Mobile PEAR transcription factors integrate positional cues to prime cambial growth

Abstract

While apical growth in plants initiates upon seed germination, radial growth is only primed during early ontogenesis in procambium cells and activated later by the vascular cambium 1 . Although it is not known how radial growth is organized and regulated in plants, this system resembles the developmental competence observed in some animal systems, in which pre-existing patterns of developmental potential are established early on 2,3 . Here we show that the initiation of radial growth occurs around early protophloem sieve element (PSE) cell files of the root procambial tissue in Arabidopsis. In this domain cytokinin signalling promotes expression of a pair of novel mobile transcription factors, PHLOEM EARLY DOF (PEAR1, PEAR2) and their four homologs (DOF6, TMO6, OBP2 and HCA2), collectively called PEAR proteins. The PEAR proteins form a short-range concentration gradient peaking at PSE and activating gene expression that promotes radial growth. The expression and function of PEAR proteins are antagonized by well-established polarity transcription factors, HD-ZIP III 4 , whose expression is concentrated in the more internal domain of radially non-dividing procambial cells by the function of auxin and mobile miR165/166. The PEAR proteins locally promote transcription of their inhibitory HD-ZIP III genes, thereby establishing a negative feedback loop that forms a robust boundary demarking the zone of cell divisions. Taken together, we have established a network, in which the PEAR -HD-ZIP III module integrates spatial information of the hormonal domains and miRNA gradients during root procambial development, to provide adjacent zones of dividing and more quiescent cells as a foundation for further radial growth. Cambial growth in plants is initiated within the procambial tissues of the apical meristems through periclinal (i.e. longitudinal) divisions associated with formation of the vascular tissues xylem and phloem 1 (Extended Data Fig. 1a). It has been established that during procambial development in Arabidopsis roots there are distinct domains for high auxin and cytokinin signalling, which mark the regions for further development of xylem and phloem/procambium, respectively 5-8 . To accurately map the spatial distribution of the periclinal divisions, we established a new nomenclature for the root procambial cells, including PSE-lateral neighbours (PSE-LN) as cells directly contacting both PSE and the pericycle, the outer procambial cells (OPC) as procambial cells adjacent to the pericycle but not contacting PSE, and SE-internal neighbours (PSE-IN) as cells located internal to and directly contacting PSE (Fig. 1a). Both the PSE cell and PSE-LN showed higher activity of periclinal cell division than the OPC and PSE-IN (Fig. 1b, Extended Data Fig. 1b-d and Supplementary Information).We observed virtually no periclinal divisions in metaxylem (MX) and internal procambial cells (IPC) (Fig. 1b). Furthermore, blocking symplastic transport genetically 9 between the PSE and the surrounding cells results in a dramatic reduct...

Tissue-specific transcriptome profiling of theArabidopsis thalianainflorescence stem reveals local cellular signatures

“…We also found activity of WUSCHEL RELATED HOMEOBOX4 (WOX4) ( Figure 4L), whose expression domain is congruent with the PXY expression domain (Hirakawa et al, 2008;Suer et al, 2011;Brackmann et al, 2018;Shi et al, 2019) to peak in PXY pro -positive nuclei. In addition, MORE LATERAL GROWTH1 (MOL1), PHLOEM EARLY DOF1 (PEAR1) and PEAR2 are expressed in SMXL5 pro -positive cells (Gursanscky et al, 2016;Miyashima et al, 2019) and peaked, as expected, in SMXL5 pro -positive nuclei ( Figures 4N-P). Although LTP1 reads were not enriched in LTP1 pro -positive nuclei ( Figure 4Q), the reads of FIDDLEHEAD (FDH) and ECERIFERUM6 (CER6), which are known to be expressed in the epidermis (Yephremov et al, 1999;Pruitt et al, 2000;Hooker et al, 2002), were most abundant in LTP1 pro -positive nuclei (Figures 4R,S).…”

The DOF Transcription Factors in Seed and Seedling Development

“…More recently, two Arabidopsis DOF proteins (DOF2.4 and DOF5.1) have been identified as mobile factors involved in root procambial development and were named PEAR1 and 2 (PHLOEM EARLY DOF1 and 2). The corresponding PEAR1 and 2 genes are highly expressed in protophloem sieve elements (PSE) [102]. PEAR1 and 2 were shown to trigger periclinal cell division by controlling genes that promote radial growth, such as SUPPRESSOR OF MAX2 1-LIKE3 (SMXL3), encoding a key regulator of phloem formation [103].…”

“…In addition, four homologues (DOF3.2, DOF5.3, DOF1.1 and DOF5.6 ( Table 2), have been identified as PSE-specifically or PSE-abundantly expressed DOF genes with a broader protein localisation. These DOF factors redundantly function in the promotion of periclinal cell divisions in PSE cells [102]. The action of PEAR factors is counteracted by the HD-ZIP III transcription factors PHB, CNA and REV (PHABULOSA, CORONA and REVOLUTA).…”

“…The action of PEAR factors is counteracted by the HD-ZIP III transcription factors PHB, CNA and REV (PHABULOSA, CORONA and REVOLUTA). In addition, this molecular network involves a double-negative feedback loop where PEAR1 induces transcription of the PHB, CNA, and REV genes and the corresponding PHB, CNA and REV proteins negatively control PEAR1 transcription and protein movement [102]. The transition from seeds' dormancy to germination involves a wide reprogramming of the transcriptome and involves chromatin remodelling.…”

From plasmodesma geometry to effective symplasmic permeability through biophysical modelling

“…A well studied example is SHORT-ROOT (SHR), an Arabidopsis thaliana GRAS family transcription factor, that moves from the stele to cortical-endodermal tissue layers to specify cell fate and root patterning ( Nakajima et al, 2001 ; Spiegelman et al, 2018 ; Wu and Gallagher, 2013, 2014 ). Other mobile factors with developmental importance include TARGET OF MONOPTEROS 7, PEAR transcription factors and miRNAs ( Lu et al, 2018 ; Miyashima et al, 2019 ; Skopelitis et al, 2018 ).…”