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Blocking the QB-binding site of photosystem II by tenuazonic acid, a non–host-specific toxin of Alternaria alternata, activates singlet oxygen-mediated and EXECUTER-dependent signalling in Arabidopsis

Blocking the QB-binding site of photosystem II by tenuazonic acid, a non–host-specific toxin of Alternaria alternata, activates singlet oxygen-mediated and EXECUTER-dependent signalling in Arabidopsis
Shiguo Chen1,2*, Chanhong Kim1,*?, Je Min Lee1?, Hyun-Ah Lee1§, Zhangjun Fei1, Liangsheng Wang1 & Klaus Apel1
1Boyce Thompson Institute for Plant Research, Ithaca, NY 14853-1801, USA, 2College of Life Science, Nanjing Agricultural University, Nanjing 210095, China

ABSTRACT
Necrotrophic fungal pathogens produce toxic compounds that induce cell death in infected plants. Often, the primary targets of these toxins and the way a plant responds to them are not known. In the present work, the effect of tenuazonic acid (TeA), a non–host-specific toxin of Alternaria alternata, on Arabidopsis thaliana has been analysed. TeA blocks the QB-binding site at the acceptor side of photosystem II (PSII). As a result, charge recombination at the reaction centre (RC) of PSII is expected to enhance the formation of the excited triplet state of the RC chlorophyll that promotes generation of singlet oxygen (1O2). 1O2 activates a signalling pathway that depends on the two EXECUTER (EX) proteins EX1 and EX2 and triggers a programmed cell death response. In
seedlings treated with TeA at half-inhibition concentration 1O2-mediated and EX-dependent signalling 1s activated as indicated by the rapid and transient up-regulation of 1O2-responsive genes in wild type, and its suppression in ex1/ex2 mutants. Lesion formation occurs when seedlings are exposed to higher concentrations of TeA for a longer period of time. Under these conditions, the programmed cell death response triggered by 1O2-mediated and EX-dependent signalling is superimposed by other events that also contribute to lesion formation.

Key-words: necrotrophic pathogens; programmed cell death; Arabidopsis thaliana; photosynthetic electron transport.