- © 2010 American Society of Plant Biologists
The development and maintenance of the procambium and cambium, together referred to as the vascular meristem, are not well understood (reviewed in Elo et al., 2009). Recently, it has come to light that, along with hormones, peptide signals are active in vascular tissue development. TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR (TDIF) is a member of the CLAVATA3/EMBRYO SURROUNDING REGION-related (CLE) family of secreted peptides. These mediate cell–cell communication in plants and are involved in regulating both the shoot and root apical meristems (reviewed in Fiers et al., 2007). In Arabidopsis thaliana, TDIF is secreted by phloem tissue and plays two roles in the maintenance of the vascular stem cell population: it inhibits xylem differentiation and promotes procambial cell proliferation (reviewed in Hirakawa et al., 2010a). The best characterized of the CLE peptides, CLAVATA3 (CLV3), regulates the shoot apical meristem via its receptor CLV1, a leucine-rich repeat receptor-like kinase (LRR-RLK). Correspondingly, the receptor for TDIF, called TDIF RECEPTOR (TDR) or PHLOEM INTERCALATED WITH XYLEM, is also an LRR-RLK.
Now, Hirakawa et al. (2010b; pages 2618–2629) have taken advantage of this similarity to CLV3 to elucidate TDIF signaling mechanisms. CLV3’s regulation of meristem size in the shoot apex involves a feedback loop with the homeodomain transcription factor WUSCHEL (WUS). Hirakawa et al. looked for targets of TDIF signaling among Arabidopsis WUSCHEL-RELATED HOMEOBOX (WOX) genes and found WOX4. WOX4 is expressed in vascular tissue throughout the plant, consistent with it being involved in TDIF/TDR regulation of the vascular meristem. Furthermore, the rapid increase in WOX4 expression caused by TDIF treatment is not seen in tdr mutants, showing that the induction of WOX4 is dependent upon TDIF signaling through TDR.
Interestingly, TDIF peptide treatment still inhibits xylem differentiation in wox4 mutants, as it does in the wild type (see figure). By contrast, tdr mutants do not form discontinuous xylem in response to TDIF. Both wox4 and tdr mutants, however, exhibit decreased procambial cell division. Together, these results suggest that TDR mediates both roles of TDIF in vascular stem cell maintenance, as expected if it functions as TDIF's receptor, whereas WOX4 functions in the promotion of procambial cell proliferation but not in inhibition of xylem differentiation.
TDIF treatment induces the formation of discontinuous xylem strands (white arrowheads) in the wild type (Col-0, top) and the wox4 mutant (bottom), but not in the tdr mutant (middle). Leaves from 10-d-old plants are shown. Veins are outlined in red, and xylem strands are in blue. Bars = 100 μm. (Adapted from Hirakawa et al. [2010].)
In this work, Hirakawa et al. have made strides toward understanding the mechanisms of TDIF signaling in the vascular meristem. They show that TDIF/TDR influences vascular development by two pathways: one WOX4-dependent and one independent of WOX4. In addition, they provide further evidence that mechanisms involving CLE peptides, LRR-RLK receptors, and homeobox transcription factors are common not only to shoot and root apical meristems, but also to lateral meristems. The details are not identical: CLV3 inhibits WUS expression, whereas TDIF induces WOX4, but the players are strikingly similar, suggesting further commonalities in meristem regulation. Given that the shoot and root apical meristems are much better studied, this could open exciting avenues for research into the vascular meristem.
