The Arabidopsis immutans Mutation Affects Plastid Differentiation and the Morphogenesis of White and Green Sectors in Variegated Plants

Abstract

The immutans (im) variegation mutant of Arabidopsis has green and white leaf sectors due to the action of a nuclear recessive gene, IMMUTANS (IM). This gene encodes the IM protein, which is a chloroplast homolog of the mitochondrial 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 on plant growth and development and is required for the differentiation of multiple plastid types, including chloroplasts, amyloplasts, and etioplasts. IM promoter activity and IM mRNAs are also expressed ubiquitously in Arabidopsis. IM transcript levels correlate with carotenoid accumulation in some, but not all, tissues. This suggests that IM function is not limited to carotenogenesis. Leaf anatomy is radically altered in the green and white sectors of im: Mesophyll cell sizes are dramatically enlarged in the green sectors and palisade cells fail to expand in the white sectors. The green im sectors also have significantly higher than normal rates of O 2 evolution and elevated chlorophyll a/b ratios, typical of those found in "sun" leaves. We conclude that the changes in structure and photosynthetic function of the green leaf sectors are part of an adaptive mechanism that attempts to compensate for a lack of photosynthesis in the white leaf sectors, while maximizing the ability of the plant to avoid photodamage.Variegation mutants provide an excellent system to explore the nature of communication between the nucleus-cytoplasm, chloroplast, and mitochondrial genetic compartments (for review, see Leó n et al., 1998; Rodermel, 2001). The leaves of these mutants have green and white (or yellow) sectors that arise as a consequence of mutations in nuclear or organellar genes (Tilney-Bassett, 1975). Whereas the green sectors contain cells with morphologically normal chloroplasts, cells in the white sectors contain plastids that lack pigments and normal lamellar structures. One common mechanism of variegation involves the induction of defective mitochondria or chloroplasts by mutations in nuclear genes for organelle proteins. This is sometimes due to transposable element activity, in which case the green and white cells have different genotypes. In other cases, the two types of cells have the same (mutant) genotype, indicating that the gene defined by the mutation codes for a product that is required for organelle biogenesis in some, but not all, cells of the mutant.Despite the large number of mutant screens that have been conducted in Arabidopsis, surprisingly few nuclear "variegation" loci have been reported. These include cab underexpressed (cue1), chloroplast mutator (chm), differential development of vascularassociated cells (dov), immutans (im), pale cress (pac), var1, and var2 (e.g.

Effects of chloroplast dysfunction in a subpopulation of leaf mesophyll cells on photosynthetic and respiratory activities of a whole leaf: A study using variegated leaves of Hedera helix L.

“…This conclusion may contradict the opinion that green sectors in variegated leaves have a higher photosynthetic capacity than wild-type green leaves to fulfill the large sink demand by neighboring white sectors that have lost photosynthetic function (Aluru et al, 2001(Aluru et al, , 2006. If photosynthetic capacity of green sectors is indeed affected by a sink demand by neighboring white sectors, a small green sector near a large white sector should exhibit higher photosynthetic capacity than a large green sector near a small white sector, because sink demand for the green sector should be larger in the former than in the latter.…”

“…In addition to these intra-cellular (inter-organellar) interactions, inter-cellular interactions may also exist; i.e., the presence of white mesophyll cells may affect photosynthetic capacities of green mesophyll cells of normal appearance present in the same leaves. In an Arabidopsis variegation mutant im, the green leaf sectors exhibited increased photosynthetic oxygen evolution rates under CO 2 -saturating conditions, compared with the wild-type, all-green leaves (Aluru et al 2001). This increase in photosynthesis may be a means of compensating for a lack of photosynthesis in white sectors.…”

“…Aluru et al (2001Aluru et al ( , 2006 measured photosynthesis rates on a chlorophyll basis, and found increased photosynthesis in green sectors of variegated leaves of Arabidopsis im mutant. However, comparison on a chlorophyll basis may not be appropriate in this case, because they also reported that im green sectors exhibit a higher chlorophyll a/b ratio than wild-type leaves.…”

Influence of plastids on light signalling and development

“…Additionally, immutans (im) had variegated leaves, attenuated biogenesis of non-photosynthetic plastids and short roots. Thus, signals from non-photosynthetic plastids appear to affect development [64]. The distinct PhANG expression phenotypes of variegated mutants are consistent with multiple plastid-to-nucleus signalling mechanisms affecting development [61,65].…”

Involvement of a Plastid Terminal Oxidase in Plastoquinone Oxidation as Evidenced by Expression of the Arabidopsis thaliana Enzyme in Tobacco

“…Overexpression of At-PTOX did not influence the leaf carotenoid content, thus indicating that the PTOX level is not a limiting factor regulating carotenoid biosynthesis. Interestingly, the IM (or PTOX) promoter was shown to be active, and IM mRNAs were expressed ubiquitously in Arabidopsis tissues and organs throughout development, arguing in favor of a more global role for this protein in plastid metabolism (48).…”