Pseudomonas putida 1290 is a model organism for the study of bacterial degradation of the plant hormone indole-3-acetic acid (IAA). This property is encoded by the iac gene cluster. Insertional inactivation and/or deletion of individual iac genes and heterologous expression of the gene cluster in Escherichia coli were combined with mass spectrometry to demonstrate that iac-based degradation of IAA is likely to involve 2-hydroxy-IAA, 3-hydroxy-2-oxo-IAA, and catechol as intermediates. The first gene of the cluster, iacA encodes for the first step in the pathway, and also can convert indole to indoxyl to produce the blue pigment indigo. Transcriptional profiling of iac genes in P. putida 1290 revealed that they were induced in the presence of IAA. Based on results with an iacR knockout, we propose that this gene codes for a repressor of iacA expression and that exposure to IAA relieves this repression. Transformation of P. putida KT2440 (which cannot degrade IAA) with the iac gene cluster conferred the ability to grow on IAA as a sole source of carbon and energy, but not the ability to chemotaxi towards IAA. We could show such tactic response for P. putida 1290, thus representing the first demonstration of bacterial chemotaxis towards IAA. We discuss the ecological significance of our findings, and specifically the following question: under what circumstances do bacteria with the ability to degrade, recognize, and move towards IAA have a selective advantage?

The effect of temperature on Fusarium wilt of lettuce (Lactuca sativa), caused by Fusarium oxysporum f. sp. lactucae, was examined in a controlled environment. Nine lettuce cultivars planted in infested potting mix (500 or 5,000 CFU/g) were maintained under high/low diurnal temperature regimes of 26/18°C, 28/20°C, or 33/26°C. Three cultivars were resistant to Fusarium wilt under all test conditions, and thus were little affected by differences in inoculum level or temperature. The remaining cultivars were more susceptible and manifested more severe symptoms at the higher inoculum level and when maintained at higher temperatures. The tendency for the disease to be more severe under warmer conditions may be due, in part, to an effect of temperature on growth of the pathogen. Radial growth rates calculated for six isolates of F. oxysporum f. sp. lactucae were observed to increase from 10°C up to an apparent maximum near 25°C. The results of this study suggest that growers can reduce the risk of damage from Fusarium wilt by avoiding susceptible cultivars during the warmest planting periods. Further, isolates of F. oxysporum f. sp. lactucae were shown to have a distinctive colony morphology in culture, which made it possible to distinguish them from nonpathogenic strains.

Lettuce cultivars adapted to Californian growing conditions were screened for resistance to fusarium wilt caused by Fusarium oxysporum f.sp. lactucae in order to determine if differences in susceptibility among currently grown cultivars might contribute to management of this disease. Based on a preliminary evaluation of 46 cultivars, eight that were among the most resistant of their horticultural type (iceberg, romaine or leaf) were selected for further testing. The relative susceptibility of these cultivars was assessed by: (i) root-dip inoculation, (ii) sowing seeds into infested potting mix and (iii) transplanting seedlings into an infested field. Evaluations of disease severity showed that both methods (i) and (ii) produced cultivar rankings that were significantly correlated with rankings from field trials [method (iii)]. Two romaine and two leaf cultivars were highly resistant to fusarium wilt (mean disease severity rating of £ 1AE3 on a 1-4 scale) under all test conditions. Other romaine and leaf cultivars, however, were highly susceptible in root-dip tests, so there was no consistent association between cultivar type and susceptibility to fusarium wilt. Likewise, there was considerable variation in susceptibility to wilt among iceberg cultivars, but all were significantly more susceptible than the most resistant romaine and leaf cultivars.

The severity of fusarium wilt is affected by inoculum density in soil, which is expected to decline during intervals when a non-susceptible crop is grown. However, the anticipated benefits of crop rotation may not be realized if the pathogen can colonize and produce inoculum on a resistant cultivar or rotation crop. The present study documented colonization of roots of broccoli, cauliflower and spinach by Fusarium oxysporum f. sp. lactucae, the cause of fusarium wilt of lettuce. The frequency of infection was significantly lower on all three rotation crops than on a susceptible lettuce cultivar, and the pathogen was restricted to the cortex of roots of broccoli. However, F. oxysporum f. sp. lactucae was isolated from the root vascular stele of 7Á4% of cauliflower plants and 50% of spinach plants that were sampled, indicating a greater potential for colonization and production of inoculum on these crops. The pathogen was also recovered from the root vascular stele of five fusarium wilt-resistant lettuce cultivars. Thus, disease-resistant plants may support growth of the pathogen and thereby contribute to an increase in soil inoculum density. Cultivars that were indistinguishable based on above-ground symptoms, differed significantly in the extent to which they were colonized by F. oxysporum f. sp. lactucae. Less extensively colonized cultivars may prove to be superior sources of resistance to fusarium wilt for use in breeding programmes.