Abstract
Stem cutting recalcitrance to adventitious root formation is a major limitation for the clonal propagation or micropropagation of elite genotypes of many forest tree species, especially at the adult stage of development. The interaction between the cell wall–plasma membrane and cytoskeleton may be involved in the maturation-related decline of adventitious root formation. Here, pine homologs of several genes encoding proteins involved in the cell wall–plasma membrane–cytoskeleton continuum were identified, and the expression levels of 70 selected genes belonging to the aforementioned group and four genes encoding auxin carrier proteins were analyzed during adventitious root formation in rooting-competent and non-competent cuttings of Pinus radiata. Variations in the expression levels of specific genes encoding cell wall components and cytoskeleton-related proteins were detected in rooting-competent and non-competent cuttings in response to wounding and auxin treatments. However, the major correlation of gene expression with competence for adventitious root formation was detected in a family of genes encoding proteins involved in sensing the cell wall and membrane disturbances, such as specific receptor-like kinases (RLKs) belonging to the lectin-type RLKs, wall-associated kinases, Catharanthus roseus RLK1-like kinases and leucine-rich repeat RLKs, as well as downstream regulators of the small guanosine triphosphate (GTP)-binding protein family. The expression of these genes was more affected by organ and age than by auxin and time of induction.
Highlights
Vegetative propagation has been employed mostly in commercial forests to propagate elite genotypes expressing yield traits of economic or ecological interest, such as wood quality or resistance to biotic or abiotic factors
To analyze the overall gene expression of rooting-competent and non-competent cuttings during development and in response to auxin during adventitious root formation, a hierarchical clustering based on Pearson’s distances and a principal component analysis (PCA) of gene expression levels in cuttings having different rooting capacities were performed at the time of excision and during adventitious root formation in response to auxin (Figure 1 and Supplementary Figures 1A,B, 2)
NcH91 and noncompetent hypocotyl (ncH91) or epicotyl (ncE91) may show different developmental stages, common expression patterns of specific genes involved in the cell wall–plasma membrane–cytoskeleton continuum, especially for the receptor-like kinases (RLKs) and guanosine triphosphate (GTP)-binding-related protein groups at the time of excision and during adventitious root formation, indicated that both types of cuttings maintain common developmental pathways and that these pathways may be associated with the capacity to root (Figures 3, 4; Supplementary Figures 3–5; and Supplementary Table 2)
Summary
Vegetative propagation has been employed mostly in commercial forests to propagate elite genotypes expressing yield traits of economic or ecological interest, such as wood quality or resistance to biotic or abiotic factors. This research has been performed using a simple experimental system that reproduce, in young seedlings and in short periods of time, the maturation-related decline of adventitious root formation in response to auxin that occurs in adult trees (Díaz-Sala, 2014, 2016) This system is based on the different rooting capacity of hypocotyl and epicotyl cuttings. The rooting capacity is lost in hypocotyl cuttings by the transition to secondary growth, in which the resin canals are included in the whole ring of the xylem These poles are no longer evident after the formation of the epicotyl, an early phase of the maturation process in which the shoot makes the transition from an embryonic to a postembryonic pattern of development, and root formation in response to auxin is rare or lengthy (Díaz-Sala et al, 1996; Greenwood et al, 2001; Sánchez et al, 2007; Solé et al, 2008; Abarca et al, 2014). The reorientation of cell-division planes during cell-plate formation is an evident change that characterizes rooting and rooting-derived cells in rooting-competent cuttings compared with the periclinal divisions of non-rooting cells or with the multiplicative periclinal divisions induced by auxin in noncompetent cuttings; (2) Asymmetric auxin distributions and overlaps in the temporal and spatial distributions of auxin and GRAS genes before the resumption of cell divisions in the rooting cells (Sánchez et al, 2007; Solé et al, 2008; Abarca et al, 2014); and (3) Antagonism between adventitious rooting and cambium proliferation along with xylem formation (Ricci et al, 2016; Díaz-Sala, 2019; Pizarro and Díaz-Sala, 2020, Díaz-Sala, 2020)
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