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.