Simple Bioassay for PAMP-Triggered Immunity in Rice Seedlings Based on Lateral Root Growth Inhibition

Pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) is an essential layer of plant disease resistance. Robust bioassays for PTI are pre-required to dissect its molecular mechanism. In this study, we established that lateral root growth inhibition as a simple and robust measurement of PTI in rice seedlings. Specifically, flg22, a well-characterized PAMP from bacterial flagellin, was used to induce PTI in rice seedlings. While flg22 treatment induced PR gene expression and mitogen-activated protein kinase activation in the roots of rice seedlings to support the PTI triggered, this treatment substantially repressed lateral root growth, but it did not alter primary root growth. Moreover, treatments with chitin (i.e., a fungal PAMP) and oligogalacturonides (i.e., classical damage-associated molecular pattern) clearly inhibited the lateral root growth, although a priming step involving ulvan was required for the chitin treatment. The bioassay developed was applicable to various rice cultivars and wild species. Thus, lateral root growth inhibition represents a simple and reliable assay for studying PTI in rice plants.