Differences in Cell Death Induction by Phytophthora Elicitins Are Determined by Signal Components Downstream of MAP Kinase Kinase in Different Species of Nicotiana and Cultivars ofBrassica rapaandRaphanus sativus

Abstract

Elicitins are small, secreted proteins produced by species of the plant-pathogenic oomycete Phytophthora. They induce hypersensitive cell death in most Nicotiana species and in some cultivars of Brassica rapa and Raphanus sativus. In this study, two true-breeding Fast Cycling B. rapa lines were established that showed severe necrosis (line 7-R) or no visible response (line 18-NR) after treatment with elicitin. Unexpectedly, microscopic examination revealed localized cell death in line 18-NR plants, and expression levels of various defense-marker genes were comparable in both lines. These results suggested that both ''responsive'' and ''nonresponsive'' plants responded to elicitin but differed in the extent of the cell death response. Expression of a constitutively active form of Arabidopsis (Arabidopsis thaliana) MAP kinase kinase 4 (AtMEK4 DD ) also induced rapid development of confluent cell death in line 7-R, whereas line 18-NR showed no visible cell death. Similarly, elicitin-responsive Nicotiana species and R. sativus cultivars showed significantly stronger cell death responses following expression of AtMEK4 DD compared with nonresponsive species/cultivars. Line 7-R also showed higher sensitivity to toxin-containing culture filtrates produced by Alternaria brassicicola, and toxin sensitivity cosegregated with elicitin responsiveness, suggesting that the downstream responses induced by elicitin and Alternaria toxin share factors that control the extent of cell death. Interestingly, elicitin responsiveness was shown to correlate with greater susceptibility to A. brassicicola (a necrotroph) in B. rapa but less susceptibility to Phytophthora nicotianae (a hemibiotroph) in Nicotiana, suggesting a more extensive cell death response could cause opposite effects on the outcomes of biotrophic versus necrotrophic plant-pathogen interactions.Plants have the ability to recognize potential pathogens and resist them by inducing various defense mechanisms. Molecules derived from pathogens are targets for plant recognition and can elicit defense responses even in the absence of the pathogen. These elicitors include nonspecific molecules, such as conserved structural components of the fungal cell wall, the bacterial outer membrane or flagella, and specific molecules produced by particular strains of pathogens such as the avirulence proteins secreted by some fungi (e.g. Avr9 and AvrL567) and type III effectors produced by some bacteria (e.g. AvrB and PopP2; Montesano et al., 2003;Dodds et al., 2004;Lahaye, 2004). Elicitins are small elicitor proteins produced by the pathogenic oomycete genera Phytophthora and Pythium, although not by all species of Pythium. Phytophthora species possess a family of elicitins and elicitin-related proteins divided into three broad classes Tyler, 2002;Baillieul et al., 2003;Qutob et al., 2003), but the term elicitins generally refers to Class I elicitins, which are secreted abundantly in culture and are well conserved among Phytophthora species. Elicitin treatment of responsive plants induces ty...

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