Carbon metabolism in developing soybean root nodules: The role of carbonicanhydrase

A full-length cDNA clone encoding carbonic anhydrase (CA) was isolated from a soybean nodule cDNA library. In situ hybridization and immunolocalizatio n were performed in order to assess the location of CA transcripts and prot ein in developing soybean nodules, CA transcripts and protein were present at high levels in all cell types of young nodules, whereas in mature nodule s they were absent from the central tissue and were concentrated in cortica l cells. The results suggested that, in the earlier stages of nodule develo pment, CA might facilitate the recycling of CO2 while at later stages it ma y facilitate the diffusion of CO2 out of the nodule system. In parallel, su crose metabolism was investigated by examination of the temporal and spatia l transcript accumulation of sucrose synthase (SS) and phosphoenolpyruvate carboxylase (PEPC) genes, with in situ hybridization. In young nodules, hig h levels of SS gene transcripts were found in the central tissue as well as in the parenchymateous cells and the vascular bundles, while in mature nod ules the levels of SS gene transcripts were much lower, with the majority o f the transcripts located in the parenchyma and the pericycle cells of the vascular bundles. High levels of expression of PEPC gene transcripts were f ound in mature nodules, in almost all cell types, while in young nodules lo wer levels of transcripts were detected, with the majority of them located in parenchymateous cells as well as in the vascular bundles. These data sug gest that breakdown of sucrose may take place in different sites during nod ule development.