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Published on June 9, 2006; 10.1104/pp.106.081463

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Received April 4, 2006
Returned for revision April 28, 2006
Accepted May 15, 2006

Biosynthesis of Ascorbic Acid in Legume Root Nodules

Manuel A. Matamoros *, Jorge Loscos , Maria J. Coronado , Javier Ramos , Shusei Sato , Pilar S. Testillano , Satoshi Tabata , and Manuel Becana

Estación Experimental de Aula Dei, Consejo Superior de Investigaciones Científicas, Apartado 202, 50080 Zaragoza, Spain
Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, Ramiro de Maeztu 9, 28040 Madrid, Spain
Kazusa DNA Research Institute, 1532-3 Yana, Kisarazu, Chiba, 292-0812, Japan

* Corresponding author; email: manumat{at}eead.csic.es.

Ascorbic acid (vitamin C) is a major antioxidant and redox buffer, but is also involved in other critical processes of plants. Recently, the hypothesis has been proposed that legume nodules are unable to synthesize ascorbate and have to import it from the shoot or root, thus providing a means by which the plant regulates nodule senescence. The last step of ascorbate biosynthesis in plants is catalyzed by L-galactono-1,4-lactone dehydrogenase (GalLDH). The mRNAs encoding GalLDH and three other enzymes involved in ascorbate biosynthesis are clearly detectable in nodules. Furthermore, an active membrane-bound GalLDH enzyme is present in nodule mitochondria. Biochemical assays on dissected nodules reveal that GalLDH activity and ascorbate are correlated in nodule tissues and predominantly localized in the infected zone, with lower levels of both parameters (relative to the infected tissues) in the apex (87%) and senescent region (43%) of indeterminate nodules and in the peripheral tissues (65%) of determinate nodules. In situ RNA hybridization showed that the GalLDH mRNA is particularly abundant in the infected zone of indeterminate and determinate nodules. Thus, our results refute the hypothesis that ascorbate is not synthesized in nodules and lend support to a previous conclusion that ascorbate in the infected zone is primarily involved in the protection of host cells against peroxide damage. Likewise, the high ascorbate and GalLDH activity levels found in the apex of indeterminate nodules strongly suggest a participation of ascorbate in additional functions during symbiosis, possibly related to cell growth and division and to molecular signaling.




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