Fig. 2. Vascular bundle structure and lignin biosynthesis in A. spinulosa.
a, A stem cross-section and mature sori (So) underneath the leaf. A wavy structure is enlarged to show the xylem (Xy), phloem (Ph) and sclerenchymatic belt (Sb) in the vascular bundle. Pi, pith. b, Segregated xylem cells showing scalariform thickening. c,d, Scanning electron micrographs of xylem for cross-section (c) and longitudinal section (d). e, microCT shows the three-dimensional arrangement of tracheids. f, Transmission electron microscopy (TEM) image of Sb. The microscopic observations in b–d,f are more than ×6. g, The histogram (top) shows the content of acid-soluble lignin and acid-insoluble lignin in Ph, Sb, Xy and spores (Sp), calculated as percentage of cell wall residue (CWR). The heat map (bottom) shows the content of lignin aromatic units (G, S and H) from the three canonical monolignols in Ph, Sb, Xy and Sp. The values are the mean ± s.d. of two independent experiments. h, Heteronuclear single-quantum coherence NMR spectra, showing that guaiacyl units are major components of lignins in Xy and Sp. Relative quantification was performed using the correlation peak volume integration (uncorrected). Side chain units are on the basis A + B + C = 100%; aromatics are on the basis S + G = 100% as H peaks overlap. pCA, p-coumarate; FA, ferulate; Phe and Tyr are phenylalanine and tyrosine units (in protein).