USE OF A LUX-BASED PROCEDURE TO RAPIDLY VISUALIZE ROOT COLONIZATION BY PSEUDOMONAS-FLUORESCENS IN THE WHEAT RHIZOSPHERE

The bioluminescently marked Pseudomonas fluorescens strain 5RL, has be en used previously to follow colonisation of soy bean roots (De Weger et al. [1991] Appl. Environ. Microbiol. 57:36-41). In the present pape r the method has been further developed and optimized for wheat roots and it is used to get a quick overview of the colonisation patterns of many different root systems at the same time. Colonisation was follow ed on wheat plants grown in our gnotobiotic sand system (Simons et al. , 1996. Mol Plant Microbe Interact 9: 600-607) and the following resul ts were obtained. (i) A spatio-temporal analysis of the colonisation o f wheat roots showed that 4 days after planting the highest bacterial activity was observed at the upper part of the root. After 6 days the high bacterial activity at the upper part was further increased, where as spot-like activities were observed on the lower root parts, possibl y due to micro-colonies. (ii) Bacterial mutations causing lack of moti lity or auxotrophy for amino acids resulted in impaired colonisation o f the lower root parts, indicating that motility and prototrophy for t he involved amino acid(s) are important factors for wheat root colonis ation by strain 5RL. (iii) Coinoculation of strain 5RL with other wild type Pseudomonas strains on the root influenced the colonisation patt ern observed for strain 5RL. Colonisation was not visually affected wh en the competing strain was a poor root coloniser, but was severely re duced when the competing strain was a good root coloniser. The results show that the spatio-temporal colonisation of wheat root by P. fluore scens strain 5RL and derivatives is similar to that of strain WCS365 o n tomato. The advantage of the use of lux-marked strains is that the r esults are obtained much quicker than when conventional methods are us ed and that the result is supplied as an image of the colonisation pat tern of many different roots.