Plant & Animal Genomes XIV Conference

January 14-18, 2006
Town & Country Convention Center
San Diego, CA

The Plant Immune System: Surprising Roles And Mechanisms For Plant Hormones In Resistance And Disease

Jonathan D G Jones¹ , Lionel Navarro^{1, 4} , Nicholas Harberd² , Mark Estelle³ , Olivier Voinnet⁴

¹  The Sainsbury Laboratory, John Innes Centre, Colney Lane, Norwich, Norfolk, NR4 7UH, UK

²  John Innes Centre, Colney Lane, Norwich, Norfolk, NR4 7UH, UK

³  Indiana University, 915 East Third Street, Bloomington, IN 47405, USA

⁴  IBMP, 12 Rue Du General Zimmer, 67084 Strasbourg Cedex, FRANCE

Plant interactions with pathogenic microbes involve an extraordinary variety of components and mechanisms. Disease resistance (R) genes play an important role in race-specific and perhaps in non-host resistance.
Tomato Cf- genes confer race-specific resistance to Cladosporium fulvum, and encode plasma membrane receptor-like proteins (RLPs), with an LRR domain that is involved in recognition, and no apparent signalling domain. In Cf-9-carrying tobacco cell cultures, provision of Avr9 results in activation of a canonical set of defence responses, including production of active oxygen, gene induction and cell death within 3–4 hrs. Expression profiling of elicited tobacco cells reveals a set of Avr9, Cf-9 Rapidly Elicited (ACRE) genes. Virus-induced gene silencing (VIGS) was used to show that ACRE genes encoding several E3 ubiquitin ligase genes, and a protein kinase, are required for cell death, and in some cases, for disease resistance.
In Arabidopsis cell cultures challenged with flagellin or a conserved flagellin peptide (flg22), similar defence responses and sets of induced genes are elicited to those triggered by Avr9/Cf-9 in tobacco. Arabidopsis orthologs of ACRE (AtACRE) genes are being investigated to assess their role in plant disease resistances.
Remarkably, we also found that several mRNAs involved in the auxin response are down-regulated during the flagellin response. This effect is mediated at least in part through a flagellin-inducible microRNA. Further studies on this microRNA and its targets, and the role of auxin in disease and suppression of defence, will be reported. Furthermore, recent investigations into the mechanism of flagellin-induced growth inhibition have also revealed a role for gibberellin in enhanced susceptibility to necrotrophic (killing) pathogens, and enhanced resistance to biotrophs.