Shlizerman, L., Department of Fruit Trees Sciences, Agricultural Research Organization, Volcani Center, PO Box 6, Bet Dagan 50250, Israel
Marsh, K., Horticultural and Food Research Institute of New Zealand, Mount Albert Research Center, 120 Mount Albert Road, Auckland 1025, New Zealand
Blumwald, E., Department of Plant Sciences, Mail Stop 5, University of California, 1 Shields Avenue, Davis, CA 95616, United States
Sadka, A., Department of Fruit Trees Sciences, Agricultural Research Organization, Volcani Center, PO Box 6, Bet Dagan 50250, Israel

Aconitase, which catalyses the conversion of citrate into isocitrate, requires Fe for its activity. The yeast and animal enzyme loses its enzymatic activity under Fe shortage and binds to RNA of genes involved in Fe homeostasis, altering their expression. Thus, the enzyme provides a regulatory link between organic acid metabolism and Fe cellular status. Roots and leaves of Fe-deficient plants show induction in organic acids, especially citrate. Although no RNA-binding activity has been so far demonstrated for the plant aconitase, whether alternations in enzyme activity by Fe could play a role in this induction remain unanswered. This question was investigated in lemon fruit [Citrus limon (L.) Burm var Eureka], characterized by the accumulation of citrate to about 0.3 M in the juice vesicles cells (pulp). Calli and isolated juice vesicles showed two- to three-fold induction in citrate level when subjected to Fe shortage. The mRNA level of aconitase exhibited no changes under reduced Fe concentrations. Analysis of aconitase isozymes demonstrated that out of two aconitase isozymes, typically detected in citrus fruit, only the cytosolic form displayed a reduced activity under low Fe concentrations. Our data support the notion of a limited Fe-availability-induced reduction in cytosolic aconitase, resulting in a slower rate of citrate breakdown and a concomitant increase in citrate levels. © Physiologia Plantarum 2007.