Transcriptional profiling of Zea mays roots reveals roles for jasmonic acid and terpenoids in resistance against Phytophthora cinnamomi

Abstract

Phytophthora cinnamomi is a soil-borne plant pathogen that has caused widespread damage to vulnerable native ecosystems and agriculture systems across the world and shows no sign of abating. Management of the pathogen in the natural environment is difficult and the options are limited. In order to discover more about how resistant plants are able to defend themselves against this generalist pathogen, a microarray study of plant gene expression following root inoculation with P. cinnamomi was undertaken. Zea mays was used as a resistant model plant, and microarray analysis was conducted using the Affymetrix GeneChip Maize Genome Array on root samples collected at 6- and 24-h post-inoculation. Over 300 genes were differentially expressed in inoculated roots compared with controls across the two time points. Following Gene Ontology enrichment analysis and REVIGO visualisation of the up-regulated genes, many were implicated in plant defence responses to biotic stress. Genes that were up-regulated included those involved in phytoalexin biosynthesis and jasmonic acid/ethylene biosynthesis and other defence-related genes including those encoding glutathione S-transferases and serine-protease inhibitors. Of particular interest was the identification of the two most highly up-regulated genes, terpene synthase11 (Tps11) and kaurene synthase2 (An2), which are both involved in production of terpenoid phytoalexins. This is the first study that has investigated gene expression at a global level in roots in response to P. cinnamomi in a model plant species and provides valuable insights into the mechanisms involved in defence.

Signals that stop the rot: Regulation of secondary metabolite defences in cereals

“…Transcriptome analysis of maize roots post-infection by P. cinnamomi also revealed over expression of several genes involved in the biosynthesis of JA and Et [69]. Furthermore, the expression of a copalyl diphosphate synthase (CPS), An2, was induced by R. microsporus and preceded kauralexin accumulation, which suggested that kauralexin production is dependent on An2 activity [27].…”

“…JA/Et synergy has been demonstrated in maize [27,69]. Application of a combination of these phytohormones resulted in a build-up of kauralexins and zealexins, novel diterpenoid and sesquiterpenoid phytoalexins, respectively, which far exceeded the accumulation induced by each hormone individually [27].…”

“…Once ROS are produced, it usually implies successful recognition of a pathogen by the host, resulting in an oxidative burst and restriction of infection. Various studies have shown an induction of ROS-related genes in conjunction with phytoalexin biosynthesis and resistance to pathogens [43,[69][70][71]. In maize, phytoalexin biosynthetic genes as well as ROS-related genes were upregulated in response to Phytophthora cinnamomi [69], and similarly in rice, where infection with bacterial blight caused accumulation of diterpenoid phytoalexins as well as regulated ROS production [72].…”

“…Furthermore, the expression of a copalyl diphosphate synthase (CPS), An2, was induced by R. microsporus and preceded kauralexin accumulation, which suggested that kauralexin production is dependent on An2 activity [27]. An2 expression has been shown in response to Fusarium verticillioides, F. graminearum, P. cinnamomi and U. maydis [69,[99][100][101]. Strikingly, kauralexins have also been shown to accumulate in maize roots as a result of abiotic stressors [102].…”

“…Strikingly, kauralexins have also been shown to accumulate in maize roots as a result of abiotic stressors [102]. Kauralexins display antimicrobial activity against a variety of maize pathogens such as R. microsporus, P. cinnamomi and C. graminicola [27,69]. Likewise, zealexin production is preceded by increased expression of the biosynthetic genes Tps6/11 [28].…”