Genetic and Phenotypic Analysis of Shoot Apical and Floral Meristem Development.

The shoot apical and floral meristems (SAM and FM, respectively) of Arabidopsis thaliana contain reservoirs of self-renewing stem cells that function as sources of progenitor cells for organ formation during development. The primary SAM produces all the aerial structures of the adult plant, while the FMs generate the four types of floral organs. Consequently, aberrant SAM and FM activity can profoundly affect vegetative and reproductive plant morphology. The embedded location and small size of Arabidopsis meristems make accessing these structures difficult, so specialized techniques have been developed to facilitate their analysis. Microscopic, histological, and molecular techniques provide both qualitative and quantitative data on meristem organization and function, which are crucial for the normal growth and development of the entire plant.

Interactive CLV3, CLE16 and CLE17 signaling mediates stem cell homeostasis in the Arabidopsis shoot apical meristem.

The ability of plants to grow and form organs throughout their lifetime is dependent on their sustained stem cell activity. These stem cell populations are maintained by intricate networks of intercellular signaling pathways. In Arabidopsis thaliana, the small secreted peptide CLAVATA3 (CLV3) controls shoot apical meristem (SAM) maintenance by activating a signal transduction pathway that modulates the expression of the homeodomain transcription factor WUSCHEL (WUS). Here, we demonstrate that two CLV3-related peptides, CLE16 and CLE17, restrict stem cell accumulation in the absence of CLV3. CLE16 and CLE17 contribute independently to SAM maintenance and organ production in clv3 plants at all stages of development. We show that CLE16 and CLE17 signal through a subset of CLV3 receptors, the BARELY ANY MERISTEM (BAM) receptor kinases, and act upstream of WUS. Our study reveals that CLE16 and CLE17 function in a mechanism that partially compensates for CLV3 to maintain stem cell homeostasis and plant resiliency, and expands the potential targets for enhancing yield traits in crop species.

A reevaluation of the role of the ASIL trihelix transcription factors as repressors of the seed maturation program.

Developmental transitions are typically tightly controlled at the transcriptional level. Two of these transitions involve the induction of the embryo maturation program midway through seed development and its repression during the vegetative phase of plant growth. Very little is known about the factors responsible for this regulation during early embryogenesis, and only a couple of transcription factors have been characterized as repressors during the postgerminative phase. Arabidopsis 6b-INTERACTING PROTEIN-LIKE1 (ASIL1), a trihelix transcription factor, has been proposed to repress maturation both embryonically and postembryonically. Preliminary data also suggested that its closest paralog, ASIL2, might play a role as well. We used a transcriptomic approach, coupled with phenotypical observations, to test the hypothesis that ASIL1 and ASIL2 redundantly turn off maturation during both phases of growth. Our results indicate that, contrary to what was previously published, neither of the ASIL genes plays a role in the regulation of maturation, at any point during plant development. Analyses of gene ontology (GO)-enriched terms and published transcriptomic datasets suggest that these genes might be involved in responses during the vegetative phase to certain biotic and abiotic stresses.

The signaling peptide-encoding genes CLE16, CLE17 and CLE27 are dispensable for Arabidopsis shoot apical meristem activity.

The shoot apical meristem produces all of the leaves, stems and flowers of a flowering plant from a reservoir of stem cells at its growing tip. In Arabidopsis, the small polypeptide signaling molecule CLAVATA3 (CLV3), a member of the CLV3/EMBRYO SURROUNDING REGION-RELATED (CLE) gene family, is a key component of a negative feedback loop that maintains stem cell activity in shoot and floral meristems throughout development. Because in some plant species multiple CLE genes are involved in regulating shoot apical meristem activity, we tested the hypothesis that CLE genes other than CLV3 might function in stem cell homeostasis in Arabidopsis. We identified three Arabidopsis CLE genes expressed in the post-embryonic shoot apical meristem, generated loss-of-function alleles using genome editing, and analyzed the meristem phenotypes of the resulting mutant plants. We found that null mutations in CLE16, CLE17 or CLE27 affected neither vegetative nor reproductive shoot meristem activity under normal growth conditions, although CLE27 appears to slightly prolong vegetative growth. Our results indicate that the CLE16, CLE17 and CLE27 genes have largely redundant roles in the Arabidopsis shoot apical meristem and/or regulate meristem activity only under specific environmental conditions.

Peptide signaling molecules CLE5 and CLE6 affect Arabidopsis leaf shape downstream of leaf patterning transcription factors and auxin.

Intercellular signaling mediated by small peptides is critical to coordinate organ formation in animals, but whether extracellular polypeptides play similar roles in plants is unknown. Here we describe a role in Arabidopsis leaf development for two members of the CLAVATA3/ESR-RELATED peptide family, CLE5 and CLE6, which lie adjacent to each other on chromosome 2. Uniquely among the CLE genes, CLE5 and CLE6 are expressed specifically at the base of developing leaves and floral organs, adjacent to the boundary with the shoot apical meristem. During vegetative development CLE5 and CLE6 transcription is regulated by the leaf patterning transcription factors BLADE-ON-PETIOLE1 (BOP1) and ASYMMETRIC LEAVES2 (AS2), as well as by the WUSCHEL-RELATED HOMEOBOX (WOX) transcription factors WOX1 and PRESSED FLOWER (PRS). Moreover, CLE5 and CLE6 transcript levels are differentially regulated in various genetic backgrounds by the phytohormone auxin. Analysis of loss-of-function mutations generated by genome engineering reveals that CLE5 and CLE6 independently and together have subtle effects on rosette leaf shape. Our study indicates that the CLE5 and CLE6 peptides function downstream of leaf patterning factors and phytohormones to modulate the final leaf morphology.