Soil and rhizosphere as habitats for Pseudomonas inoculants: new knowledgeon distribution, activity and physiological state derived from micro-scaleand single-cell studies

Pseudomonas spp. comprise an important group of bacteria used for biologica l control of microfungi in the plant rhizosphere. Successful performance of microbial inoculants requires both establishment, proliferation and activi ty under in situ conditions. To identify the factors controlling fate and p erformance of the inoculants, small-scale analyses are needed due to the he terogeneity characterizing the complex soil and rhizosphere environments. D irect staining techniques and advanced microscopy have provided the first d etailed single-cell images of root colonization by these bacteria using flu orescent antibodies, fluorescent in situ hybridization and marker gene tech nology. These tracking methods have, in conjunction with activity assays, p rovided high-resolution data on the metabolic activity and growth of the in oculants. Finally, Pseudomonas reporter bacteria constructed to sense phosp horus, nitrogen, iron, and oxygen limitations have provided new insight int o the significance of growth-limiting factors in the soil and along the roo t. The present work reviews the current knowledge on Pseudomonas inoculants in soil and rhizosphere based on these modern techniques. Finally, some pe rspectives for future studies are discussed.