Specification of Arabidopsis floral meristem identity by repression of flowering time genes

Flowering plants produce floral meristems in response to intrinsic and extrinsic flowering inductive signals. In Arabidopsis, the floral meristem identity genes LEAFY (LFY) and APETALA1 (AP1) are activated to play a pivotal role in specifying floral meristems during floral transition. We show here that the emerging floral meristems require AP1 to partly specify their floral identities by directly repressing a group of flowering time genes, including SHORT VEGETATIVE PHASE (SVP), AGAMOUS-LIKE 24 (AGL24) and SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1). In wild-type plants, these flowering time genes are normally downregulated in emerging floral meristems. In the absence of AP1, these genes are ectopically expressed, transforming floral meristems into shoot meristems. By post-translational activation of an AP1-GR fusion protein and chromatin immunoprecipitation assays, we further demonstrate the repression of these flowering time genes by induced AP1 activity and in vivo AP1 binding to the cis-regulatory regions of these genes. These findings indicate that once AP1 is activated during the floral transition, it acts partly as a master repressor in floral meristems by directly suppressing the expression of flowering time genes, thus preventing the continuation of the shoot developmental program.     

APETALA2 regulates the stem cell niche in the Arabidopsis shoot meristem

Postembryonic organ formation in higher plants relies on the activity of stem cell niches in shoot and root meristems where differentiation of the resident cells is repressed by signals from surrounding cells. We searched for mutations affecting stem cell maintenance and isolated the semidominant l28 mutant, which displays premature termination of the shoot meristem and differentiation of the stem cells. Allele competition experiments suggest that l28 is a dominant-negative allele of the APETALA2 (AP2) gene, which previously has been implicated in floral patterning and seed development. Expression of both WUSCHEL (WUS) and CLAVATA3 (CLV3) genes, which regulate stem cell maintenance in the wild type, were disrupted in l28 shoot apices from early stages on. Unlike in floral patterning, AP2 mRNA is active in the center of the shoot meristem and acts via a mechanism independent of AGAMOUS, which is a repressor of WUS and stem cell maintenance in the floral meristem. Genetic analysis shows that termination of the primary shoot meristem in l28 mutants requires an active CLV signaling pathway, indicating that AP2 functions in stem cell maintenance by modifying the WUS-CLV3 feedback loop.     

POLTERGEIST functions to regulate meristem development downstream of the CLAVATA loci

Mutations at the CLAVATA loci (CLV1, CLV2 and CLV3) result in the accumulation of undifferentiated cells at the shoot and floral meristems. We have isolated three mutant alleles of a novel locus, POLTERGEIST (POL), as suppressors of clv1, clv2 and clv3 phenotypes. All pol mutants were nearly indistinguishable from wild-type plants; however, pol mutations provided recessive, partial suppression of meristem defects in strong clv1 and clv3 mutants, and nearly complete suppression of weak clv1 mutants. pol mutations partially suppressed clv2 floral and pedicel defects in a dominant fashion, and almost completely suppressed clv2 phenotypes in a recessive manner. These observations, along with dominant interactions observed between the pol and wuschel (wus) mutations, indicate that POL functions as a critical regulator of meristem development downstream of the CLV loci and redundantly with WUS. Consistent with this, pol mutations do not suppress clv3 phenotypes by altering CLV1 receptor activation.     

Interactions among APETALA1, LEAFY, and TERMINAL FLOWER1 specify meristem fate.

Upon floral induction, the primary shoot meristem of an Arabidopsis plant begins to produce flower meristems rather than leaf primordia on its flanks. Assignment of floral fate to lateral meristems is primarily due to the cooperative activity of the flower meristem identity genes LEAFY (LFY), APETALA1 (AP1), and CAULIFLOWER. We present evidence here that AP1 expression in lateral meristems is activated by at least two independent pathways, one of which is regulated by LFY. In lfy mutants, the onset of AP1 expression is delayed, indicating that LFY is formally a positive regulator of AP1. We have found that AP1, in turn, can positively regulate LFY, because LFY is expressed prematurely in the converted floral meristems of plants constitutively expressing AP1. Shoot meristems maintain an identity distinct from that of flower meristems, in part through the action of genes such as TERMINAL FLOWER1 (TFL1), which bars AP1 and LFY expression from the influorescence shoot meristem. We show here that this negative regulation can be mutual because TFL1 expression is downregulated in plants constitutively expressing AP1. Therefore, the normally sharp phase transition between the production of leaves with associated shoots and formation of the flowers, which occurs upon floral induction, is promoted by positive feedback interactions between LFY and AP1, together with negative interactions of these two genes with TFL1.     

Genetic control of shoot and flower meristem behavior

Flowers and shoots are derived from specialized groups of stem cells termed meristems. Recent studies in Arabidopsis have identified factors that contribute to meristem structure and identity, such as CLAVATA1, CLAVATA3, and SHOOTMERISTEMLESS, which act in both shoot and flower meristems, as well as LEAFY and APETALA1 which specifically determine a floral fate.     

