title: Iron-shortage-induced increase in citric acid content and reduction of cytosolic aconitase activity in Citrus fruit vesicles and calli.
abstract: 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.
authors.affiliation: The Department of Fruit Trees Sciences, Agricultural Research Organization, the Volcani Center, PO Box 6, Bet Dagan 50250, Israel.
authors.forename: Lyudmila
authors.initials: L
authors.lastname: Shlizerman
authors.forename: Ken
authors.initials: K
authors.lastname: Marsh
authors.forename: Eduardo
authors.initials: E
authors.lastname: Blumwald
authors.forename: Avi
authors.initials: A
authors.lastname: Sadka
mesh_terms: D000154:Aconitate Hydratase / Q000378:metabolism*; D002384:Catalysis / Q000187:drug effects; D019343:Citric Acid / Q000378:metabolism*; D002957:Citrus / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism*; D003600:Cytosol / Q000187:drug effects / Q000201:enzymology; D007501:Iron / Q000494:pharmacology; D000090463:Iron Deficiencies; D018515:Plant Leaves / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism; D018517:Plant Roots / Q000201:enzymology / Q000254:growth & development / Q000378:metabolism; D046509:Tissue Culture Techniques
chemical_list: D019343:Citric Acid; D007501:Iron; D000154:Aconitate Hydratase