An expression quantitative trait locus (eQTL) study in a full-sib F1 population of the shrub willow Salix suchowensis (Chen et al., 2023) predicted five master trans-regulators of wood formation (FLA13, CAGE1, SRF6, PGF11, CSE) and key candidate genes (CLV1, BAM1, CYCB2;4, CDKB2;1, JAZ1, HAT22, MYB36), conjectured that a WUSCHEL/CLAVATA-like stem-cell regulatory mechanism operates in the vascular cambium, and found enrichment of targets in five phytohormone pathways (auxin, jasmonate, abscisate, brassinosteroid, ethylene). Whether these predicted regulators actually control wood formation in vivo in S. suchowensis remains untested: no transgenic or CRISPR-based functional study of any of these candidates has been reported in S. suchowensis or any other willow, stable genetic transformation has been demonstrated only in other Salix species, and cambial stem-cell control has been functionally dissected mainly in Populus, Arabidopsis, pine, and birch. The open problem is to determine, by direct functional-genetic evidence in S. suchowensis, the regulatory roles of these genes in cambium proliferation and secondary cell wall deposition, whether a WUS/CLV-like module operates in its vascular cambium, and how the five hormones jointly coordinate secondary growth.

Background

Wood (secondary xylem) of trees and shrubs is produced by the vascular cambium, a cylindrical meristem (tissue of dividing, undifferentiated cells) between the inner bark and the wood that annually generates xylem inward and phloem outward; this radial growth is called secondary growth. Wood formation is a quantitative trait controlled by many genes, and its regulatory genetics has been studied mostly in poplar (Populus), where functional tools such as stable transformation and CRISPR editing are established.

Expression quantitative trait locus (eQTL) mapping locates genomic regions whose sequence variants are associated with variation in the expression level of genes, rather than with an external trait; a trans-eQTL is a distantly located regulator, a cis-eQTL acts near the gene it controls. Chen et al. (2023) performed the first comprehensive eQTL analysis of wood formation in Salix suchowensis, a fast-growing shrub willow used for biomass, wicker, and ecological restoration, using RNA sequencing of young stems from a full-sibling F1 population, with genetic variants identified by simplified genome sequencing. They mapped 16,487 eQTL blocks controlling 5,505 genes, identified eQTL hotspots (genomic windows enriched for eQTLs), and combined eQTL mapping with gene co-expression networks to predict master trans-regulators: FLA13 (a fasciclin-like arabinogalactan protein, a cell-wall-associated protein family implicated in poplar tension wood), CAGE1 (a Golgi-localized GT31 glycosyltransferase required for normal cellulose biosynthesis in Arabidopsis), SRF6 (a STRUBBELIG-family receptor-like kinase; the Arabidopsis STRUBBELIG pathway mediates responses to cellulose deficiency), PGF11 (a polygalacturonase clade enzyme), and CSE (caffeoyl shikimate esterase, a lignin-biosynthesis enzyme whose CRISPR knockouts impair lignification in Populus). Network analysis additionally nominated CLV1 and BAM1 (CLAVATA1-family receptor-like kinases that control stem-cell niches in shoot and root meristems and root xylem patterning in Arabidopsis), the cell-cycle regulators CYCB2;4 and CDKB2;1 (whose transcription accompanies cambium reactivation in poplar), and the transcription factors JAZ1 (a jasmonate-signaling repressor), HAT22 (an HD-Zip II transcription repressor; its poplar ortholog PtrHAT22, when overexpressed, reduces lignin and cellulose content, thins secondary cell walls, and inhibits stem thickening), and MYB36 (linked to lignification and Casparian-strip formation in Arabidopsis roots).

In the shoot and root apical meristems, stem-cell homeostasis is maintained by the WUSCHEL/CLAVATA (WUS/CLV) module: the WUS homeobox transcription factor promotes stem-cell identity, and the mobile CLV3 peptide, perceived by the CLV1 receptor, feedback-inhibits WUS to balance proliferation against differentiation. Whether an analogous module controls the stem cells of the vascular cambium has been a long-standing question ever since a high-resolution transcript profile across the poplar wood-forming meristem (Schrader et al., 2004) identified cambial markers but could not establish the mechanism. Current functional evidence, mainly from Arabidopsis and Populus, supports a related but distinct cambial module — the TDIF/CLE41-family peptide–PXY receptor–WOX4 transcription factor axis — so the existence of a WUS/CLV-like mechanism in the cambium of a given woody species remains genuinely open. Chen et al. (2023) observed that their eQTL/co-expression results implicate CLV1 and BAM1 in S. suchowensis cambium activity and suggested there may be a WUS/CLV-like regulatory mechanism in the vascular cambium; they also found their regulated targets enriched in auxin (IAA), jasmonate (JA), abscisate (ABA), brassinosteroid (BR), and ethylene (ETH) pathways, and noted that combining eQTL mapping with GWAS (genome-wide association studies), QTL (quantitative trait locus) mapping, and haplotype analysis would be needed to pinpoint causal variants. All of these regulatory predictions, however, rest solely on statistical association and co-expression — none has been tested by direct genetic perturbation in S. suchowensis. Stable genetic transformation has not yet been established in S. suchowensis, having been demonstrated only in other Salix species.

Problem Statement

Determine, by direct functional-genetic evidence in Salix suchowensis, whether and how the expression quantitative trait locus (eQTL)-predicted regulators of wood formation actually regulate secondary growth. Specifically: (i) establish the in vivo regulatory roles of the predicted master trans-regulators (FLA13, CAGE1, SRF6, PGF11, CSE) and the key candidate genes (CLV1, BAM1, CYCB2;4, CDKB2;1, JAZ1, HAT22, MYB36) in cambium cell proliferation, xylem differentiation, and secondary cell wall (cellulose, hemicellulose, lignin) biosynthesis, with phenotypic consequences for wood traits; (ii) determine whether a WUS/CLV-like stem-cell regulatory mechanism operates in the vascular cambium of S. suchowensis, as conjectured from the implication of CLV1 and BAM1 in cambium activity, and if so characterize its molecular components; and (iii) determine how the five phytohormones auxin (IAA), jasmonate (JA), abscisate (ABA), brassinosteroid (BR), and ethylene (ETH) jointly coordinate secondary growth in this species, including the crosstalk among their pathways in regulating the predicted target genes (the hormone-pathway-enriched eQTL target genes of the predicted master trans-regulators reported by Chen et al., 2023). The three parts (i), (ii) and (iii) are jointly required.

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