Aconitase activity and expression during the development of lemon fruit

Citrus fruits are characterized by the accumulation of high levels of citri c acid in the juice sac cells and a decline in acid level toward maturation . It has been suggested that changes in mitochondrial aconitase (EC 4.2.1.3 ) activity affect fruit acidity. Recently, a cytosolic aconitase (cyt-Aco) homologous to mammalian iron-regulated proteins was identified in plants, l eading us to re-evaluate the role of aconitase in acid accumulation. Aconit ase activity was studied in 2 contrasting citrus varieties, sweet Lime (Cit rus limettioides Tan., low acid) and sour lemon (Citrus limon var. Eureka, high acid). Two aconitase isozymes were detected. One declined early in sou r lemon fruit development, but was constant throughout sweet lime fruit dev elopment. Its reduction in sour lemon was associated with a decrease in aco nitase activity in the mitochondrial fraction. Another isozyme was detected in sour lemon toward maturation, and was associated with an increase in ac onitase activity in the soluble fraction, suggesting a cytosolic localizati on. The cyt-Aco was cloned from lemon juice sac cells, but in contrast to t he changes in isozyme activity, its expression was constant during fruit de velopment. We present a model, which suggests that reduction of the mitocho ndrial aconitase activity plays a role in acid accumulation, while an incre ase in the cyt-Aco activity reduces acid level toward fruit maturation.