Possible relation between level of cyclic hydroxamate and lesion formation

The presence of glucoside of the cyclic hydroxamate 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) and its role in resistance to many pathogens and insects has been reported by several workers. The concentration of DIMBOA was correlated with lesion development in the plant during pathogenesis. The dominant disease lesion mutants which mimic disease symptoms would be a model system to study the role of this glucoside in lesion formation. The three lesion mutants, Les1, Les*-1438 and Les*-1453, which show lesion development at 200 C, were used for estimation of DIMBOA. The plants were grown either in growth chambers maintained at 200 C with 14 hr day and 10 hr night or in the greenhouse for 21 days and then transferred to a 200 growth chamber. Initiation of lesions was observed after 4 to 5 days. Stem tissue (200-400 mg) from single plants was used in glucoside analysis. The DIMBOA analysis was done by the rapid procedure developed by Long et al. (Crop Science 14:601-603). The results are given below:
 
  Mean DIMBOA concentration in mg/g fresh weight
Expt. No. Sample Number Lesion sib Sample Number Normal sib
    Les1    
1 3 0.016 2 0.832
2 3 0.508 3 0.419
3 3 0.377 3 0.218
4 1 0.622 3 0.865
5 3 0.578 4 0.698
6 2 0.570 4 0.521
7 4 0.549 4 0.550
  19 Av. 0.460 ± 0.079 23 0.584 ±0.086
         
    Les*-1438    
1 1 0.615 1 0.959
2 3 0.822 3 0.830
3 5 0.541 3 0.554
  9 Av. 0.659 ± 0.084 7 0.781±0.119
         
    Les*-1453    
1 2 0.430 2 0.909
2 2 0.319 0.651
  4 Av. 0.374 ± 0.055 4 0.780 ±0.129

Our initial results (experiment 1 with each) were striking and highly significant. Clearly, normal sibs had almost twice as much DIMBOA as did the mutant sibs. Succeeding trials were erratic and less instructive although still showing the same trend. The reasons for this ambiguity are not known but one factor involved may be the age and general health of the plants used. Initially we were forced to use the same material which was overgrown and distinctly unhealthy from culture conditions. Other causes for variation in results may be (1) undetected day to day fluctuation in glucoside level, (2) individual plant differences and (3) variations due to environmental conditions beyond our control. In regard to item 3 we found that even though we can regulate lesion formation by varying temperature, this control is not as precise as we would like. Other factors such as humidity, light and daily temperature range which we did not properly control may be involved. However, it does appear that the level of DIMBOA is higher in normal sibs than in mutant plants at some stage in development. The level of DIMBOA concentration may be one of the factors responsible for the lesion initiation.

S. E. Pawar and M. G. Neuffer


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