Introduction
Wood serves three essential functions in plants: providing mechanical support, positioning leaves and reproductive organs for photosynthesis and pollination/dispersion; conducting water and nutrients; and storing water and nutrients (Baas et al. , 2004). The evolution of cellular specialization in Angiosperm wood, or secondary xylem, has led to a division of labor among different cell types. Fibers primarily provide mechanical support, vessels conduct water and minerals, and axial and radial parenchyma store resources (Tyree & Zimmermann, 2002). Secondary xylem formation occurs through the activity of the cambium, a lateral meristem, producing the different cell types of the secondary xylem toward the inside of the stem, and secondary phloem toward the exterior (Evert, 2006).
Studies on model species, like Arabidopsis thaliana andPopulus have significantly advanced the understanding of cambium function and activity, and xylem differentiation (Groover, 2005; Robischon et al. , 2011; Ye & Zhong, 2015). Crucial molecular components, such as the TDIF/CLE41/CLE44-TDR/PXY-WOX4 module controlling cambial maintenance and proliferation (Hirakawa et al. , 2010; Zhang et al. , 2019), and the secondary cell wall (SCW) biosynthesis master transcription factors NACs (NST1, XND1, VND6, VND7, and SND1 ) and MYBs (Kubo et al ., 2005; Zhonget al ., 2006, 2007; Zhang et al. , 2020), have been characterized in A. thaliana.These findings have proven to be conserved among diverse herbaceous and woody species (Hu et al ., 2010; Zhong et al ., 2011; Hirakawa & Bowman, 2015). However, our understanding of the molecular regulation underlying the cellular composition, arrangement, and dimensions of secondary xylem, crucial for wood functions, remains limited (Ziemińska et al. , 2015; Beeckman, 2016).
Advancements in technologies, like high-throughput sequencing have enabled molecular studies in non-model species, revealing unique features and processes absent in model species organisms (Carpentieret al. , 2008; Wang et al ., 2009). Within this context, lianas exhibit a distinctive set of wood anatomical characteristics associated with high flexibility, conduction efficiency, and intraindividual plasticity in wood traits (Fig. 1). These attributes make lianas an excellent model for investigating vascular system differentiation. Despite evolving independently in various plant groups, most lianas share convergent secondary xylem anatomical features, including reduced fibers, wide vessels (up to 500 μm in diameter) associated with small vessels (referred to as vessel dimorphism by Carlquist, 1981), and the presence of soft tissues interspersed in the xylem (Schenck, 1893; Obaton, 1960; Carlquist, 1985). These common features are collectively known as the “lianescent vascular syndrome” (Angyalossy et al. , 2015).
Most lianas, however, show a dense fibrous xylem with small vessels, resembling the xylem of self-supporting species at the beginning of secondary development (Schenck, 1893; Obaton, 1960; Caballé, 1998). The transition from the formation of this dense xylem (termed as self-supporting xylem hereafter) to the development of that showing the lianescent vascular syndrome (termed as lianescent xylem hereafter) occurs abruptly, as seen in adult stem cross-sections (Fig. 1b). Importantly, self-supporting and lianescent xylems are produced simultaneously along liana stems. While older sections produce lianescent xylem, younger parts continue to form self-supporting xylem. Furthermore, recent findings indicate that attachment to supports triggers the production of lianescent xylem in the twining liana Condylocarpon guianensis(Soffiatti et al ., 2022). However, it remains unclear if this triggering factor is consistent across lianas from different lineages or with different climbing methods.