CD2-1, the C-Terminal Region of Flagellin, Modulates the Induction of Immune Responses in Rice

Flagellin from the rice avirulent N1141 strain of Acidovorax avenae functions as a pathogen-associated molecular pattern (PAMP) and induces PAMP-triggered immunity (PTI) in rice. To study the recognition mechanism of flagellin in rice, we attempted to define one or more regions of the flagellin protein required to activate the PTI response. Based on domain classification, we produced four fragments of N1141 flagellin: N-terminal D0, D1. and D2 domains (ND0-2), N-terminal D2, D3, and C-terminal D2 domains (ND2-CD2), C-terminal D2, D1, and D0 domains (CD2-0), and C-terminal D2 and D1 domains (CD2-1). The C-terminal CD2-1 and CD2-0 fragments induced PTI responses in cultured rice cells. Synthetic flg22, which is sufficient to produce the flagellin response in Arabidopsis, and the N-terminal flagellin fragments containing flg22 region elicited very weak immune responses in rice. OsFLS2, the rice ortholog of AtFLS2, which mediates flg22 recognition, was not involved in CD2-0 or CD2-1 recognition in rice. In addition, CD2-0 triggered resistance to coinfection with pathogenic bacteria. Taken together, these data suggest that rice mainly recognizes flagellin CD2-1 by a receptor distinct from OsFLS2 and that this epitope recognition leads to PTI responses.

Analysis of Flagellin Perception Mediated by flg22 Receptor OsFLS2 in Rice

Plants have sensitive perception systems that recognize various pathogen-derived molecules. We previously reported that rice detects flagellin from a rice-incompatible strain of gram-negative phytopathogenic bacterium, Acidovorax avenae, which induces subsequent immune responses involving cell death. The mechanism of flagellin perception in rice, however, has remained obscure. In this study, we found that flg22, a peptide derived from the flagellin N-terminus, induced weak immune responses without cell death in cultured rice cells. To elucidate the mechanism by which flg22 induced signaling in rice, we characterized OsFLS2, the rice ortholog of AtFLS2, which mediates flg22 perception. Heterologous expression of OsFLS2 functions in Arabidopsis, showing the conservation of the flg22 signaling pathway across divergent plant taxa. OsFLS2-overexpressing rice cultured cells generated stronger immune responses with the induction of cell death following stimulation with flg22 and flagellin. However, examination of the growth rate of the compatible strain in inoculated OsFLS2-overexpressing rice could not confirm bacterial growth suppression compared with wild-type rice. These results suggest that rice possesses a conserved flagellin perception system utilizing the FLS2 receptor which, when upregulated, hardly affects resistance against compatible A. avenae.