PIP and bacterial infectious disease: Second, evidence suggests that plants specifically mediate free soluble lysine upon bacterial infection of leaves, first increasing lysine import then massively upregulating lysine catabolism to produce an excess of pipecolic acid (Pip), which then acts as the major metabolic regulator/intensifier of SAR defense (Yang and Ludewig, 2014; Yang et al., 2014) and downregulating lysine synthesis in leaves, reducing this limiting amino acid in roots.