Control of plant stem cell function by conserved interacting transcriptional regulators.

Plant stem cells in the shoot apical meristem (SAM) and root apical meristem are necessary for postembryonic development of aboveground tissues and roots, respectively, while secondary vascular stem cells sustain vascular development. WUSCHEL (WUS), a homeodomain transcription factor expressed in the rib meristem of the Arabidopsis SAM, is a key regulatory factor controlling SAM stem cell populations, and is thought to establish the shoot stem cell niche through a feedback circuit involving the CLAVATA3 (CLV3) peptide signalling pathway. WUSCHEL-RELATED HOMEOBOX 5 (WOX5), which is specifically expressed in the root quiescent centre, defines quiescent centre identity and functions interchangeably with WUS in the control of shoot and root stem cell niches. WOX4, expressed in Arabidopsis procambial cells, defines the vascular stem cell niche. WUS/WOX family proteins are evolutionarily and functionally conserved throughout the plant kingdom and emerge as key actors in the specification and maintenance of stem cells within all meristems. However, the nature of the genetic regime in stem cell niches that centre on WOX gene function has been elusive, and molecular links underlying conserved WUS/WOX function in stem cell niches remain unknown. Here we demonstrate that the Arabidopsis HAIRY MERISTEM (HAM) family of transcription regulators act as conserved interacting cofactors with WUS/WOX proteins. HAM and WUS share common targets in vivo and their physical interaction is important in driving downstream transcriptional programs and in promoting shoot stem cell proliferation. Differences in the overlapping expression patterns of WOX and HAM family members underlie the formation of diverse stem cell niche locations, and the HAM family is essential for all of these stem cell niches. These findings establish a new framework for the control of stem cell production during plant development.

Arabidopsis homologs of the petunia hairy meristem gene are required for maintenance of shoot and root indeterminacy.

Maintenance of indeterminacy is fundamental to the generation of plant architecture and a central component of the plant life strategy. Indeterminacy in plants is a characteristic of shoot and root meristems, which must balance maintenance of indeterminacy with organogenesis. The Petunia hybrida HAIRY MERISTEM (HAM) gene, a member of the GRAS family of transcriptional regulators, promotes shoot indeterminacy by an undefined non-cell-autonomous signaling mechanism(s). Here, we report that Arabidopsis (Arabidopsis thaliana) mutants triply homozygous for knockout alleles in three Arabidopsis HAM orthologs (Atham1,2,3 mutants) exhibit loss of indeterminacy in both the shoot and root. In the shoot, the degree of penetrance of the loss-of-indeterminacy phenotype of Atham1,2,3 mutants varies among shoot systems, with arrest of the primary vegetative shoot meristem occurring rarely or never, secondary shoot meristems typically arresting prior to initiating organogenesis, and inflorescence and flower meristems exhibiting a phenotypic range extending from wild type (flowers) to meristem arrest preempting organogenesis (flowers and inflorescence). Atham1,2,3 mutants also exhibit aberrant shoot phyllotaxis, lateral organ abnormalities, and altered meristem morphology in functioning meristems of both rosette and inflorescence. Root meristems of Atham1,2,3 mutants are significantly smaller than in the wild type in both longitudinal and radial axes, a consequence of reduced rates of meristem cell division that culminate in root meristem arrest. Atham1,2,3 phenotypes are unlikely to reflect complete loss of HAM function, as a fourth, more distantly related Arabidopsis HAM homolog, AtHAM4, exhibits overlapping function with AtHAM1 and AtHAM2 in promoting shoot indeterminacy.

HAM proteins promote organ indeterminacy: but how?

