Regulation of expression of the mitochondrial and cytosolic forms of aconitase in maize leaves via phytochrome.

Regulation of expression and methylation of promoters of two aconitase (EC 4.2.1.3) genes by light have been investigated in maize (Zea mays L.) in relation to the involvement of phytochrome. Transferring of plants from light to darkness resulted in the stimulation of aconitase activity in mitochondria and in its suppression in the cytosol. Irradiation by red light reversed aconitase activity to the levels observed under white light while far red light reverted the effect of red light. Electrophoretic staining of aconitase activity revealed the preference of the cytosolic form in white and red light and of the mitochondrial form in darkness and in far red light. Both forms of aconitase were purified, the mitochondrial form revealed lower affinity to citrate and higher to isocitrate as compared to the cytosolic form. The study of the aconitase gene Aco1 encoding the mitochondrial form revealed its low expression and high promoter methylation in the light and upon irradiation by red light as compared to high expression and low promoter methylation in darkness and in far red light. The pattern of expression and promoter methylation of the gene Aco2 encoding the cytosolic form was opposite. It is concluded that expression of the mitochondrial and cytosolic forms of aconitase is under control of light via phytochrome in opposite ways at the level of promoter methylation. Light inhibits expression of the mitochondrial aconitase, while it stimulates expression of the cytosolic aconitase which is important for directing citrate exported from mitochondria to the synthesis of amino acids.