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The shading treatment which decreases light intensity affects the ornamental characteristics in herbaceous peony (Paeonia lactiflora Pall.), including flower color, leaf shape and stem straightness. However, the morphological, microstructural and lignin-related responses of its stems to shading have not been systematically and deeply studied. Here, we focused on these responses to shading treatment in herbaceous peony stems. Morphological results showed that shading treatment significantly bended stems, decreased stem diameters, lightened stem weights and reduced stem strength. Microstructural observations demonstrated that shading treatment decreased vascular bundle number, xylem area, xylem ratio and secondary cell wall (SCW) thickness. Lignin-related analysis revealed that shading treatment markedly decreased lignin content in cell walls especially S-lignin and G-lignin, deposition in SCWs and related enzyme activities. Furthermore, transcriptomic analysis showed that shading treatment suppressed lignin-related processes, lignin biosynthetic pathway and lignin-related transcriptional activators. Collectively, shading treatment suppressed lignin biosynthesis and reduced lignin deposition in SCW, leading to undeveloped secondary xylem and bender stems. These findings provide a fundamental understanding of the morphological, microstructural and lignin-related responses of herbaceous peony stem to shading. • Shading treatment led to slender and bender stems in herbaceous peony. • Shading treatment inhibited xylem and SCW development, resulting in bender stems. • Shading treatment reduced lignin content, deposition, composition and related enzyme activities. • Shading treatment suppressed lignin biosynthesis and its transcriptional regulation. [ABSTRACT FROM AUTHOR]

• Herbaceous peony stem diameter could be used to estimate stem strength. • Thickness of secondary cell wall contributed a lot to stem strength. • Three lignin monomers affected herbaceous peony stem strength. • Lignin deposition and secondary cell wall of the stem were observed. • CAD was a key gene controlling herbaceous peony lignin accumulation. Straight stems are highly important to herbaceous peony (Paeonia lactiflora Pall.) cut flowers. To clarify whether the microstructural and lignin characteristics are associated with P. lactiflora stem mechanical strength, eight cultivars were used to study their anatomical structure, secondary cell wall, lignin deposition, lignin content and structure, lignin biosynthetic enzyme activity and related gene expression levels. The results showed that stem diameter was a direct indicator estimating its mechanical strength. Simultaneously, the number of vascular bundles, area of vascular bundles, number of duct perforations, ratio of xylem to cross-section and thickness of secondary cell wall had significant positive correlations with stem mechanical strength. Moreover, three lignin monomers were all detected in the stem, and the contents of S-lignin, G-lignin and total lignin in high stem mechanical strength cultivars were higher than those in low stem mechanical strength cultivars as a whole, and lignin was deposited in the cell walls of the sclerenchyma, vascular bundle sheath and duct. Additionally, cinnamate 4-hydroxylase (C4H), cinnamyl alcohol dehydrogenase (CAD) and polyphenol oxidase (PPO) activities in high stem mechanical strength cultivars were higher than those in low stem mechanical strength cultivars as a whole, and CAD might be the key gene among detected lignin biosynthetic genes. Consequently, microstructural and lignin characteristics were closely correlated with stem mechanical strength in P. lactiflora. These results would provide useful information concerning the physiological mechanism of stem strength. [ABSTRACT FROM AUTHOR]

Herbaceous peony (Paeonia lactiflora Pall.) is a popular high-end cut flower, but stem bending caused by low stem strength severely decreases its quality. To enhance stem strength, the regulatory effects of exogenous silicon were investigated in P. lactiflora. The results showed that silicon application enhanced stem strength by increasing the thickness of secondary cell walls and the layers of thickened secondary cells. Moreover, more lignin accumulated, particularly G-lignin and S-lignin, and the activities of lignin biosynthetic enzymes increased with silicon application. In addition, based on transcriptome analysis, silicon application induced the expression of genes participating in lignin biosynthesis pathway. Among them, hydroxycinnamoyl-CoA: shikimate hydroxycinnamoyl transferase gene (HCT1) was isolated from P. lactiflora and found to be mainly localized in the cytoplasm of cells. Overexpression of PlHCT1 increased the layers of thickened secondary cells and lignin accumulation in tobacco, resulting in enhanced stem strength and demonstrably straight stems. Finally, silicon content, lignin content and PlHCT1 expression in P. lactiflora cultivars with high stem strengths were totally higher than those in cultivars with low stem strengths. These results indicated that silicon application enhanced stem strength by promoting lignin accumulation in P. lactiflora , which has prospects for stem quality improvement in general. [Display omitted] • Silicon increased the net photosynthesis rate and SPAD value. • Silicon enhanced stem strength by promoting lignin accumulation. • Silicon increased lignin biosynthetic enzyme activities and gene expression. • HCT1 expression was induced and localized in the cytoplasm of cells. • Overexpression of HCT1 increased lignin, resulting in enhanced stem strength. [ABSTRACT FROM AUTHOR]

• CCoAOMT could respond to drought stress in Paeonia ostii. • CCoAOMT was isolated, and its characteristics were clarified. • Overexpression of CCoAOMT conferred drought stress tolerance. • CCoAOMT could significantly altered G-lignin content. • Stress tolerance was caused by promoting lignin synthesis and ROS scavenging. Paeonia ostii is an emerging woody oil crop, but drought severely inhibits its growth and promotion in arid or semiarid areas, and little is known about the mechanism governing this inhibition. In this study, the full-length cDNA of a caffeoyl-CoA O -methyltransferase gene (CCoAOMT) from P. ostii was isolated, and determined to be comprised of 987 bp. PoCCoAOMT encoded a 247-amino acid protein, which was located in the nucleus and cytosol. Significantly higher PoCCoAOMT transcription was detected in P. ostii treated with drought stress. Subsequently, the constitutive overexpression of PoCCoAOMT in tobacco significantly conferred drought stress tolerance. Under drought stress, transgenic lines exhibited lower reactive oxygen species (ROS) accumulation, and higher antioxidant enzyme activities and photosynthesis. Moreover, the expression levels of senescence-associated genes were significantly downregulated, whereas the expression levels of lignin biosynthetic genes and PoCCoAOMT were significantly upregulated in transgenic lines. Similarly, transgenic lines produced significantly higher lignin, especially guaiacyl-lignin. These results suggest that PoCCoAOMT is a vital gene in promoting lignin synthesis and ROS scavenging to confer drought stress tolerance in P. ostii. [ABSTRACT FROM AUTHOR]