WOX4 Imparts Auxin Responsiveness to Cambium Cells in Arabidopsis      

Abstract

Multipotent stem cell populations, the meristems, are fundamental for the indeterminate growth of plant bodies. One of these meristems, the cambium, is responsible for extended root and stem thickening. Strikingly, although the pivotal role of the plant hormone auxin in promoting cambium activity has been known for decades, the molecular basis of auxin responsiveness on the level of cambium cells has so far been elusive. Here, we reveal that auxin-dependent cambium stimulation requires the homeobox transcription factor WOX4. In Arabidopsis thaliana inflorescence stems, 1-N-naphthylphthalamic acid-induced auxin accumulation stimulates cambium activity in the wild type but not in wox4 mutants, although basal cambium activity is not abolished. This conclusion is confirmed by the analysis of cellular markers and genome-wide transcriptional profiling, which revealed only a small overlap between WOX4-dependent and cambiumspecific genes. Furthermore, the receptor-like kinase PXY is required for a stable auxin-dependent increase in WOX4 mRNA abundance and the stimulation of cambium activity, suggesting a concerted role of PXY and WOX4 in auxin-dependent cambium stimulation. Thus, in spite of large anatomical differences, our findings uncover parallels between the regulation of lateral and apical plant meristems by demonstrating the requirement for a WOX family member for auxin-dependent regulation of lateral plant growth.

In the absence of BYPASS1-related gene function, the bps signal disrupts embryogenesis by an auxin-independent mechanism

“…Consistent with this possibility, the pattern of PIN1-GFP distribution during bps triple mutant embryogenesis is similar to that of mutants with defects in two protein phosphatase 2A genes, POLTERGEIST (POL) and POLTERGEIST-LIKE (PLL), which function downstream from CLV1 (Song et al, 2008). Recently, the CLE41/TDIF-PXY/TDR-WOX4 module has been shown to be required for vascular development and procambium auxin responsiveness (Hirakawa et al, 2010;Ji et al, 2010;Suer et al, 2011), and is a possible candidate for the developmental target of the bps signal. Alternative developmental regulators that might be attractive candidates for the bps signal include KANADI and the Class II HD-ZIP transcription factors (Ito et al, 2006;Hirakawa et al, 2010;Ilegems et al, 2010), which also function upstream of auxin, and in vascular patterning pathways.…”

A comparative transcriptomic approach to understanding the formation of cork

“…ARK1/STM and WOX4 have been related to stem cell maintenance in cambium (Groover et al 2006;Hirakawa et al 2010). ARK1/STM prevents the differentiation of meristematic cells (Groover et al 2006), whereas WOX4 has been shown to trigger auxin responsiveness in the cambial meristem to promote cambial proliferation (Ji et al 2010;Suer et al 2011). Taking into account the greater capacity of cork oak to produce phellem tissue, it is tempting to speculate that the stronger expression of cork oak orthologs of ARK1/STM and WOX4 in cork could be required to maintain phellogen meristematic activity for a longer period, thus allowing the formation of more phellem daughter cells.…”

Signaling Peptides in Plants

“…TDR/PXY is mainly expressed in procambium and cambium cells, whereas CLE41 is transcribed in neighboring phloem cells, indicating that CLE41 must be secreted towards the procambium where it is perceived by TDR/PXY [60,62,63]. Intriguingly, another WUS-related homeobox gene, WOX4, is essential for the regulatory function of TDIF in the vascular cell fate, but not for TDIF inhibition of xylem differentiation [64,65]. The signaling peptides CLE9 and CLE10 have also been reported to be produced in the vasculature and to function in vascular development.…”