Tillering onion AcPAL and AcALL genes regulate trans‐cinnamic acid and allicin accumulation to improve cucumber environmental adaptability and growth

Abstract

Putative allelochemicals released by tillering onion promote cucumber growth in intercropping systems. However, the underlying molecular mechanisms remain unclear. This study focused on PHENYLALANINE AMMONIA‐LYASE (PAL) and ALLIINASE (ALL), which play crucial roles in phenylpropanoid biosynthesis and organic sulfur metabolism, respectively. The expression levels of AcPAL and AcALL, their enzyme activities, and trans‐cinnamic acid (t‐CA) and allicin content were analyzed in the donor tillering onions. Exogenous application of t‐CA and allicin was conducted on target cucumbers to investigate seedling growth, and differentially expressed genes (DEGs) were screened using transcriptome analysis to determine the response metabolic pathways. The results showed that intercropping with cucumber up‐regulated the expression of AcPAL and AcALL and increased the enzymatic activities of AcPAL and AcALL, resulting in increased accumulation of t‐CA and allicin in the tillering onion and rhizosphere soil, particularly in the roots of accession L‐hh, with strong allelopathic potential, compared to monocropping. Application of t‐CA at 25 μg mL−1 and allicin at 50 μg mL−1 significantly enhanced root fresh weight, shoot length, and shoot fresh weight in cucumber seedlings, whereas transcriptome analysis revealed that the DEGs were significantly enriched in plant hormone signal transduction, phenylpropanoid biosynthesis, MAPK signaling pathway, phenylalanine pathway, and starch and sucrose metabolism. In conclusion, AcPAL and AcALL of tillering onions participate in the accumulation of t‐CA and allicin, inducing the expression of genes involved in phenylpropanoid biosynthesis and MAPK signaling pathways to enhance environmental adaptability as well as auxin‐related genes to promote cucumber seedling growth.