Exogenous Auxin Enhances Stem Straightness in Paeonia lactiflora Cultivar by Modulating Cellulose Development and Vascular Tissue

The straightness of Paeonia lactiflora stems is pivotal for their use in cut flowers, often influenced by cellulose and other cell wall materials. Variances in auxin (IAA) content between straight and bending cultivars have been noted, yet the underlying mechanism remains elusive. Here, we treated the bending cultivar ‘Qi Hualushuang’ with 150 mg·L⁻¹ IAA across five developmental stages, examining morphological, anatomical, and cellulose metabolism-related changes. The results showed that exogenous IAA significantly improved stem straightness, plant height, and stem diameter. This improvement coincided with the early development of vascular tissue at the early and mid-stage of stem growth. IAA application down-regulated the expression of cellulose synthase genes, altering sucrose synthase and cellulase activities, ultimately resulting in higher cellulose content during the bud-breeding stage. Notably, an early accumulation of cellulose in vascular tissue was observed, characterized by earlier and more orderly arranged cellulose microfibrils, oriented at a smaller angle to the growth direction. This observation suggests that early cellulose development is advantageous for achieving stem straightness. Our study also found that the promotional effect of IAA on stem growth was mainly in the early stages, and other phytohormones may be jointly involved in the regulation. In summary, our study suggests that IAA enhances stem straightness by modulating the development of cell wall cellulose and vascular tissue. These findings provide a theoretical basis for the cultivation and trait improvement of P. lactiflora cut flower cultivars.