Aminotransferase involved in local and systemic acquired resistance (SAR) to the bacterial pathogen P.syringae. Required for salicylic acid (SA) and camalexin accumulation upon pathogen infection. Possesses aminotransferase activity in vitro and may generate amino-acid-derived defense signals in vivo. May be involved in ethylene-induced senescence signaling. Involved in the biosynthesis of pipecolate (Pip), a metabolite that orchestrates defense amplification, positive regulation of SA biosynthesis, and priming to guarantee effective local resistance induction and the establishment of SAR (PubMed:23221596, PubMed:27758894). Converts lysine to alpha-keto-epsilon-aminocaproate, which then can spontaneously cyclize to form delta-(1)-piperideine-2-carboxylate (P2C). P2C is converted to Pip by SARD4 (PubMed:27758894). May produce non-Pip metabolites that play roles in immunity. Involved in the synthesis of distinct metabolite signals that affect basal and early defenses, and later defense responses (PubMed:25372120). Binds 1 pyridoxal phosphate per subunit. Highly expressed in senescing leaves, flowers, siliques and seeds. By ozone, benzothiadiazole (BTH) and infection with the bacterial pathogen P.syringae pv. maculicola. Down-regulated by nitric oxide. No visible phenotype under normal growth conditions, but during infection with virulent strain of P.syringae, mutant plants have reduced levels of SA and camalexin, and show increased susceptibility to pathogen. Leaves show decreased ethylene-induced senescence. Treatment with exogenous pipecolate (Pip) enhances disease resistance to Pseudomonas syringae pv. maculicola (PubMed:23221596). Plants over-expressing ALD1 exhibit resistance to Pseudomonas syringae pv. maculicola (PubMed:25372120). Belongs to the class-I pyridoxal-phosphate-dependent aminotransferase family. LL-diaminopimelate aminotransferase subfamily.