Ornithine enhances common bean growth and defense against white mold disease via interfering with SsOAH and diminishing the biosynthesis of oxalic acid in Sclerotinia sclerotiorum

The necrotrophic fungal phytopathogen, <i>Sclerotinia sclerotiorum</i> (Lib.) de Bary, employs a multilayered strategy to infect a wide range of host plants. The current study proposed the diamine _L-ornithine, a non-proteinogenic amino acid that promotes the synthesis of other essential amino acids, as an alternative management strategy to boost the molecular, physiological, and biochemical responses of common bean (<i>Phaseolus vulgaris</i> L.) against white mold disease caused by <i>S. sclerotiorum</i>. <i>In vitro</i> experiments showed that _L-ornithine significantly inhibited the mycelial growth of <i>S. sclerotiorum</i> in a dose-dependent manner. Moreover, it markedly diminished the white mold severity under greenhouse conditions. Moreover, _L-ornithine stimulated the growth of treated plants suggesting that the tested concentration of _L-ornithine has no phytotoxicity on treated plants. Additionally, _L-ornithine enhanced the non-enzymatic antioxidants (total soluble phenolics and flavonoids), the enzymatic antioxidants (CAT, POX, and PPO), and upregulated the gene expression of three antioxidant-associated genes (<i>PvCAT1</i>, <i>PvSOD</i>, and <i>PvGR</i>). Moreover, <i>in silico</i> analysis showed that the genome of <i>S. sclerotiorum</i> possesses a putative oxaloacetate acetylhydrolase (<i>SsOAH</i>) protein that is highly similar in its functional analysis, conserved domains, and topology with OAH from <i>Aspergillus fijiensis</i> (<i>AfOAH</i>) and <i>Penicillium lagena</i> (<i>PlOAH</i>). Interestingly, the addition of _L-ornithine to the potato dextrose broth (PDB) medium significantly down-regulated the gene expression of <i>SsOAH</i> in the mycelium of <i>S. sclerotiorum</i>. Likewise, exogenous application of _L-ornithine significantly down-regulated the gene expression of <i>SsOAH</i> in the fungal mycelia collected from treated plants. Finally, _L-ornithine application significantly diminished the secretion of oxalic acid in the PDB medium as well as infected leaves. Collectively, _L-ornithine plays a pivotal role in maintaining the redox status, in addition to boosting the defense responses of infected plants. The current study provides insights that may lead to innovative eco-friendly approaches for controlling white mold disease and mitigating its impact on common bean cultivation particularly, and other crops in general.