The role of the Les1 gene in the production of discrete necrotic lesions in leaves is being investigated. One question which is being asked is whether the Les1 gene is required in all stages of lesion formation. It is possible that the Les1 gene only triggers a cellular event which results in the production of cytotoxic compound(s). These compounds then diffuse to neighboring cells resulting in the observed lesion phenotype. Thus, Les1 could be required only for the initiation of lesions. Another possibility is that Les1 is also required for the enlargement of lesions. In this case, cells not containing the Les1 gene could not respond to a Les1 initiated lesion.

One method to study this question is the use of X-ray induced sectors of normal tissue in Les1 leaves. Les1 is located on the short arm of chromosome 2, approximately 5 map units proximal to B. It is therefore possible to use B to follow the loss of Les1 in leaf tissue. Seeds from the cross B Les1/b +, Pl x b +/b +, Pl were treated with 10,000 r of filtered X-rays in the dry seed state. Plants were observed for sectorial losses of B (green sectors in a purple leaf) and, hopefully, concomitant loss of Les1. None of the 173 plants grown in Missouri showed a loss of B or apparent sectoring for Les1. However, one out of 100 plants grown at Stanford-University in California showed a single sectorial loss of B and, presumably, Les1. The sector occurred in the second leaf from the top and was approximately 1/10 of the width of the leaf. The sector occurred only in this one leaf and over the entire length of the leaf. No lesions were observed to initiate within the green tissue of the sector, nor did lesions that initiated just outside of the sector's border extend within the green tissue. The rest of the leaf expressed lesions in the normal manner. Thus, if this sector is the result of the loss of B and Les1, then the Les1 gene is a cell autonomous trait for all aspects of lesion expression--requiring its presence for initiation of lesions as well as enlargement to form mature lesions. This experiment is being repeated next summer in order to observe further sectors. Also, in order to maintain plants which could form sectorial losses of these genes as well as produce a larger number of sectors in one plant, the production of a ring chromosome which contains B and Les1 is being attempted.

David Hoisington, Virginia Walbot and M. G. Neuffer

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