Auxotrophy in rhizobia revisited

Abstract

Among the various types of mutations studied in rhizobia, the auxotrophic mutations (which confer on the mutants the inability to synthesize certain essential substances such as amino acids, vitamins and nucleic acids), are the most favoured ones as these can be used as suitable markers for genetic analysis. An important property of rhizobia is their effectiveness i.e. their ability to fi x atmospheric nitrogen into ammonia within the nodule. Special interest in this category of mutations by rhizobial geneticists is due to the fact that there is a strong correlation between the metabolic defects and the ineffectiveness (Nod -and/or Fix -) of the rhizobial strains. Auxotrophic mutants of various species of rhizobia with defects in the synthesis of nucleic bases, vitamins and amino acids have been obtained by mutagenising with physical, chemical and Tn5 mutagens. These mutants have been used in mapping studies as well as in establishing a correlation between its metabolic requirement and symbiotic relationship with the host plant. The present review deals with the isolation of auxotrophs, and their genetic, biochemical and symbiotic characterization. The review also encompasses the studies on the elucidation of biosynthetic pathways of nutritional substances in rhizobia.

Elevated atmospheric CO2 increases microbial growth rates in soil: results of three CO2 enrichment experiments

“…Glucose is commonly used to induce the respiratory activity of soil heterotrophic microorganisms (Anderson & Domsch, 1978). Note that soil contains not only oligotrophic species capable of efficient growth on glucose, but also auxotrophic microorganisms, those that cannot grow in the absence of vitamins, amino acids, and growth factors (Alexander, 1991; Yadav, 2007). Accordingly, we applied – along with glucose and mineral salts – mixtures containing yeast extract in order to stimulate a broader spectrum of soil microorganisms with diverse nutrient requirements.…”

Proteomic analysis of free-living Bradyrhizobium diazoefficiens: highlighting potential determinants of a successful symbiosis

“…Proteins associated with amino-acid and lipid metabolism may be particularly important for free-living cells, and several of those proteins identified in CPAC 7 have been previously reported in B. japonicum CPAC 15 [ 23 ]. Beyond their main functions, proteins in these two categories may also play important roles at various stages of the symbiotic interaction, since auxotrophic mutants in both of them are defective in both nodulation and in N 2 -fixation abilities [ 24 ].…”