The extremely large number of leaves produced by terrestrial and aquatic plants provide habitats for colonization by a diversity of microorganisms. This review focuses on the bacterial component of leaf microbial communities, with emphasis on Pseudomonas syringae—a species that participates in leaf ecosystems as a pathogen, ice nucleus, and epiphyte. Among the diversity of bacteria that colonize leaves, none has received wider attention than P. syringae, as it gained notoriety for being the first recombinant organism (Ice− P. syringae) to be deliberately introduced into the environment. We focus on P. syringae to illustrate the attractiveness and somewhat unique opportunities provided by leaf ecosystems for addressing fundamental questions of microbial population dynamics and mechanisms of plant-bacterium interactions. Leaf ecosystems are dynamic and ephemeral. The physical environment surrounding phyllosphere microbes changes continuously with daily cycles in temperature, radiation, relative humidity, wind velocity, and leaf wetness. Slightly longer-term changes occur as weather systems pass. Seasonal climatic changes impose still a longer cycle. The physical and physiological characteristics of leaves change as they expand, mature, and senesce and as host phenology changes. Many of these factors influence the development of populations of P. syringae upon populations of leaves. P. syringae was first studied for its ability to cause disease on plants. However, disease causation is but one aspect of its life strategy. The bacterium can be found in association with healthy leaves, growing and surviving for many generations on the surfaces of leaves as an epiphyte. A number of genes and traits have been identified that contribute to the fitness of P. syringae in the phyllosphere. While still in their infancy, such research efforts demonstrate that the P. syringae-leaf ecosystem is a particularly attractive system with which to bridge the gap between what is known about the molecular biology of genes linked to pathogenicity and the ecology and epidemiology of associated diseases as they occur in natural settings, the field.

Bacteria in the leaf ecosystem with emphasis on Pseudomonas syringae-a pathogen, ice nucleus, and epiphyte.

Publisher Summary The role of microscopically small organisms in numerous soil processes, particularly those affecting plant productivity, is well recognized. Less well known is the quantity of recently gathered information concerning microorganisms as the components of major plant communities and the extent to which they participate in the total energy flow therein. Although the soil microflora is the single most important group in the annual turnover of energy trapped by photosynthesis, no broad general review of the microflora of grassland has heretofore been compiled. There is a rich microflora associated with the surfaces of living plants that is either nonpathogenic or at most functioning at an extremely marginal level of pathogenicity. This microflora feeds primarily on exudates and sloughed cellular material. It commonly is divided into two categories: (a) the aboveground, shoot-associated microflora, or broadly referred as the “phyllosphere” and (b) the belowground, root-associated microflora, broadly referred as the “rhizophere.” This chapter examines the microflora of grassland litter, the microflora of grassland soils, biomass and bioactivity measurements, the humic component of grassland soil, and nitrogen transformations in grassland soils.

The microflora of grassland.

Biological nitrogen fixation is a characteristic of certain micro-organisms, which may be free-living or occur in symbiotic association with higher plants The purpose of this paper is to summarize some of the biological and ecological aspects of nitrogen-fixation by free-living forms Biochemical aspects have been reviewed in other contributions to this discussion by Drs Wilson, Burris, and Cox & Fay Nitrogen fixation by heterotrophic micro-organisms has been considered by Jensen (1965); nitrogen fixation by blue-green algae by Fogg & Stewart (1965), and by Stewart (1966, 1969), while Moore (1966) has evaluated the contribution of nitrogen-fixing micro-organisms to soil fertility

Biological and ecological aspects of nitrogen fixation by free-living micro-organisms

Summary: Populations of heterotrophic bacteria occurring on healthy green leaves of Lolium perenne were estimated using a dilution technique. Bacterial populations in spring and summer were relatively low compared with counts obtained in September when the highest maximum and monthly mean air temperature was recorded. The area of leaf surface colonized rose from about 0·0001 % in May to approximately 0·1 % in September. Over 600 isolates were examined and most found to be Gram-negative and chromogenic. The bacterial populations consisted of only a few taxa and the variations in total numbers were due to changes in the relative abundance of these. The predominant flora were identified to the genera Chromobacterium, Corynebacterium, Pseudomonas and Xanthomonas. Leaf impression and overpour techniques gave some indication of the micro-ecology of bacteria on the leaf, and showed the presence of actinomycetes which were rarely observed on dilution plates.

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Quantitative and Qualitative Studies of Phylloplane Bacteria from Lolium perenne

A phyllospheric bacterial culture, previously reported to partially replace nitrogen fertilizer (B. R. Patti and A. K. Chandra, Plant Soil 61:419-427, 1981) was found to contain a fluorescent pseudomonas which was identified as Pseudomonas putida and a Corynebacterium sp. The P. putida isolate was found to produce an extracellular cutinase when grown in a medium containing cutin, the polyester structural component of plant cuticle. The Corynebacterium sp. grew on nitrogen-free medium but could not produce cutinase under any induction conditions tested, whereas P. putida could not grow on nitrogen-free medium. When cocultured with the nitrogen-fixing Corynebacterium sp., the P. putida isolate grew in a nitrogen-free medium, suggesting that the former provided fixed N2 for the latter. These results suggest that the two species coexist on the plant surface, with one providing carbon and the other providing reduced nitrogen for their growth. The presence of cutin in the medium induced cutinase production by P. putida. However, unlike the previously studied fungal systems, cutin hydrolysate did not induce cutinase. Thin-layer chromatographic analysis of the products released from labeled apple fruit cutin showed that the extracellular enzyme released all classes of cutin monomers. This enzyme also catalyzed hydrolysis of the model ester substrates, p-nitrophenyl esters of fatty acids, and optimal conditions were determined for a spectrophotometric assay with p-nitrophenyl butyrate as the substrate. It did not hydrolyze triacyl glycerols, indicating that the cutinase activity was not due to a nonspecific lipase. It showed a broad pH optimum between 8.0 and 10.5 with 3H-labeled apple cutin as the substrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Discovery of a cutinase-producing Pseudomonas sp. cohabiting with an apparently nitrogen-fixing Corynebacterium sp. in the phyllosphere

The mulberry leaves were shown to harbour substantial populations of bacteria, streptomycetes, yeasts, and moulds. Azotobacter and Beijerinckia were observed to contribute to nearly 5 to 10 per cent of the bacterial population. When grown in water culture under sterile conditions, Azotobacter inoculation on the leaf or root surface was found to increase plant growth, dry wt, and nitrogen content of the mulberry. The beneficial effect of Azotobacter was largely influenced by the presence of a carbon source in the plant nutrient solution. The root inoculation in comparison to leaf application was found to confer greater benefits to the growing plant. The presence of carbohydrates and amino acids in the leaf leachates of mulberry was shown. The mutual beneficial nature of the association of the plant and Azotobacter has been brought to light.

JV Bhat

Papers

Interrelations of micro-organisms and mulberryII. Phyllosphere microflora and nitrogen fixation in leaf and root surfaces