Arabidopsis PDK1 activity is regulated by binding to the lipid phosphatidic acid (PA) resulting in activation of the oxidative stress-response protein kinase OXI1/AGC2-1. Thus there is an inferred link between lipid signaling and oxidative stress signaling modules. Among a panel of hormones and stresses tested, we found that, in addition to PA, the fungal elicitor xylanase activated PDK1, suggesting that PDK1 has a role in plant pathogen defense mechanisms. The downstream OXI1 was activated by additional stress factors, including PA, H 2 O 2 , and partially by xylanase. We have isolated an interacting partner of OXI1, a Ser/Thr kinase (PTI1-2), which is downstream of OXI1. Its sequence closely resembles the tomato Pti kinase, which has been implicated in the hypersensitive response, a localized programmed cell death that occurs at the site of pathogen infection. PTI1-2 is activated by the same stresses/elicitors as OXI1 and additionally flagellin. We have used RNA interference to knock out the expression of PDK1 and OXI1 and to study the effects on PTI1-2 activity. We show that specific lipid signaling pathways converge on PTI1-2 via the PDK1-OXI1 axis, whereas H 2 O 2 and flagellin signals to OXI1-PTI1-2 via a PDK1-independent pathway. PTI1-2 represents a new downstream component that integrates diverse lipid and reactive oxygen stress signals and functions closely with OXI1. It is now clear that reactive oxygen species (ROS)2 play an important signaling role in plants controlling processes such as growth, development, programmed cell death, and responses to biotic and abiotic environmental stimuli (1). Current evidence supports the concept that ROS represent a significant point of convergence between pathways that respond to biotic and abiotic stresses (2). Nevertheless, our current understanding of ROS participation in cross-talk between these pathways is very limited (2). OXI1/AGC2-1 is a protein kinase that was identified as a downstream signaling component to the PDK1 (3-phosphinositide-dependent protein kinase 1) and as a protein kinase that is required for oxidative burst-mediated signaling in Arabidopsis, hereafter referred to as OXI1 (3, 4). OXI1 is critical for at least two very different ROS-mediated processes, basal resistance to Peronospora parasitica infection and root hair growth (3, 4). OXI1 is a member of the AGC family of protein kinases, and we have reported previously that OXI1 is activated by PDK1-mediated phosphorylation. In addition PDK1 acts upstream of other AGC kinases and regulates diverse signaling pathways, involved in root hair growth, auxin regulation, and plant cell death (3, 5, 6). PDK1 enzyme activity is regulated by binding the lipid signaling molecule, phosphatidic acid (PA), to its pleckstrin homology (PH) domain (3). PA is produced in response to many different stresses, including absisic acid (ABA)-induced stomatal closure, pathogen attack, and oxidative stress (7,8). It is generated via two distinct phospholipase pathways, either directly by phospholipase D (PLD) or the s...
Many studies on aspects of the biology of plant-parasitic nematodes can be facilitated by using the information and resources available for the model species Caenorhabditis elegans. Comparative genomics of shared processes can provide insights into plant-parasitic nematode biology that would otherwise be intractable. In this article we consider some of the resources available for C. elegans. We describe the practical utility of C. elegans and the use of available information to facilitate the characterisation of neurobiological processes in plant-parasitic nematodes.Model organisms, such as Saccharomyces cerevisiae, Caenorhabditis elegans, Drosophila melanogaster and Arabidopsis thaliana, share a number of common characteristics, including a short life cycle, small adult size, easy maintenance in large numbers and tractability. Their simplicity of structure, in terms of both anatomy and genome organisation often lends itself readily to investigation (Bolker, 1995). Many basic biological processes are shared between model and target organisms. Having identified genes, proteins or processes of interest in a model organism it is then possible to study the target organism comparatively. Model organisms are often more amenable to technical manipulation than the target organisms. These factors lead to an increasing wealth of information and technical resources that pertain to certain model species. The free-living nematode, C. elegans, has become the model organism of choice both for other nematode species and for higher metazoans. The small size and obligate biotrophic life cycles of plant-parasitic nematodes make them refractory to many biological and genetic studies. Plant-parasitic nematodes and C. elegans share many similarities despite their disparate modes of existence. In certain areas of investigation, C. elegans can be used as a powerful model for the study of plantparasitic nematodes. Biology of plant-parasitic nematodesPlant-parasitic nematodes comprise more than 50 genera, primarily within the orders Tylenchida and Dory-
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