Identification of a maize (Zea mays) chitinase allele sequence suitable for a role in ear rot fungal resistance

Authors: Dowd, Patrick; Naumann, Todd; Price, Neil; Johnson, Eric

Submitted to: AGRI GENE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/2017
Publication Date: 10/14/2017
Citation: Dowd, P.F., Naumann, T.A., Price, N.P., Johnson, E.T. 2017. Identification of a maize (Zea mays) chitinase allele sequence suitable for a role in ear rot fungal resistance. AGRI GENE. 7:15-22. http://dx.doi.org/10.1016/j.aggene.2017.10.001.
DOI: https://doi.org/10.1016/j.aggene.2017.10.001

Interpretive Summary: Molds that rot corn ears cause millions of dollars of damage in the U.S. Some ear rot molds also produce toxins harmful to people and animals. More information on the genes responsible for resistance to ear rots is needed in order to more rapidly develop improved corn lines. A gene potentially responsible for degrading ear rot mold “skin” was sequenced in several inbred lines with reported resistance and susceptibility to ear rot molds. The gene sequence of most of the resistant inbreds was likely to produce a fully functional protein, while the opposite was true for the susceptible inbreds. One gene form likely to be fully functional was transgenically expressed in yeast, and found to produce a protein that was active, resistant to degradation by fungal proteases, and enhanced the toxicity of fungicides to an ear mold. Corn cell clumps that transgenically expressed the same gene form were also more resistant to the ear mold than cell clumps that didn’t. Introduction of the fully functional form of this gene into corn lines is likely to increase the quality and safety of corn produced for producers, users, and consumers.

Technical Abstract: Chitinases are thought to play a role in plant resistance to pathogens, but the extent of this role is unknown. The gene for a maize chitinase “chitinase 2” previously reported to be induced by two ear rot pathogens in one maize inbred, was cloned from mRNA isolated from milk stage kernels of several different inbreds reported to be susceptible or resistant to ear rot pathogens. The chitinase gene sequence of some of the susceptible inbreds had frame shifts which would result in a nonfunctional protein. Other susceptible inbreds, and a few resistant inbreds, were missing regions that would contribute to chitin binding. A putative functional clone from a resistant inbred was expressed in yeast, and produced a protein with chitinase activity against different oligomers of N-acetyl glucosamine and modified chitin. The yeast-produced chitinase was also resistant to degradation by proteases from maize ear rot fungi, and enhanced antifungal activity of miconazole towards Fusarium graminearum. When introduced in maize callus transgenically, the callus expressing the chitinase 2 gene had significantly less growth of the ear rot pathogen, F. graminearium, than callus which did not express the gene. This information suggests susceptibility and resistance to ear rot pathogens is influenced not only by expression levels, but also by the sequence of putative resistance genes. Previous reports of increased expression of putative resistance genes need to be tempered with the realization that they may play no role in resistance if the genes code for nonfunctional or reduced function proteins.