The transcriptome of Arabidopsis thaliana during systemic acquired resistance.

Infected plants undergo transcriptional reprogramming during initiation of both local defence and systemic acquired resistance (SAR). We monitored gene-expression changes in Arabidopsis thaliana under 14 different SAR-inducing or SAR-repressing conditions using a DNA microarray representing approximately 25-30% of all A. thaliana genes. We derived groups of genes with common regulation patterns, or regulons. The regulon containing PR-1, a reliable marker gene for SAR in A. thaliana, contains known PR genes and novel genes likely to function during SAR and disease resistance. We identified a common promoter element in genes of this regulon that binds members of a plant-specific transcription factor family. Our results extend expression profiling to definition of regulatory networks and gene discovery in plants.

Induced plant defence responses: scientific and commercial development possibilities.

Recent work has demonstrated that plants have endogenous defence mechanisms that can be induced as a response to attack by insects and pathogens. There are two well-studied examples of these induced defence responses. Systemic acquired resistance (SAR) results in increased resistance to a broad spectrum of pathogens throughout a plant in response to localized necrosis caused by pathogen infection. The second example is the systemic induction of proteinase inhibitors to deter feeding by herbivores following an initial event of feeding. In addition, there is now preliminary evidence for other induced defence response pathways. By understanding the breadth of induced defence responses and the mechanisms used to control these pathways, novel plant protection strategies may be developed for use in agronomic settings. Rather than reducing crop losses caused by pests or pathogens by using chemicals that are designed to kill the offending organism, the plant's own defence mechanisms can be used to limit damage due to pests. Novel crop protection strategies based on genetic or chemical regulation of these induced responses show great potential. The first example of a crop protection product that acts by inducing an endogenous defence response pathway is now on the market. Bion reduces the level of pathogen infection in plants by activating SAR.

Defense on multiple fronts: how do plants cope with diverse enemies?

Plants have evolved an array of defense mechanisms to protect themselves against the wide variety of pathogens and pests with which they are confronted. Included in these defense mechanisms are inducible responses that are turned on systemically in the plant in response to attempted infection or predation. The two most studied inducible responses are systemic acquired resistance, which provides enhanced resistance to pathogen infection, and the wound response pathway, resulting in enhanced resistance to insect feeding. Recent research suggests that the two pathways are not completely independent, and the induction of one might affect the expression of the other. However, the evidence for cross-talk between different induced defense response pathways is somewhat confusing, and at times contradictory. Here, we review recent advances in our understanding of how the different pathways might interact.

Inhibition of protoporphyrinogen oxidase expression in Arabidopsis causes a lesion-mimic phenotype that induces systemic acquired resistance.

We have used an antisense expression technology in Arabidopsis based on the yeast GAL4/UAS transactivation system (Guyer et al., Genetics, 1998; 149:633-639) to reduce levels of protoporphyrinogen IX oxidase (PPO), the last common enzyme of the biosynthesis of the haem group and chlorophyll. Plants expressing the antisense PPO gene presented growth alterations and their leaves showed necrotic lesions that appeared similar to lesions characteristic of the pathogen-induced hypersensitive reaction, and seen in the so-called lesion-mimic mutants. Plants expressing the antisense gene also had high endogenous salicylic acid levels, constitutive expression of the PR-1 gene, and were resistant to Peronospora parasitica, consistent with the activation of systemic acquired resistance (SAR). Treatment of wild-type plants with sublethal concentrations of herbicides that inhibit PPO also induced defence responses that conferred enhanced tolerance to P. parasitica. This effect was not observed in NahG and nim1 plants, which are compromised in their ability to activate SAR. These results demonstrate that genetic or chemical disruption of a metabolic pathway can lead to the induction of a set of defence responses including activation of SAR.