Spatio-temporal sequence of cross-regulatory events in root meristem growth

Abstract

A central question in developmental biology is how multicellular organisms coordinate cell division and differentiation to determine organ size. In Arabidopsis roots, this balance is controlled by cytokinin-induced expression of SHORT HYPOCOTYL 2 (SHY2) in the so-called transition zone of the meristem, where SHY2 negatively regulates auxin response factors (ARFs) by protein-protein interaction. The resulting down-regulation of PIN-FORMED (PIN) auxin efflux carriers is considered the key event in promoting differentiation of meristematic cells. Here we show that this regulation involves additional, intermediary factors and is spatio-temporally constrained. We found that the described cytokinin-auxin crosstalk antagonizes BREVIS RADIX (BRX) activity in the developing protophloem. BRX is an auxin-responsive target of the prototypical ARF MONOPTEROS (MP), a key promoter of vascular development, and transiently enhances PIN3 expression to promote meristem growth in young roots. At later stages, cytokinin induction of SHY2 in the vascular transition zone restricts BRX expression to down-regulate PIN3 and thus limit meristem growth. Interestingly, proper SHY2 expression requires BRX, which could reflect feedback on the auxin responsiveness of SHY2 because BRX protein can directly interact with MP, likely acting as a cofactor. Thus, cross-regulatory antagonism between BRX and SHY2 could determine ARF activity in the protophloem. Our data suggest a model in which the regulatory interactions favor BRX expression in the early proximal meristem and SHY2 prevails because of supplementary cytokinin induction in the later distal meristem. The complex equilibrium of this regulatory module might represent a universal switch in the transition toward differentiation in various developmental contexts.

Molecular genetic framework for protophloem formation

“…A similar phenotype is observed in loss-of-function mutants of BREVIS RADIX (BRX) (11), another polar localized plasma membrane-associated protein that is also thought to act in the nucleus (11,12). The brx short root phenotype is rescued by second site mutation in BARELY ANY MERISTEM 3 (BAM3), which encodes a receptor-like kinase that is required for suppression of protophloem differentiation on treatment with the peptide ligand, CLAVATA3/EMBRYO SURROUNDING REGION 45 (CLE45) (13).…”

“…4 E and F), suggesting it is particularly sensitive to changes in auxin activity. Given that BRX has been implicated in potentiating auxin response (11,24), this would also explain why the penetrance of the observed phenotypes was highest in brx mutants. Remarkably, however, root growth rate and meristem size were only mildly impaired in auxinole-treated roots, despite the absence of the incipient metaphloem cell file (Fig.…”

“…Plant tissue culture, CLE peptide treatments, plant transformation, histology, light microscopy, and confocal microscopy, as well as molecular biology experiments such as genomic DNA isolation, genotyping, or sequencing, were performed according to standard procedures, as described previously (11,13).…”

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).…”

Natural Variation of Root Traits: From Development to Nutrient Uptake

“…Subsequent analysis showed that BRX mediates the interaction between brassinosteroids and auxin pathways, which is required for root growth (Mouchel et al, 2006). Later analysis suggested that BRX is also involved in cytokinin-mediated inhibition of LR development (Li et al, 2009) and furthermore, the auxin-cytokinin cross talk that coordinates cell division and differentiation in the root tip (Scacchi et al, 2010). Combining high-throughput confocal microscopy imaging, GWAS, and expression analysis, a then-uncharacterized gene encoding an F-box protein was found that underlies the natural variation of root development in a broad set of accessions (Meijón et al, 2014).…”