Salicylic acid-mediated competitive regulation of xylem vessel formation and defense responses in Arabidopsis thaliana.

In vascular plants, xylem vessels transport water and contribute to structural integrity. As part of vessel formation, xylem cells deposit secondary cell walls (SCWs), which are composed of cellulose, hemicellulose, and lignin polymers. Under environmental challenges such as pathogen attack, a growth-defense trade-off limits xylem vessel development. Understanding the mechanism regulating this trade-off has implications for understanding of plant strategy to utilize their carbon resources because SCWs contain large amounts of densely packed high-carbon compounds. Here, we investigated the effect of pathogen responses induced by the peptide defense elicitor flagellin22 (flg22) on VASCULAR-RELATED NAC-DOMAIN7 (VND7)-dependent xylem vessel formation in Arabidopsis (Arabidopsis thaliana). Treatment with flg22 decreased xylem transport and delayed ectopic xylem vessel differentiation, including SCW deposition, in plants with a dexamethasone-inducible VND7 system, suggesting that the plant prioritized defense responses over xylem formation. Consistent with this, transcriptome analysis revealed that flg22 treatment suppressed SCW-related genes and activated immune-related genes. Salicylic acid (SA) treatment had a similar inhibitory effect on VND7-dependent xylem vessel formation, and the sid2-2 and npr1-1 mutants, which are defective in SA biosynthesis and signaling, respectively, did not exhibit flg22-induced inhibition of xylem transport activity; these highlight the role of SA, which is biosynthesized from phenylalanine during immune responses, as a key mediator of the competition between xylem vessel formation and immune responses. These findings provide insights into how plants flexibly regulate xylem development under biotic stress to optimize their development.