HAIRY MERISTEM (HAM) proteins, members of the GRAS family of transcriptional regulators, are essential for maintenance of indeterminate growth in flowering plant shoots, loss-of-function ham mutants exhibiting a strikingly novel phenotype of shoot meristem arrest and differentiation. Specific cellular/molecular functions of HAM proteins underlying meristem maintenance are unknown. In this review, I highlight findings from recent analyses of Arabidopsis ham (Atham) loss-of-function phenotypes, including that HAM function limits the generation of clonally-derived meristem layers and that HAM function regulates CLAVATA3 expression. I consider how this new information both refines our understanding of the role of HAM proteins in regulating meristem structure and function, and may also suggest possible downstream HAM protein transcriptional targets. Finally, I note the significant phenotypic overlap between Atham phenotypes, and aintegumenta/anintegumenta-like6 double mutant phenotypes, suggesting meristem regulatory functions common to, and possible genetic interactions between, HAM and AINTEGUMENTA.

Phylogenetic analysis of GRAS proteins from moss, lycophyte and vascular plant lineages reveals that GRAS genes arose and underwent substantial diversification in the ancestral lineage common to bryophytes and vascular plants.

GRAS genes are a large family of streptophyte specific transcription factors that function in a diverse set of physiological and developmental processes. GRAS proteins of the HAIRY MERISTEM (HAM) sub-family are required for maintenance of shoot and root indeterminacy. The transcriptional targets of HAM proteins and the signaling inputs regulating HAM activity are completely unknown. Understanding the relationship of HAM proteins to other members of the GRAS family may inform hypotheses relating to cellular level HAM functions. I here report a phylogenetic analysis of GRAS proteins employing the complete set of known and probable GRAS proteins from the sequenced genomes of the flowering plants Arabidopsis and Rice, the lycophyte Selaginella moellendorffii, and the bryophyte Physcomitrella patens. HAM proteins are most closely related to DELLA proteins, key components of gibberellin perception. However, GRAS proteins diversified into a minimum of twelve discreet monophyletic lineages, including the HAM and DELLA subfamilies, prior to divergence of the moss and flowering plant lineages. Substantial diversification of GRAS proteins at so early a point in land plant evolution suggests that relative relatedness sequence homology among GRAS proteins sub-families may not substantially reflect shared protein function. 

The Selaginella genome identifies genetic changes associated with the evolution of vascular plants.

Vascular plants appeared ~410 million years ago, then diverged into several lineages of which only two survive: the euphyllophytes (ferns and seed plants) and the lycophytes. We report here the genome sequence of the lycophyte Selaginella moellendorffii (Selaginella), the first nonseed vascular plant genome reported. By comparing gene content in evolutionarily diverse taxa, we found that the transition from a gametophyte- to a sporophyte-dominated life cycle required far fewer new genes than the transition from a nonseed vascular to a flowering plant, whereas secondary metabolic genes expanded extensively and in parallel in the lycophyte and angiosperm lineages. Selaginella differs in posttranscriptional gene regulation, including small RNA regulation of repetitive elements, an absence of the trans-acting small interfering RNA pathway, and extensive RNA editing of organellar genes.

Pharmacological analysis of nod factor-induced calcium spiking in Medicago truncatula. Evidence for the requirement of type IIA calcium pumps and phosphoinositide signaling.

Bacterial Nod factors trigger a number of cellular responses in root hairs of compatible legume hosts, which include periodic, transient increases in cytosolic calcium levels, termed calcium spiking. We screened 13 pharmaceutical modulators of eukaryotic signal transduction for effects on Nod factor-induced calcium spiking. The purpose of this screening was 2-fold: to implicate enzymes required for Nod factor-induced calcium spiking in Medicago sp., and to identify inhibitors of calcium spiking suitable for correlating calcium spiking to other Nod factor responses to begin to understand the function of calcium spiking in Nod factor signal transduction. 2-Aminoethoxydiphenylborate, caffeine, cyclopiazonic acid (CPA), 2,5-di-(t-butyl)-1,4-hydroquinone, and U-73122 inhibit Nod factor-induced calcium spiking. CPA and U-73122 are inhibitors of plant type IIA calcium pumps and phospholipase C, respectively, and implicate the requirement for these enzymes in Nod factor-induced calcium spiking. CPA and U-73122 inhibit Nod factor-induced calcium spiking robustly at concentrations with no apparent toxicity to root hairs, making CPA and U-73122 suitable for testing whether calcium spiking is causal to subsequent Nod factor responses.