We investigated two forms of Betula pendula Roth: common silver birch (B. pendula var. pendula) and Karelian birch (B. pendula var. carelica) differing in wood texture. Activity of different forms of

Activity of sucrose synthase (SS) and the content of starch were studied in the xylem and phloem of two forms of 40-year-old trees of Betula pendula Roth: common silver birch (B. pendula var. pendula) and Karelian birch (B. pendula var. carelica). In order to reveal the initial signs of wood patterning, we used 8-year-old trees of Karelian birch that did not yet exhibit visible deviations from normal growth (trees with unpatterned wood) and those where formation of structural trunk anomalies already commenced (trees with patterned wood). Investigation was conducted during the period of cambial growth: in June growth was active and in July growth was suppressed by weather conditions. In June we found a high SS activity in the xylem of 40-year-old plants of common birch. Retardation of growth of common birch in July was accompanied by the decrease in SS activity in the xylem and by accumulation of starch, especially in the phloem. In 8-year-old trees of Karelian birch with unpatterned wood, the SS activity in the xylem was lower than in common birch trees, while the content of starch in the phloem was higher. In the xylem of trees with patterned wood, the highest activity of SS was approximately 2.5 times lower than in the trees with unpatterned wood. The lower SS activity in the xylem of patterned trunk trees was not associated with starch accumulation in the phloem. We propose that low SS activity during cambial growth reduces sink capacity of xylem tissues, thus leading to a considerable increase in the sucrose content in the phloem, which can alter the program of cell development in the cambial zone of Karelian birch.

WUSCHEL (WUS)-related homeobox (WOX) protein family members play important roles in the maintenance and proliferation of the stem cells in the cambium, the lateral meristem that forms all the wood structural elements. Most studies have examined the function of these genes in angiosperms, and very little was known about coniferous trees. Pine is one of the most critical forest-forming conifers globally, and in this research, we studied the distribution of WOX4, WOX13, and WOXG genes expression in Pinus sylvestris L. trunk tissues. Further, we considered the role of TDIF(CLE41/44)/TDR(PXY) signaling in regulating Scots pine cambial activity. The distribution of CLE41/44-PXY-WOXs gene expression in Scots pine trunk tissues was studied: (1) depending on the stage of ontogenesis (the first group of objects); and (2) depending on the stage of cambial growth (the second group of objects). The first group of objects is lingonberry pine forests of different ages (30-, 80-, and 180-year-old stands) in the middle taiga subzone. At the time of selection, all the trees of the studied groups were at the same seasonal stage of development: the formation of late phloem and early xylem was occurring in the trunk. The second group of objects is 40-year-old pine trees that were selected growing in the forest seed orchard. We took the trunk tissue samples on 27 May 2022, 21 June 2022, and 21 July 2022. We have indicated the spatial separation expressed of PsCLE41/44 and PsPXY in pine trunk tissues. PsCLE41/44 was differentially expressed in Fraction 1, including phloem cells and cambial zone. Maximum expression of the PsPXY gene occurred in Fraction 2, including differentiating xylem cells. The maximum expression of the PsCLE41/44 gene occurred on 27 May, when the number of cells in the cambial zone was the highest, and then it decreased to almost zero. The PsPXY gene transcript level increased from May to the end of July. We found that the highest transcript level of the PsWOX4 gene was during the period of active cell proliferation in the cambial zone, and also in the trees with the cambial age 63 years, which were characterized by the largest number of cell layers in the cambial zone. In this study, we have examined the expression profiles of genes belonging to the ancient clade (PsWOXG and PsWOX13) in stem tissues in Scots pine for the first time. We found that, in contrast to PsWOX4 (high expression that was observed during the period of active formation of early tracheids), the expression of genes of the ancient clade of the WOX genes was observed during the period of decreased cambial activity in the second half of the growing season. We found that PsWOX13 expression was shifted to Fraction 1 in most cases and increased from the phloem side, while PsWOXG expression was not clearly bound to a certain fraction. Based on the data, the role of the CLE41/44-PXY-WOX signaling module in regulating P. sylvestris cambial growth is discussed.

The processes of cambial activity and secondary xylem and phloem differentiation are completely dependent on the influx of photoassimates. Trunk girdling is a frequently used method for studying cambial growth under conditions of different assimilate supply. We girdled 20-year-old birch trees (Betula pendula Roth) and took samples 1 cm (AG1) and 35 cm above the girdle (AG35). Tissues of ungirdled trees served as a control. A sharp increase in carbohydrates level (AG1) inhibited xylogenesis and stimulated phloemogenesis. A moderate increase (AG35) also stimulated phloemogenesis; however, xylogenesis continued. The activity of the APL gene encoding a phloem-specific transcription factor correlates with the active phloemogenesis, as it was 2.18 (AG1) and 3 (AG35) times higher than in the control. The SUC gene encoding the transmembrane sucrose transporter was up-regulated in the AG1 and AG35 zones by 2.24 and 2.51 times, respectively, compared with the control, which indicates an active sucrose loads into the cells and correlates with the preferential differentiation of parenchyma. The activity of the PIN1 gene encoding the auxin transporter was highest in zone AG35 (2.1 times higher than in the control). In zone AG1, the PIN1 activity was 1.7 times lower than that of AG35, which corresponds to the impaired differentiation of the phloem sieve tubes. The data obtained can be useful for a better understanding of physiological processes and predicting changes in the forest’s productivity under conditions of a changing climate.