MICROSCOPIC STUDY TO DETERMINE THE COLONIZATION PATTERN OF PLANT GROWTH-PROMOTING Bacillus sp. (ZK) AND Rhizobium sp. (8.1.2.1) IN MAIZE AND TOMATO SEEDLINGS

The use of plant growth-promoting bacteria (PGPB) raise as alternative to decrease of use of chemical inputs in agriculture, and for a better exploitation of the beneficial plant-bacteria interactions, the plant colonization patterns by inoculant strains are fundamental to determine the mechanisms of growth-promotion and the compatibility level between the associative pair. Studies carried out by the Research Group in Development and Application of Biofertilizers (UEL) identified new bacterial strains (Rhizobium sp. strain 8.1.2.1 and Bacillus sp. strain ZK) with high potential of use to promote the growth of important crops, such as maize, soybean and tomato. In spite of this, no information is currently available to describe the relationships between these strains and inoculated plants. Aiming to clarify this point, this work applied microscopic observations in monoxenic plant-bacteria systems to determine the root colonization pattern of maize and tomato plants by the PGPB Rhizobium sp. and Bacillus sp. Maize and tomato seedlings obtained from surface-sterilized seeds were grown in test tubes (90 mL) containing 40 mL of agar-water (1% w/v) and inoculated with bacterial suspensions (1 mL at 106 cells mL-1) at the 7th day of germination. Plants were grown at constant temperature of 25 ± 2 °C and 12 h photoperiod for additional 14 days (maize) or 21 days (tomato). At the end of the growth period, roots from both plants were sampled (0.5 – 1.0 cm) and fixed with paraformaldehyde (4% in PBS buffer 1.0 M) for 24 h following wash (PBS 1.0 M) and storage at -20 °C in ethanol:PBS (1:1, v/v) solution until visualization. Root samples were stained with DAPI (1µg ml-1) for 5 min and mounted in microscopic slides to visualization under an epifluorescence microscope Axio Imager 2 (Carl Zeiss, Germany). Micrographs of root surface were processed using the software AxioVision 4TM. The results demonstrated the exclusive colonization of rhizoplane in both plants, demonstrating the non-endophytic character of studied PGPB strains. Bacillus sp. was observed in microcolonies close to root hairs and forming biofilms on the root surface, with increased density of bacterial cells in association with tomato. The same pattern was observed for Rhizobium, although a denser colonization was observed in association with maize. The results demonstrated the ability of both PGPB to colonize the root surface of maize and tomato plants in high population density, while indicates a differential compatibility when considering the different plant genotypes.