Integration of Plant and Bacterial Metabolism in Nitrogen Fixing Systems

Promiscuity of hosting nitrogen fixation in rice: an overview from the legume perspective

The subject area of this review provides extraordinary challenges and opportunities. The challenges relate to the fact that the integration of various fields such as microbiology, biochemistry, plant physiology, eukaryotic as well as bacterial genetics, and applied plant sciences are required to assess the disposition of rice, an alien host, for establishing such a unique phenomenon as biological nitrogen fixation. The opportunities signify that, if successful, the breakthrough will have a significant impact on the global economy and will help improve the environment. This review highlights the literature related to the area of legume-rhizobia interactions, particularly those aspects whose understanding is of particular interest in the perspective of rice. This review also discusses the progress achieved so far in this area of rice research and the possibility of built-in nitrogen fixation in rice in the future. However, it is to be borne in mind that such research does not ensure any success at this point. It provides a unique opportunity to broaden our knowledge and understanding about many aspects of plant growth regulation in general.

METABOLITE TRANSPORT ACROSS SYMBIOTIC MEMBRANES OF LEGUME NODULES

Infection of legume roots or stems with soil bacteria of the Rhizobiaceae results in the formation of nodules that become symbiotic nitrogen-fixing organs. Within the infected cells of these nodules, bacteria are enveloped in a membrane of plant origin, called the peribacteroid membrane (PBM), and divide and differentiate to form nitrogen-fixing bacteroids. The organelle-like structure comprised of PBM and bacteroids is termed the symbiosome, and is the basic nitrogen-fixing unit of the nodule. The major exchange of nutrients between the symbiotic partners is reduced carbon from the plant, to fuel nitrogenase activity in the bacteroid, and fixed nitrogen from the bacteroid, which is assimilated in the plant cytoplasm. However, many other metabolites are also exchanged. The metabolic interaction between the plant and the bacteroids is regulated by a series of transporters and channels on the PBM and the bacteroid membrane, and these form the focus of this review.

The Bradyrhizobium japonicum aconitase gene (acnA) is important for free-living growth but not for an effective root nodule symbiosis

The Bradyrhizobium japonicum acnA gene encoding the tricarboxylic acid cycle enzyme aconitase was cloned and characterized. The gene was mapped immediately upstream of the cytochrome c biogenesis gene cycV and found to be transcribed in the opposite direction. The nucleotide sequence of acnA was determined; the derived amino acid sequence shared a significant similarity with bacterial aconitases and with the human iron-responsive-element-binding protein. The level of expression of the acnA gene under aerobic growth conditions was 10-fold higher than that under anaerobic conditions. The start of transcription was mapped by primer extension experiments, and the putative promoter was found to contain a typical -10 but no -35 consensus sequence for a sigma70-type RNA polymerase. A 5' deletion removing all but 19 nucleotides upstream of the start of transcription completely abolished gene expression. An acnA mutant was constructed by gene disruption, and the mutant phenotype was characterized. Growth of the mutant was severely affected and could not be corrected by the addition of glutamate as a supplement. Although aconitase activity in free-living cells was decreased by more than 70%, the ability of the mutant to establish an effective root nodule symbiosis with soybean plants was not affected. This suggested either the existence of a second aconitase or the compensation for the mutant defect by symbiosis-specific metabolites synthesized in the root nodules.