Submitted to: Maize Genetics Cooperation Newsletter
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2001
Publication Date: 5/15/2002
Citation: SIMMONS, C.R., TOSSBERG, J.T., SANDAHL, G.A., MARSH, W.A., DOWD, P.F., DUVICK, J.P., BRIGGS, S.P. MAIZE PATHOGEN DEFENSES ACTIVATED BY AVIRULENCE GENE AVRRXV. MAIZE GENETICS COOPERATION NEWS LETTER. 2002. v. 76. p. 40-41 Interpretive Summary: Diseases of crop plants cause hundreds of millions of dollars in direct losses in the U.S. each year and can also be responsible for contaminating foods with toxins or carcinogens. Plants have many defenses against disease, but little information is available on how this occurs or can be manipulated when desired. A method was devised to introduce a gene "switch" into corn tissue that is turned on by application of a test chemical. When the chemical was applied, several defensive systems were turned on. Application of this methodology should result in a more precise and environmentally acceptable (compared to chemical pesticides) method for activating natural plant defenses only when diseases are present or forecast. This application should result in greater profits for farmers, and a higher quality and safer product for consumers and other corn end users.
Technical Abstract: The avirulence gene avrRxv from Xanthomonas campestris pv vesicatoria controls gene-for-gene host resistance in tomato and pepper, and non-host resistance in various species including maize (Zea mays L.). Transient avrRxv expression in maize embryos and leaves suppresses coexpression of a reporter gene system, suggesting an avrRxv-mediated defense system activation, as related experiments have indicated. Consistent with this, PR1 protein accumulation is induced in these tissues. A survey of diverse genotypes demonstrates that this response is widespread and dominant or semi-dominant. Using an estrogen receptor/estrogen response element promoter construct driving avrRxv expression, healthy transgenic callus and cell suspensions were generated. Upon estradiol treatment, avrRxv mRNA was dramatically increased; and there occurred changes consistent with defense activation, namely slowed tissue growth, tissue browning, metabolite accumulation, and induced levels of PR1, chitinase, and cationic peroxidase. In addition, changes in mRNA profiles were investigated using a 1500-gene Affymetrix microarray. Twelve genes with three-fold or more avrRxv-induced mRNA abundance were identified, all members of known or suspected defense-related gene classes. These results reveal further details of the molecular nature of a maize defense response and indicate that a chemically-inducible maize defense system activation has been achieved using an avirulence gene from a heterologous host-