Molecular genetic framework for protophloem formation

Abstract

The phloem performs essential systemic functions in tracheophytes, yet little is known about its molecular genetic specification. Here we show that application of the peptide ligand CLAVATA3/EMBRYO SURROUNDING REGION 45 (CLE45) specifically inhibits specification of protophloem in Arabidopsis roots by locking the sieve element precursor cell in its preceding developmental state. CLE45 treatment, as well as viable transgenic expression of a weak CLE45 G6T variant, interferes not only with commitment to sieve element fate but also with the formative sieve element precursor cell division that creates protophloem and metaphloem cell files. However, the absence of this division appears to be a secondary effect of discontinuous sieve element files and subsequent systemically reduced auxin signaling in the root meristem. In the absence of the formative sieve element precursor cell division, metaphloem identity is seemingly adopted by the normally procambial cell file instead, pointing to possibly independent positional cues for metaphloem formation. The protophloem formation and differentiation defects in brevis radix (brx) and octopus (ops) mutants are similar to those observed in transgenic seedlings with increased CLE45 activity and can be rescued by loss of function of a putative CLE45 receptor, BARELY ANY MERI-STEM 3 (BAM3). Conversely, a dominant gain-of-function ops allele or mild OPS dosage increase suppresses brx defects and confers CLE45 resistance. Thus, our data suggest that delicate quantitative interplay between the opposing activities of BAM3-mediated CLE45 signals and OPS-dependent signals determines cellular commitment to protophloem sieve element fate, with OPS acting as a positive, quantitative master regulator of phloem fate. stem cell | division plane switching

AspWood: High-Spatial-Resolution Transcriptome Profiles Reveal Uncharacterized Modularity of Wood Formation in Populus tremula

“…The two plasma membrane-associated proteins OCTOPUS (OPS) and BREVIS RADIX (BRX) are expressed in procambium and protophloem precursors, respectively, and are required for the correct specification of protophloem cells (Scacchi et al, 2010;Truernit et al, 2012). In sieve tube element precursors, the CLAVATA3/EMBRYO SURROUNDING REGION45 (CLE45) peptide inhibits early protophloem development by signaling through the LRR-RLK receptor BARELY ANY MERISTEM3 (BAM3) (Depuydt et al, 2013;Rodriguez-Villalon et al, 2014). In support of these genes having a similar role during secondary vascular development, we identified that the closest homologs of OPS (PtOPSs; Supplemental Data Set 9) were highly expressed across the cambium, whereas genes homologous to BRX, CLE45, and BAM3 were highly expressed in the differentiating phloem (sample cluster i; Figure 4A).…”

Primary root protophloem differentiation requires balanced phosphatidylinositol-4,5-biphosphate levels and systemically affects root branching

“…Each phloem pole comprises two distinct cell types: the sieve elements, which are the actual conductive cells of the phloem; and the companion cells, which provide essential metabolic functions to the enucleated sieve elements (Lucas et al, 2013). The early, initial phloem is called protophloem and is the first tissue to differentiate in the root meristem (Furuta et al, 2014;Rodriguez-Villalon et al, 2014). It reaches from close to the stem cell niche up into the maturation zone of the meristem, where its function is gradually taken over by the neighboring metaphloem (which differentiates once the protophloem is mature) (Fig.…”

“…Sieve element differentiation is a complex process during which nuclei are broken down and cell walls are thickened (Furuta et al, 2014;RodriguezVillalon et al, 2014;Truernit et al, 2012). Genetic studies in Arabidopsis have identified two genes that promote the commitment of precursor cells to the sieve element differentiation program (Rodriguez-Villalon et al, 2014;Truernit et al, 2012): BREVIS RADIX (BRX) and OCTOPUS (OPS). Both encode different plantspecific plasma membrane-associated proteins, which are expressed in the developing protophloem and polar localized at the root-ward (BRX) or shoot-ward (OPS) end of cells (Scacchi et al, 2009;Truernit et al, 2012).…”

“…In both brx-and ops-null mutants, root meristem protophloem cells frequently fail to differentiate properly. These so-called 'gap cells' disrupt the continuity of the protophloem sieve element strands, resulting in reduced phloem cargo delivery to the meristem (Depuydt et al, 2013;Rodriguez-Villalon et al, 2014;Truernit et al, 2012). This manifests in, for example, a systemically reduced auxin response throughout the meristem (Gujas et al, 2012;Rodriguez-Villalon et al, 2014), which is likely responsible for its reduced activity and size, and thus the decreased overall root growth observed in the mutants.…”

“…These so-called 'gap cells' disrupt the continuity of the protophloem sieve element strands, resulting in reduced phloem cargo delivery to the meristem (Depuydt et al, 2013;Rodriguez-Villalon et al, 2014;Truernit et al, 2012). This manifests in, for example, a systemically reduced auxin response throughout the meristem (Gujas et al, 2012;Rodriguez-Villalon et al, 2014), which is likely responsible for its reduced activity and size, and thus the decreased overall root growth observed in the mutants. Recent studies suggest that OPS is a master regulator of the commitment to sieve element fate, because extra OPS dose or an ops gain-of-function allele can rescue brx defects and can overcome hyperactivity of the BARELY ANY MERISTEM 3 (BAM3)-CLAVATA3/EMBRYO SURROUNDING REGION 45 (CLE45) receptor-ligand module (Rodriguez-Villalon et al, 2014), a powerful inhibitor of sieve element specification that is de-regulated in brx roots (Depuydt et al, 2013).…”