Stem Cell Basis of Shoot Branching

During their postembryonic development, plants continuously form branches to conquer more space and adapt to changing environments. In seed plants, this is achieved by lateral branching, in which axillary meristems (AMs) initiate at the leaf axils to form axillary buds. The developmental potential of AMs to form shoot branches is the same as that of embryonic shoot apical meristems (SAMs). Recent studies in Arabidopsis thaliana have revealed the cellular origin of AMs and have identified transcription factors and phytohormones that regulate sequential steps leading to AM initiation. In particular, a group of meristematic cells detached from the SAM are key to AM initiation, which constitutes an excellent system for understanding stem cell fate and de novo meristem formation.

Influence of different types of explants in chickpea regeneration using thidiazuron seed-priming

A comparative regeneration of three types of explants prepared from axillary meristems, plumular apices and hypocotyls of chickpea (Cicer arietinum) was carried out using four thidiazuron (TDZ) treatment methods. The first and third ones included the short-term 20 μM TDZ pre-treatment for all three explant types followed by non-supplementation or supplementation of TDZ (4 μM) into the shoot induction medium (SIM), while the second and fourth ones lacked TDZ pre-treatment followed by non-addition or addition of 4 μM TDZ in the SIM. Axillary meristem explants produced the best results with seed pre-treatment using 20 μM TDZ without TDZ in SIM and showed the highest rate of regeneration efficiency (71.33 ± 1.5%) after 20 days. Concurrently, plumular apex explants from TDZ-primed seeds was ranked second, exhibiting a regeneration percentage of 54.33 ± 2.3% in SIM without supplementation of TDZ, whereas explants from hypocotyls generated from seeds subjected to any of the TDZ treatments were not regenerated on any SIMs after 20 days.

Developmental genetics of maize vegetative shoot architecture

More than 1.1 billion tonnes of maize grain were harvested across 197 million hectares in 2019 (FAOSTAT 2020). The vast global productivity of maize is largely driven by denser planting practices, higher yield potential per area of land, and increased yield potential per plant. Shoot architecture, the three-dimensional structural arrangement of the above-ground plant body, is critical to maize grain yield and biomass. Structure of the shoot is integral to all aspects of modern agronomic practices. Here, the developmental genetics of the maize vegetative shoot is reviewed. Plant architecture is ultimately determined by meristem activity, developmental patterning, and growth. The following topics are discussed: shoot apical meristem, leaf architecture, axillary meristem and shoot branching, and intercalary meristem and stem activity. Where possible, classical and current studies in maize developmental genetics, as well as recent advances leveraged by "-omics" analyses, are highlighted within these sections.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-021-01208-1.

The AtMYB2 inhibits the formation of axillary meristem in Arabidopsis by repressing RAX1 gene under environmental stresses

AtMYB2 protein represses the formation of axillary meristems in response to environmental stresses so that plants can undergo a shorter vegetative development stage under environmental stresses. Shoot branching is an important event determined by endogenous factors during the development of plants. The formation of axillary meristem is also significantly repressed by environmental stresses and the underlying mechanism is largely unknown. The REGULATOR OF AXILLARY MERISTEMS (RAX) genes encode the R2R3 MYB transcription factors that have been shown to regulate the formation of axillary meristems in Arabidopsis. The AtMYB2 is also a member of R2R3 MYB gene family whose expression is usually induced by the environmental stresses. In this study, our results showed that AtMYB2 protein plays a pivotal negative regulatory role in the formation of axillary meristem. AtMYB2 is mainly expressed in the leaf axils as that of RAX1. The environmental stresses can increase the expression of AtMYB2 protein which further inhibits the expression of RAX1 gene by binding to its promoter. Therefore, AtMYB2 protein represses the formation of axillary meristems in response to environmental stresses so that plants can undergo a shorter vegetative development stage under environmental stresses.

