Role of PMR4 and PDLP1 in priming of early acting penetration defence by resistance-inducing β-amino acids.

R-β-homoserine (RBH) and β-aminobutyric acid (BABA) induce resistance against the oomycete Hyaloperonospora arabidopsidis (Hpa) in Arabidopsis, which is based on priming of multiple defence layers, including early-acting penetration resistance at the cell wall. Here, we have examined the molecular basis of RBH- and BABA-primed defence by cell wall papillae against Hpa. Three-dimensional reconstruction of Hpa-induced papillae by confocal microscopy revealed no structural differences between control-, RBH- and BABA-treated plants after Hpa challenge. However, mutation of POWDERY MILDEW RESISTANT4 and PLASMODESMATA LOCATED PROTEINS (PDLPs) only impaired BABA-induced penetration resistance and not RBH-induced penetration resistance. Furthermore, PDLP1 over-expression mimicked primed penetration resistance, while the intensity of GFP-tagged PDLP1 at germinating Hpa conidiospores was increased in BABA-primed plants but not RBH-primed plants. Our study reveals new regulatory layers of immune priming by β-amino acids and supports the notion that penetration resistance is a multifaceted defence layer that can be achieved through distinct pathways.