1.3.39
THE MAIZE LETHAL LEAF SPOT 1MUTANT RESISTS FUNGAL INFECTION AT THE LEAF EPIDERMIS


SABINE HANTKEl, CARL SIMMONSl, SUSAN GRANTl, GURI JOHAL2 and STEVE BRIGGSl

lPioneer Hi-Bred International, Inc., 7300 N.W. 62nd Avenue, Johnston, IA 50131; 2Department of Agronomy, University of Missouri, Columbia, MO 65211

Background and objectives
The maize lethal leaf spot 1 (lls1) mutant is a recessive lesion mimic that forms large necrotic lesions upon ageing and environmental stresses such as light, wounding, and pathogen attack. The Llsl gene was cloned by transposon tagging and encodes a novel protein that is highly conserved in plants. The predicted LLS1 protein has homology to ring-hydroxylating dioxygenases and may function to degrade a phenolic mediator of cell death [1]. Expression analysis and pathology studies were used to find out more about the function of the Lls1 gene product and its possible involvement in the defense pathway.

Results and conclusions
In situ hybridization revealed that the Lls1 gene is strongly expressed in upper and lower epidermis, but not in the mesophyll of wild-type leaves. Based on this expression pattern a preferential function of LLS1 in the epidermis was postulated. To test if lack of Llsl gene product leads to enhanced resistance to pathogens at the epidermis, the lls1 mutant was challenged with two fungal pathogens: Cochliobolus heterostrophus, which is a necrotroph that enters directly through the epidermis, and Puccinia sorghi which is a biotroph that enters through the stomates and colonizes the mesophyll. Disease resistance was measured by the three major criteria: reduced lesion number, reduced presence of viable pathogen in the lesion (lesion sterility) and reduced fungal growth rate on the epidermis, all as compared to infected wild-type piants.
Lls1 mutants exhibit enhanced resistance to C. heterostrophus, meeting all the criteria tested: lesion number is reduced about 40% upon infection, and the lesions that do form are usually sterile. The reduced lesion number is correlated with a marked reduction of C. heterostrophus germination, hyphal growth, haustoria formation, and lesion initiation on the leaf epidermis. Resistance of the Llsl mutant to the rust pathogen P. sorghi meets two of the criteria: pustule formation on lls1 is reduced, and the fungal germination and hyphal growth is slowed on the epidermis. However, P. sorghi was observed to form spore-filled pustules on llsl and even green islands within lls1-type necrotic tissue. This indicates that the llsl mutant is not resistant to P. sorghi in the third manner, the formation of sterile lesions. This difference is most likely based on the invasion strategy that enables P. sorghi to colonize the mesophyll. The site of successful colonization correlates with the lack of Lls1 expression in wild-type mesophyll, indicating that an antifungal factor is produced where Lls1 is normally expressed.

References
1. Gray, J., Close, P.S., Briggs, S.P., Johal, G.S. 1997. Cell 89, 25-31.