| Summary: | Silver birch (<i>Betula pendula</i> Roth) is an economically important species in Northern Europe. The current research focused on the molecular background of different xylogenesis scenarios in the birch trunks. The study objects were two forms of silver birch, silver birch trees, and Karelian birch trees; the latter form is characterized by the formation of two types of wood, non-figured (straight-grained) and figured, respectively, while it is currently not clear which factors cause this difference. We identified <i>VND</i>/<i>NST</i>/<i>SND</i> genes that regulate secondary cell wall biosynthesis in the birch genome and revealed differences in their expression in association with the formation of xylem with different ratios of structural elements. High expression levels of <i>BpVND7</i> accompanied differentiation of the type of xylem which is characteristic of the species. At the same time, the appearance of figured wood was accompanied by the low expression levels of the <i>VND</i> genes and increased levels of expression of <i>NST</i> and <i>SND</i> genes. We identified <i>BpARF5</i> as a crucial regulator of auxin-dependent vascular patterning and its direct target—<i>BpHB8</i>. A decrease in the <i>BpARF5</i> level expression in differentiating xylem was a specific characteristic of both Karelian birch with figured and non-figured wood. Decreased <i>BpARF5</i> level expression in non-figured trees accompanied by decreased <i>BpHB8</i> and <i>VND/NST/SND</i> expression levels compared to figured Karelian birch trees. According to the results obtained, we suggested silver birch forms differing in wood anatomy as valuable objects in studying the regulation of xylogenesis.
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