Identification and Expression Profile of <i>CLE41/44-PXY-WOX</i> Genes in Adult Trees <i>Pinus sylvestris</i> L. Trunk Tissues during Cambial Activity

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 <i>WOX4</i>, <i>WOX13</i>, and <i>WOXG</i> genes expression in <i>Pinus sylvestris</i> L. trunk tissues. Further, we considered the role of TDIF(<i>CLE41/44</i>)/TDR(<i>PXY</i>) signaling in regulating Scots pine cambial activity. The distribution of <i>CLE41/44-PXY-WOXs</i> 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 <i>PsCLE41/44</i> and <i>PsPXY</i> in pine trunk tissues. <i>PsCLE41/44</i> was differentially expressed in Fraction 1, including phloem cells and cambial zone. Maximum expression of the <i>PsPXY</i> gene occurred in Fraction 2, including differentiating xylem cells. The maximum expression of the <i>PsCLE41/44</i> 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 <i>PsPXY</i> gene transcript level increased from May to the end of July. We found that the highest transcript level of the <i>PsWOX4</i> 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 (<i>PsWOXG</i> and <i>PsWOX13</i>) in stem tissues in Scots pine for the first time. We found that, in contrast to <i>PsWOX4</i> (high expression that was observed during the period of active formation of early tracheids), the expression of genes of the ancient clade of the <i>WOX</i> genes was observed during the period of decreased cambial activity in the second half of the growing season. We found that <i>PsWOX13</i> expression was shifted to Fraction 1 in most cases and increased from the phloem side, while <i>PsWOXG</i> 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 <i>P. sylvestris</i> cambial growth is discussed.