors are part of an adaptive mechanism that attempts to com pensate for a lack of photosynthesis in the white leaf sectors, while maxim izing the ability of the plant to avoid photodamage.