Maize callus tissue derived from hybrids Mo17 x B73 and A632 x A619 and inbred A632 were inoculated with Corynebacterium michiganense ssp. nebraskense colony forming units (CFU) and monitored for population growth at time intervals. Mo17 x B73 is field-resistant to Goss's wilt whereas A632 x A619 and A632 are susceptible. Seed from each genotype was germinated for five days in the dark at 30 C. Slices from the scutellar node were placed on modified Murashige and Skoog medium and routinely transferred. Callus pieces (approx. 0.1 gram) were abraded with microforceps prior to application of 2 or 5 µl of inoculum. At sampling times, callus pieces were homogenized in 5 ml of phosphate buffer. This mixture was serially diluted and plated onto nutrient medium. Results (Tables 1 and 2) indicate that bacterial populations at most sampling points were lower in callus initiated from the resistant genotype than in callus initiated from the susceptible genotype.
Another experiment tested callus initiated from immature embryos of inbreds W64A and WF9T, both resistant to Goss's wilt. Differences between these two genotypes were not detected. Bacterial growth in these cultures was much lower than anticipated. Callus initiated from the scutellar node of A632 was also inoculated and assayed at one time interval as a comparative check. At 88 hours A632 callus contained 1 x 109 CFU/gram tissue, whereas W64A and WF9T callus contained no more than 3 x 106 CFU/gram tissue. The different method of callus initiation may have affected bacterial growth in W64A and WF9T. Alternatively, these genotypes may inhibit bacterial growth in callus to a greater extent than Mo17 x B73. Bacterial growth in callus initiated from immature embryos of both resistant and susceptible genotypes will be examined. The results indicate some promise for use of callus cultures in screening for resistance to this bacterial pathogen. The time interval of 48-96 hours after inoculation currently appears most suitable for detection of bacterial population differences.
Table 1. Estimates of Corynebacterium nebraskense cell concentration in 1 ml of phosphate buffer used to homogenize callus.
Table 2. Estimates of Corynebacterium nebraskense cell concentration per gram of callus tissue.
T. R. Rocheford, A. K. Vidaver and C. O. Gardner
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