Root-specific CLE19 overexpression and the sol1/2 suppressors implicate a CLV-like pathway in the control of Arabidopsis root meristem maintenance

In the Arabidopsis shoot apical meristem, an organizing center signals in a non-cell-autonomous manner to specify the overlying stem cells. Stem cells express the small, secreted protein CLAVATA3 (CLV3; ) that activates the CLV1-CLV2 receptor complex, which negatively controls the size of the organizing center. Consistently, CLV3 overexpression restricts shoot meristem size. The root meristem also contains a stem cell organizer, and here we show that localized overexpression in roots of CLE19, encoding a CLV3 homolog, restricts the size of the root meristem. This suggests that CLE19 acts by overactivating an endogenous CLV-like pathway involved in root meristem maintenance. Surprisingly, CLE19 restricts meristem size without directly interfering with organizer and stem cell specification. We isolated mutations in two loci, SOL1 and SOL2, which suppress the CLE19 overexpression phenotype. sol2 plants display floral phenotypes reminiscent of clv weak alleles; these phenotypes suggest that components of a CLV pathway are shared in roots and shoots. SOL1 encodes a putative Zn(2+)-carboxypeptidase, which may be involved in ligand processing.     

A novel function of TDIF-related peptides: promotion of axillary bud formation

Small peptides derived from the CLAVATA3/EMBRYO SURROUNDING REGION-related (CLE) gene family play a key role in various cell-cell communications in land plants. Among them, tracheary element differentiation inhibition factor (TDIF; CLE41/CLE44 peptide) and CLE42 peptide of Arabidopsis have almost identical amino acid sequences and act as inhibitors of tracheary element differentiation. In this study, we report a novel function of TDIF and CLE42. We found by the GUS (β-glucuronidase) reporter gene assay that while CLE41 and CLE44 are expressed preferentially in vascular bundles, CLE42 is expressed strongly in the shoot apical meristem (SAM) and axillary meristems. Overexpression of CLE42 and CLE41 enhanced axillary bud formation in the leaf and cotyledon axils. Before floral transition, the emergence of axillary buds in these plants occurred in an acropetal order. Exogenous supply of either TDIF or CLE42 peptide to the wild type induced similar excess bud emergence. In vascular bundles, the TDIF RECEPTOR (TDR) acts as the main receptor for TDIF. The axillary bud emergence of tdr mutants was little affected by either of the peptides. It was confirmed by scanning electron microscopy that peptide-treated wild-type plants form an axillary meristem-like structure earlier than non-treated plants. SHOOT MERISTEMLESS (STM), a marker gene for meristems, was up-regulated in peptide-treated plants before the axillary meristem becomes morphologically distinguishable. These results indicate that CLE42 peptide and TDIF have an activity to enhance axillary bud formation via the TDR. Judging from its expression pattern, CLE42 may play an important role in the regulation of secondary shoot development.     

thick tassel dwarf1 encodes a putative maize ortholog of the Arabidopsis CLAVATA1 leucine-rich repeat receptor-like kinase

Development in higher plants depends on the activity of meristems, formative regions that continuously initiate new organs at their flanks. Meristems must maintain a balance between stem cell renewal and organ initiation. In fasciated mutants, organ initiation fails to keep pace with meristem proliferation. The thick tassel dwarf1 (td1) mutation of maize affects both male and female inflorescence development. The female inflorescence, which results in the ear, is fasciated, with extra rows of kernels. The male inflorescence, or tassel, shows an increase in spikelet density. Floral meristems are also affected in td1 mutants; for example, male florets have an increase in stamen number. These results suggest that td1 functions in the inflorescence to limit meristem size. In addition, td1 mutants are slightly shorter than normal siblings, indicating that td1 also plays a role in vegetative development. td1 encodes a leucine-rich repeat receptor-like kinase (LRR-RLK) that is a putative ortholog of the Arabidopsis CLAVATA1 protein. These results complement previous work showing that fasciated ear2 encodes a CLAVATA2-like protein, and suggest that the CLAVATA signaling pathway is conserved in monocots. td1 maps in the vicinity of quantitative trait loci that affect seed row number, spikelet density and plant height. We discuss the possible selection pressures on td1 during maize domestication.     

Herbivory could unlock mutations sequestered in stratified shoot apices of genetic mosaics

Many higher plants have shoot apical meristems that possess discrete cell layers, only one of which normally gives rise to gametes following the transition from vegetative meristem to floral meristem. Consequently, when mutations occur in the meristems of sexually reproducing plants, they may or may not have an evolutionary impact, depending on the apical layer in which they reside. In order to determine whether developmentally sequestered mutations could be released by herbivory (i.e., meristem destruction), a characterized genetic mosaic was subjected to simulated herbivory. Many plants develop two shoot meristems in the leaf axils of some nodes, here referred to as the primary and secondary axillary meristems. Destruction of the terminal and primary axillary meristems led to the outgrowth of secondary axillary meristems. Seed derived from secondary axillary meristems was not always descended from the second apical cell layer of the terminal shoot meristem as is expected for terminal and primary shoot meristems. Vegetative and reproductive analysis indicated that secondary meristems did not maintain the same order of cell layers present in the terminal shoot meristem. In secondary meristems reproductively sequestered cell layers possessing mutant cells can be repositioned into gamete-forming cell layers, thereby adding mutant genes into the gene pool. Herbivores feeding on shoot tips may influence plant evolution by causing the outgrowth of secondary axillary meristems.