SD3, an Arabidopsis thaliana homolog of TIM21, affects intracellular ATP levels and seedling development.

It is poorly understood how plants control their growth by cell division, elongation, and differentiation. We have characterized a seedling-lethal mutant segregation distortion 3 (sd3) that showed a very dwarf phenotype when grown in the light and, in the dark, had short hypocotyls with reduced ploidy levels. The corresponding gene of SD3 encodes a protein with high similarity to yeast translocase on the inner mitochondrial membrane 21 (TIM21), which is a component of the TIM23 complex. Indeed, SD3 protein fused to GFP localized in the mitochondria. SD3 overexpression increased cotyledon size in the light and hypocotyl thickness in the dark. The expression of genes for several subunits of the respiratory-chain complexes III and IV was up-regulated in SD3-overexpressing plants. Furthermore, these plants showed high levels of ATP whereas those of sd3 were low. These results suggested that SD3 induced an increase in cell size by raising the expression of the respiratory-chain subunit genes and hence increased the intracellular ATP levels. We propose that intracellular ATP levels regulated by mitochondria control plant organ size.