Bacteria associated with the plant rhizosphere may have beneficial effects on plant growth by providing nutrients and growth factors, or by producing antibiotics and siderophores, which antagonize phytopathogenic fungi and bacteria. There is considerable experimental support for the idea that plant growth promoting bacteria may be used as bio–fertilizers or biological disease control agents to increase agricultural yields. Recent advances in our understanding of the molecular biology of the systems responsible for plant growth stimulation are opening the way to strain improvement by genetic engineering.

Plant Beneficial Bacteria

Nucleoside antibiotics are found as diverse groups of secondary metabolites of microbial origin. They include a variety of structural modifications of nucleosides and nucleotides, often leading to intricate molecules. Their biological activities are also wide ranging, including antibacterial, antifungal, antitrypanosomal, antitumor, antiviral, herbicidal, insecticidal, immunostimulating, and often immunosuppressive properties. It is not surprising that nucleoside antibiotics exhibit such diverse biological activities, because nucleosides and nucleotides play pleiotropic roles in most fundamental cellular metabolic pathways such as metabolite carriers, energy donors, secondary messengers, and co factors for various enzymes. Thus, not only nucleic acid synthesis but also protein synthesis, glycan synthesis, and glycoprotein synthesis are targets of nucleoside antibiotics. The protein kinases, key enzymes for cell proliferation and differentiation, also require nucleotides as phosphate donors.

Nucleoside antibiotics: structure, biological activity, and biosynthesis

On the basis of ribosomal 16S sequence comparison, Brucella abortus has been found to be a member of the alpha-2 subdivision of the class Proteobacteria (formerly named purple photosynthetic bacteria and their nonphototrophic relatives). Within the alpha-2 subgroup, brucellae are specifically related to rickettsiae, agrobacteria, and rhizobiae, organisms that also have the faculty or the obligation of living in close association to eucaryotic cells. The composition of Brucella lipid A suggests a close phylogenetical relationship with members of the alpha-2 group. The chemical analysis of the lipid A fraction revealed that Brucella species contain both glucosamine and diaminoglucose, thus suggesting the presence of a so-called mixed lipid A type. The serological analysis with polyclonal and monoclonal antibodies is in agreement with the existence of mixed lipid A type in B. abortus. The amide-linked fatty acid present as acyl-oxyacyl residues were 3-O-C(16:0)12:0, 3-O-C(16:0)13:0, 3-O-C(16:0)14:0, and 3-O-C(18:0)14:0. The only amide-linked unsubstituted fatty acid detected was 3-OH-C16:0. The ester-linked fatty acids are 3-OH-C16:0, 3-OH-C18:0, C16:0, C17:0, and C18:0. Significant amounts of the large-chain 27-OH-C28:0 were detected together with traces of 25-OH-C26:0 and 29-OH-C30:0. Comparison of the Brucella lipid composition with that of the other Proteobacteria also suggests a close phylogenetical relationship with members of the alpha-2 subdivision. The genealogical grouping of Brucella species with pericellular and intracellular plant and animal pathogens as well as with intracellular plant symbionts suggests a possible evolution of Brucella species from plant-arthropod-associated bacteria.

Brucella abortus 16S rRNA and lipid A reveal a phylogenetic relationship with members of the alpha-2 subdivision of the class Proteobacteria.

https://mmbr.asm.org/content/mmbr/50/2/193.full.pdf

Signal exchange in plant-microbe interactions.

Publisher Summary The most important characteristic that immediately distinguishes agrobacteria from most other bacteria is their ability to induce tumor formation in plants. Agrobacteria transform “conditioned” plant cells into tumor cells by introducing into them a piece of genetic information which in established plant tumor cells can be recovered as T-DNA integrated into the nuclear plant DNA. Tumor cells characteristically contain a set of unusual amino acid derivatives that have never been found in normal plant cells. Crown gall cells carry a piece of DNA that is homologous to a part of A. turnefuciens DNA integrated into their nuclear DNA. The T-DNA is responsible for the phytohormone-independent growth of tumor cells and encodes enzymes that synthesize opines in the tumor cells. Moreover, the finding that the T-DNA found in different tumor tissues is of a fixed size poses the question of whether T-DNA integrates into plant DNA via a mechanism similar to that by which transposable elements integrate into foreign DNA. The answer to this question is not known. However, it is known that the ends of the T-region are important for tumor induction.

The Molecular Genetics Of Crown Gall Tumorigenesis

Studies on the mode of action of agrocin 84

Cell surface carbohydrates of Agrobacteriumtumefaciens involved in adherence during crown gall tumor initiation

Studies have been carried out in detail on the lipid make up of Agro-bacterium tumefaciens using virulent agrocin-sensitive and avirulent agrocin-resistant strains. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylmonomethylethanolamine, and phosphatidylglycerol are the major phospholipids present in A. tumefaciens. Although the total lipid content is increased due to development of resistance towards agrocin, the proportion of phospholipids and neutral lipids in the total lipid fraction remains almost the same either in the normal or in the resistant strains. The relative proportion of most of the individual phospholipid and neutral lipid components also remains unaltered due to development of resistance towards agrocin. With the exception of the proportion of neutral lipid, those of total lipid and phospholipid decrease steadily with an increase in the age of the culture. It is further revealed that agrocin resistance brought about an increase in the amount of unsaturated fatty acids with concomitant decrease of saturated fatty acid content in A. tumefaciens as obtained by gas-liquid chromatographic analysis.

Lipid profile of the strains of agrobacterium tumefaciens in relation to agrocin resistance

Physical & chemical studies on lipopolysaccharide of Agrobacterium tumefaciens.

The isoenzyme patterns of several hydrolytic enzymes, viz., acid phosphatase, alkaline phosphatase, and catalase, in virulent and avirulent, agrocin-resistant Agrobacterium tumefaciens have been studied. Agrocin resistance brought about genetic repression in regard to the synthesis of one of the isoenzymes in each of these three enzymes. Avirulent resistant strains also showed a reduction in the activities of all the hydrolases. Although Mg 2+ stimulated the activity of alkaline phosphatase, no absolute requirement of any metal ions for the activity of the enzyme was observed.

Isoenzyme patterns of several hydrolytic enzymes of agrobacterium tumefaciens

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