Shade avoidance response to the reduced ratio of red:far-red (R:FR) light was studied in a white aspen Populus alba clone 'Bolleana' using two filter systems: a clear plastic filter system that allows a R:FR ratio less than 1.0 to pass from adjacent border plant reflection; and a special commercial plastic that blocks FR light and creates a R:FR ratio above 3.0. The response to low R:FR The reduced R:FR signals enhanced the stem elongation in response to competition at the expense of relative stem diameter growth. Trees grown inside clear chambers were 27% taller and 22% heavier in stem dry weight than trees grown inside the FR-blocking filter chambers. Stem taper of clear chamber trees was 16% less than the FR-blocking filter trees. Low R:FR also induced 13% greater petiole length per leaf compared to the FR-blocking filter trees.;The immutans (im) variegation mutant of Arabidopsis has green and white leaf sectors due to the action of a nuclear recessive gene. IM is a chloroplast homolog of the mitochondrial alternative oxidase. The ghost (gh) variegation mutant of tomato bears phenotypic similarities to im. We show that the im and gh phenotypes arise from mutations in orthologous genes. Structural analyses reveal that AOX, IM and GH are RNR R2 di-iron carboxylate proteins with perfectly conserved Fe-coordinating ligands that define a quinol-binding catalytic site. IM has a global impact on plant growth and development and that it is required for the differentiation of multiple plastid types. IM transcript levels do not necessarily correlate with carotenoid pool sizes, raising the possibility that IM function is not limited to carotenogenesis. Leaf anatomy is radically altered in the green and white sectors of im. The green im sectors have significantly higher than normal rates of O2 evolution and significantly elevated chlorophyl a/b ratios, typical of those found in "sun" leaves. We conclude that IM and GH are plastid quinol oxidases that act downstream from a quinone pool to dissipate electrons in plastids. In addition, im interrupts plastid-to-nucleus signaling pathways that control Arabidopsis leaf developmental programming.