Observations on the effect of light on the progression of lethal leaf-spot lesions
--P.S. Close, J. Gray and G. Johal

We examined the effect of light on lesion development in the lethal leaf spot1 (lls1) mutation of maize. Precedent for this approach was established by Craig Echt (MNL 60:49, 1986), who reported that light is necessary for the initiation of Les1 lesions but not for subsequent necrosis. It was also found that the relationship between photosynthetic activity and Les1 lesion development was not straightforward since lesions formed quite well in the white sectors of wd1, ring-9 (Wd, C-I) or j1 plants (Dave Hoisington loc. cit. ).

Using aluminium foil strips or thick paper covering to eliminate or reduce light immanent on the leaf surface, we found that lls1 lesions do not initiate in the dark and also that existing lesions do not propagate outwards. Lesions continue to form and propagate on upper and lower leaf parts that remain open to light. Lesions that are initiated due to wounding were also found to require light for continued propagation.

Using plexiglass filters of varying transmittance values we also found that wavelengths between 650 and 700 nm were required for lesion formation, and other wavelengths were not adequate. This region of the light spectrum spans the action spectra of both phytochrome-mediated processes and the 700 nm photosynthetic reaction center. We cannot at present determine the relative contribution of these mechanisms to the lesion development process.

Using an lls1-ij1 double mutant we examined the requirement for a functional chloroplast in lesion formation. In contrast to Les1 type lesions lls1 lesions were found to initiate only in the pale or dark green sectors of these plants but failed to develop in the albino sectors. Interestingly, it appears that a lesion initiated in a green sector separated by a narrow albino sector from a second green sector can "traverse" the albino sector and continue to propagate in the second green sector.

These results are consistent with the hypothesis that free radical generation is part of the process that leads to lesion initiation and/or propagation in the lls1 mutant plant. Both wounding and photosynthetic activity may contribute to free radical generation but the ability to dismute active radicals may be diminished in lls1 plants. Studies with free radical scavengers and free radical generators as well as the creation of other double mutants are being conducted to further investigate this phenomenon. 

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