Live Imaging of Arabidopsis Axillary Meristems

Axillary meristems (AMs) are established postembryonically at the leaf axils and can develop into lateral branches. The initiation of AMs establishes new growth axis and is of primary importance for understanding plant development. Understanding plant development requires live imaging of morphogenesis and gene expression. However, AMs are embedded in the leaf axil, making it challenging to perform live imaging. In this chapter, we describe how to prepare and culture Arabidopsis thaliana leaves in vitro, to perform one-time or time-lapse imaging of AM initiation with a confocal microscope.

Control of cell fate during axillary meristem initiation

Axillary meristems (AMs) are located in the leaf axil and can establish new growth axes. Whereas their neighboring cells are differentiated, the undifferentiated cells in the AM endow the AM with the same developmental potential as the shoot apical meristem. The AM is, therefore, an excellent system to study stem cell fate maintenance in plants. In this review, we summarize the current knowledge of AM initiation. Recent findings have shown that AMs derive from a stem cell lineage that is maintained in the leaf axil. This review covers AM progenitor cell fate maintenance, reactivation, and meristem establishment. We also highlight recent work that links transcription factors, phytohormones, and epigenetic regulation to AM initiation.

Spatiotemporal control of axillary meristem formation by interacting transcriptional regulators

Branching is a common feature of plant development. In seed plants, axillary meristems (AMs) initiate in leaf axils to enable lateral shoot branching. AM initiation requires a high level of expression of the meristem marker SHOOT MERISTEMLESS (STM) in the leaf axil. Here, we show that modules of interacting transcriptional regulators control STM expression and AM initiation. Two redundant AP2-type transcription factors, DORNRÖSCHEN (DRN) and DORNRÖSCHEN-LIKE (DRNL), control AM initiation by regulating STM expression. DRN and DRNL directly upregulate STM expression in leaf axil meristematic cells, as does another transcription factor, REVOLUTA (REV). The activation of STM expression by DRN/DRNL depends on REV, and vice versa. DRN/DRNL and REV have overlapping expression patterns and protein interactions in the leaf axil, which are required for the upregulation of STM expression. Furthermore, LITTLE ZIPPER3, another REV-interacting protein, is expressed in the leaf axil and interferes with the DRN/DRNL-REV interaction to negatively modulate STM expression. Our results support a model in which interacting transcriptional regulators fine-tune the expression of STM to precisely regulate AM initiation. Thus, shoot branching recruits the same conserved protein complexes used in embryogenesis and leaf polarity patterning.

Summary: Shoot branching uses interacting transcriptional regulators to fine-tune the spatiotemporal expression of STM and, thus, to precisely regulate axillary meristem initiation.

Flowering pathway is regulated by bulb size in Lilium longiflorum (Easter lily)

Lilium longiflorum (Easter lily) vegetative propagation occurs through production of underground bulbs containing apical and axillary meristems. In addition, sexual reproduction is achieved by flowering of elongated shoots above the bulb. It is generally accepted that L. longiflorum has an obligatory requirement for vernalisation and that long day (LD) regime hastens flowering. However, the effect of bulb size and origin, with respect to axillary or apical meristems on flowering, as well as the interactions between these meristems are largely unknown. The aim of this study was to explore the effect of bulb size, vernalisation and photoperiod on L. longiflorum flowering. To this end, we applied vernalisation and photoperiod treatments to the different bulb sizes and used a system of constant ambient temperature of 25 °C, above vernalisation spectrum, to avoid cold-dependent floral induction during plant growth. Vernalisation and LD hasten flowering in all bulbs. Large, non-vernalised bulbs invariably remained at a vegetative stage. However, small non-vernalised bulbs flowered under LD conditions. These results demonstrate for the first time that cold exposure is not an obligatory prerequisite for L. longiflorum flowering, and that an alternative flowering pathway can bypass vernalisation in small bulbs. We suggest that apical dominance interactions determine the distinct flowering pathways of the apical and axillary meristems. Similar floral induction is achieved in propagated bulblets from scaling. These innovative findings in the field of geophyte floral induction represent valuable applicative knowledge for lily production.