Reduced callose modulates stem mechanical strength in Paeonia lactiflora: Insights from xylem fiber cell formation, carbon transport and material metabolism

Paeonia lactiflora is an important garden ornamental flower and has become a new cut flower in the international market in recent years. Stem mechanical strength is an important index of cut flower quality. P. lactiflora cultivars possess varying stem mechanical strength, which directly result in different cut P. lactiflora quality. In our previous study, a significant difference in phloem callose deposition was detected between two P. lactiflora cultivars with varying stem mechanical strength. However, the relationship and interaction mechanism between the phloem callose and mechanical strength in P. lactiflora stems are still unclear. This study aimed to analyze the effects of reduced phloem callose on the mechanical strength in P. lactiflora stems. The results revealed a negative correlation between the callose content and mechanical strength in P. lactiflora stems during early developmental stages. Exogenous 2DDG treatment led to a 13.6% reduction in the callose content, concomitantly enhancing the stem mechanical strength by 14.2% via facilitating the formation and secondary cell wall (SCW) thickening of xylem fiber cells. Further investigations showed that exogenous 2DDG treatment promoted carbon transport in P. lactiflora stems, thereby increasing the carbon content. Additionally, it induced alterations in the metabolism of lipids, amino acids, and organic acids in the stems and spurred the diffusion of lipids from the pith to the xylem. Consequently, reducing the content of callose improved the formation and SCW thickening of xylem fiber cells by modifying material transport and metabolism, thereby enhancing the stem mechanical strength of P. lactiflora . This study offers novel perspectives regarding the regulatory mechanism of P. lactiflora stem mechanical strength.