Distribution of flagellin CD2-1, flg22, and flgII-28 recognition systems in plant species and regulation of plant immune responses through these recognition systems.

The first layer of active plant immunity relies upon the recognition of pathogen-associated molecular patterns (PAMPs), and the induction of PTI. Flagellin is the major protein component of the bacterial flagellum. Flagellin-derived peptide fragments such as CD2-1, flg22, and flgII-28 function as PAMPs in most higher plants. To determine the distribution of CD2-1, flg22, and flgII-28 recognition systems within plant species, the inducibility of PTI by CD2-1, flg22, and flgII-28 in 8 plant species, including monocotyledonous and dicotyledonous plants, was investigated. CD2-1 caused PTI responses in Oryza sativa, Brachypodium distachyon, and Asparagus persicus; flg22 caused PTI responses in Phyllostachys nigra, A. persicus, Arabidopsis thaliana, Nicotiana tabacum, Solanum lycopersicum, and Lotus japonicus; and flgII-28 caused PTI responses only in S. lycopersicum. Furthermore, quantitative analysis of FLS2 receptor revealed that the responsiveness of flg22 in plants was dependent on the expression level of the receptor.