Natural variation in the pattern-triggered immunity response in plants: Investigations, implications and applications

Abstract

The pattern-triggered immunity (PTI) response is triggered at the plant cell surface by the recognition of microbe-derived molecules known as microbe- or pathogen-associated molecular patterns or molecules derived from compromised host cells called damage-associated molecular patterns. Membrane-localized receptor proteins, known as pattern recognition receptors, are responsible for this recognition. Although much of the machinery of PTI is conserved, natural variation for the PTI response exists within and across species with respect to the components responsible for pattern recognition, activation of the response, and the strength of the response induced. This review describes what is known about this variation. We discuss how variation in the PTI response can be measured and how this knowledge might be utilized in the control of plant disease and in developing plant varieties with enhanced disease resistance.

Keywords: pattern‐triggered immunity; plant disease resistance.

FIGURE 1

Variations in mechanisms of flagellin perception between different species. In this cartoon, the ovals in the apoplastic region represent leucine‐rich repeats while the rectangles in the cytosol represent kinase domains. (a) In Arabidopsis, FLS2 is the receptor for the flg22 epitope of flagellin (Boller & Felix, ; Cheng et al., 2020) with BAK1 (also known as SERK3) functioning as an essential co‐receptor (Chinchilla et al., 2007). The kinase domains of the two proteins do not participate directly in receptor/co‐receptor complex formation (Chen et al., 2014). (b) The tomato orthologue of FLS2, SlFLS2, appears to operate in a similar way as in Arabidopsis (Robatzek et al., 2007), though SlFLS2 can additionally recognize flg15, an N‐terminally shortened version of flg22 (Chinchilla et al., ; Robatzek et al., 2007), which is not an effective elicitor of pattern‐triggered immunity (PTI) in Arabidopsis. Another pattern recognition receptor (PRR), FLAGELLIN‐SENSING 3 (FLS3), which is specific to certain groups of solanaceous species, is activated by a different flagellin epitope, flgII‐28, and also uses BAK1 as a co‐receptor (Hind et al., 2016). (c) The rice FLS2 orthologue, OsFLS2, can recognize flg22 and can complement FLS2 mutations in Arabidopsis (Takai et al., 2008); however, it interacts as a co‐receptor with OsSERK2, which is related to, but distinct from, BAK1. In contrast to FLS2/BAK1 in Arabidopsis, the OsFLS2 and OsSERK2 kinase domains are involved in receptor/co‐receptor complex formation (Chen et al., 2014). The C‐terminal fragments of flagellin, CD2‐0 and CD2‐1, both trigger PTI in rice, but not in Arabidopsis or tomato (Murakami et al., 2022). The mechanism of recognition of CD2‐0 and CD2‐1 is unknown (presumptive receptor represented as ‘X’).