4. Plant–microbe interactions
Gage et al. (1996) labelled Rhizobium meliloti MB5OI with the gfp marker via the broad-host-range plasmid, pTB93F. The heterologous gfp was expressed constitutively in this microorganism. Visualization of the R. meliloti cells during infection of the root and subsequent nodulation was observed. The gfp system has also been used to visualize bacterial distributions on root surfaces (Fig. 1d). Bloemberg et al. (1997) constructed a gfp plasmid (pGBS) which was maintained in Pseudornonas fluorescens WC5365 cells for 7 days without antibiotic selective pressure. The gfp was constitutively expressed in the cells without nutrients being supplied. Under laboratory conditions, the gfp-labelled WCS36S cells were observed to form microcolonies at the borders of adjacent epithelial cells on the root surface of tomato seedlings. Pseudomonas fluorescens A506 has been marked chromosomally with a red-shift gfp variant (Tombolini et al., 1997). Because of the improved fluorescent intensity of the red-shift gfp and a strong constitutive promoter (PpsbA from Amaranthus hybridus), individual cells were visualized on the root surface of Lotus japonicus plants using epifluorescent confocal laser scanning microscopy.