Bioluminescent pv. as a Tool to Quantify Bacteria , Screen Germplasm, and Identify Infection Routes on Leaf Surfaces.

Imaging technology can provide insight into biological processes governing plant-pathogen interactions. We created and used a bioluminescent strain of pv. (Xg) to quantify infection processes in plants using tomato as a model. An pv. is one of the four species that causes bacterial spots in tomatoes. We used Xg to quantify bacterial growth , to assess disease severity in resistant and susceptible tomato lines, and to observe infection routes in leaves. A positive and significant linear correlation r (67) = 0.57, ≤ 0.0001 was observed between bioluminescence signals emitted by Xg i and bacterial populations determined through dilution plating. Based on bioluminescence imaging, resistant and susceptible tomato lines had significantly different average radiances. In addition, there was a positive and significant correlation r = 0.45, p = 0.024 between pv. -inoculated tomato lines evaluated by bioluminescence imaging and tomatoes rated in the field using the Horsfall-Barrat Scale. Heritability was calculated to compare the genetic variance for disease severity using bioluminescence imaging and classical field ratings. The genetic variances were 25 and 63% for bioluminescence imaging and field ratings, respectively. The disadvantage of lower heritability attained by bioluminescence imaging may be offset by the ability to complete germplasm evaluation experiments within 30 days rather than 90-120 days in field trials. We further explored pv. infection routes on leaves using spray and dip inoculation techniques. Patterns of bioluminescence demonstrated that the inoculation technique affected the distribution of bacteria, an observation verified using scanning electron microscopy (SEM). We found significant non-random distributions of pv. on leaf surfaces with the method of inoculation affecting bacterial distribution on leaf surfaces at 4 h postinoculation (hpi). At 18 hpi, regardless of inoculation method, pv. localized on leaf edges near hydathodes based on bioluminescence imaging and confirmed by electron microscopy. These findings demonstrated the utility of bioluminescent pv. to estimate bacterial populations , to select for resistant germplasm, and to detect likely points of infection.