|Background: Rp1 is a complex locus of maize, which carries a set of genes controlling race-specific resistance to
the common rust fungus, Puccinia sorghi. The resistance response includes the “Hypersensitive response” (HR), a
rapid response triggered by a pathogen recognition event that includes localized cell death at the point of
pathogen penetration and the induction of pathogenesis associated genes. The Rp1-D21gene is an autoactive allelic
variant at the Rp1 locus, causing spontaneous activation of the HR response, in the absence of pathogenesis.
Previously we have shown that the severity of the phenotype conferred by Rp1-D21 is highly dependent on
Results: In this study we show that the phenotype conferred by Rp1-D21 is highly dependent on temperature, with
lower temperatures favoring the expression of the HR lesion phenotype. This temperature effect was observed in all
the 14 genetic backgrounds tested. Significant interactions between the temperature effects and genetic
background were observed. When plants were grown at temperatures above 30°C, the spontaneous HR phenotype
conferred by Rp1-D21 was entirely suppressed. Furthermore, this phenotype could be restored or suppressed by
alternately reducing and increasing the temperature appropriately. Light was also required for the expression of this
phenotype. By examining the expression of genes associated with the defense response we showed that, at
temperatures above 30°C, the Rp1-D21 phenotype was suppressed at both the phenotypic and molecular level.
Conclusions: We have shown that the lesion phenotype conferred by maize autoactive resistance gene Rp1-D21 is
temperature sensitive in a reversible manner, that the temperature-sensitivity phenotype interacts with genetic
background and that the phenotype is light sensitive. This is the first detailed demonstration of this phenomenon
in monocots and also the first demonstration of the interaction of this effect with genetic background. The use of
temperature shifts to induce a massive and synchronous HR in plants carrying the Rp1-D21 genes will be valuable
in identifying components of the defense response pathway.