Systemic acquired resistance (SAR) is a widely distributed plant defense system that confers broad-spectrum disease resistance and is accompanied by coordinate expression of the so-called SAR genes. This type of resistance and SAR gene expression can be mimicked with chemical inducers of resistance. Here, we report that chemical inducers of resistance are active in maize. Chemical induction increases resistance to downy mildew and activates expression of the maize PR-1 and PR-5 genes. These genes are also coordinately activated by pathogen infection and function as indicators of the defense reaction. Specifically, after pathogen infection, the PR-1 and PR-5 genes are induced more rapidly and more strongly in an incompatible than in a compatible interaction. In addition, we show that monocot lesion mimic plants also express these defense-related genes and that they have increased levels of salicylic acid after lesions develop, similar to pathogeninfected maize plants. The existence of chemically inducible disease resistance and PR-1 and PR-5 gene expression in maize indicates that maize is similar to dicots in many aspects of induced resistance. This reinforces the notion of an ancient plant-inducible defense pathway against pathogen attack that is shared between monocots and dicots.

Summary Systemic acquired resistance (SAR) appears to be a ubiquitous higher plant defence response. Resistance is dependent on the high‐level expression of SAR genes. Mutations, transgenes or chemicals that cause high‐level SAR gene expression also cause resistance to pathogens. Furthermore, when SAR is eliminated, through the reduction of salicylic acid accumulation or activity, generalized severe disease‐susceptibility occurs. Therefore, SAR appears to be an essential component of the defensive repertoire that ensures plant health in nature. Chemicals that activate SAR in the field, as well as crop varieties with constitutive SAR gene expression, will provide new solutions to disease problems for growers in the near future.

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