Lesion Target Spots: a Model for Cell Death Signaling

Disease lesion (Les) mimics are the most frequently occurring dominant mutants from EMS mutagenesis in maize. From over 50,000 screened M1 plants we have been able to identify 51 separate dominant cases.

Mutants range widely in expression, but have the common phenotype of leaf lesions that are strikingly similar to those caused by various leaf blight diseases. Phenotypes occur in the absence of a pathogen. Lesions are initiated by sunlight and certain chemicals, and can be chlorotic, necrotic, or sequentially both. Lesions of different mutants vary in size, shape, color, frequency, distribution, time of onset, position, etc. In some mutants, lesions expand to cover the leaf, resulting in its senescence. Particular cells on the leaf surface may be highly susceptible to damage by sunlight, at specific developmental stages and within a certain temperature range.

There may be two signals involved. The first arises from dissolution of the cell membrane, which releases highly active cell contents that cause lethal damage to neighboring cells. Damage spreads continuously outward, forming a necrotic lesion that stops growing when conditions change. The second signal is revealed by the “Target Spot” oscillatory phenotype --- a central spot of dead tissue surrounded by alternating rings of healthy and dead tissue. This phenotype suggests there is signaling between dead and living cells, across living tissue, that causes lesion formation.  This signal proceeds more rapidly than the first, through several ranks of normal cells without damaging them, during a diurnal cycle of conditions that do not favor lesion formation.