Cell cycle reentry from the late S phase: implications from stem cell formation in the moss Physcomitrella patens

Plant regeneration in the new era: from molecular mechanisms to biotechnology applications

Plants or tissues can be regenerated through various pathways. Like animal regeneration, cell totipotency and pluripotency are the molecular basis of plant regeneration. Detailed systematic studies on Arabidopsis thaliana gradually unravel the fundamental mechanisms and principles underlying plant regeneration. Specifically, plant hormones, cell division, epigenetic remodeling, and transcription factors play crucial roles in reprogramming somatic cells and reestablishing meristematic cells. Recent research on basal non-vascular plants and monocot crops has revealed that plant regeneration differs among species, with various plant species using distinct mechanisms and displaying significant differences in regenerative capacity. Conducting multi-omics studies at the single-cell level, tracking plant regeneration processes in real-time, and deciphering the natural variation in regenerative capacity will ultimately help understand the essence of plant regeneration, improve crop regeneration efficiency, and contribute to future crop design.

Evolution of wound-activated regeneration pathways in the plant kingdom

Regeneration serves as a self-protective mechanism that allows a tissue or organ to recover its entire form and function after suffering damage. However, the regenerative capacity varies greatly within the plant kingdom. Primitive plants frequently display an amazing regenerative ability as they have developed a complex system and strategy for long-term survival under extreme stress conditions. The regenerative ability of dicot species is highly variable, but that of monocots often exhibits extreme recalcitrance to tissue replenishment. Recent studies have revealed key factors and signals that affect cell fate during plant regeneration, some of which are conserved among the plant lineage. Among these, several members of the ETHYLENE RESPONSE FACTOR (ERF) transcription factors have been implicated in wound signaling, playing crucial roles in the regenerative mechanisms after different types of wounding. An understanding of plant regeneration may ultimately lead to an increased regenerative potential of recalcitrant species, producing more high-yielding, multi-resistant and environmentally friendly crops and ensuring the long-term development of global agriculture.

Molecular mechanisms of reprogramming of differentiated cells into stem cells in the moss Physcomitrium patens

Plant and animal stem cells can self-renew and give rise to differentiated cells to form tissues or organs. Unlike differentiated cells in animals, those in land plants can be readily reprogrammed into stem cells, reflecting the plasticity of plant cell identity. The moss Physcomitrium patens (synonym: Physcomitrella patens) is highly regenerable, and its leaf cells can be reprogrammed into stem cells in response to wounding or by transient DNA damage without wounding. Wounding and DNA damage induce STEM CELL-INDUCING FACTOR 1, an APETALA2/ETHYLENE RESPONSE FACTOR. Here, we summarize the genetic networks that regulate cellular reprogramming in P. patens and the roles of STEMIN1 and discuss the generality and divergence of the molecular mechanisms underlying cellular reprogramming in land plants and animals.

The Role of Histone Acetyltransferases and Histone Deacetylases in Photoreceptor Differentiation and Degeneration

Photoreceptors are critical components of the retina and play a role in the first step of the conversion of light to electrical signals. The differentiation and degeneration of photoreceptors are regulated by specific genes and proteins. With the development of epigenetic approaches, scientists have discovered that histone modifications, such as acetylation, methylation, ubiquitylation, and phosphorylation, may modulate the processes of photoreceptor differentiation and degeneration. Histone acetylation is regulated by two opposing classes of enzymes, namely, histone acetyltransferases (HATs) and histone deacetylases (HDACs), which add and remove acetyl groups to and from target histones, respectively, causing changes in transcriptional activity. Herein, we review the effects of HATs and HDACs on the differentiation and degeneration of photoreceptors and discuss the underlying mechanisms of these effects.

Cell-specific transcriptomic analyses of three-dimensional shoot development in the moss Physcomitrella patens

Haploid moss gametophytes harbor distinct stem cell types, including tip cells that divide in single planes to generate filamentous protonemata, and bud cells that divide in three planes to yield axial gametophore shoots. This transition from filamentous to triplanar growth occurs progressively during the moss life cycle, and is thought to mirror evolution of the first terrestrial plants from Charophycean green algal ancestors. The innovation of morphologically complex plant body plans facilitated colonization of the vertical landscape, and enabled development of complex vegetative and reproductive plant morphologies. Despite its profound evolutionary significance, the molecular programs involved in this transition from filamentous to triplanar meristematic plant growth are poorly understood. In this study, we used single-cell type transcriptomics to identify more than 4000 differentially expressed genes that distinguish uniplanar protonematal tip cells from multiplanar gametophore bud cells in the moss Physcomitrella patens. While the transcriptomes of both tip and bud cells show molecular signatures of proliferative cells, the bud cell transcriptome exhibits a wider variety of genes with significantly increased transcript abundances. Our data suggest that combined expression of genes involved in shoot patterning and asymmetric cell division accompanies the transition from uniplanar to triplanar meristematic growth in moss.

Phytochrome-mediated regulation of cell division and growth during regeneration and sporeling development in the liverwort Marchantia polymorpha

Light regulates various aspects of development throughout the life cycle of sessile land plants. Photoreceptors, such as the red (R) and far-red (FR) light receptors phytochromes, play pivotal roles in modulating developmental programs. Reflecting high developmental plasticity, plants can regenerate tissues, organs, and whole bodies from varieties of cells. Among land plants, bryophytes exhibit extraordinary competency of regeneration under hormone-free conditions. As an environmental factor, light plays critical roles in regeneration of bryophytes. However, how light regulates regeneration remains unknown. Here we show that using the liverwort Marchantia polymorpha, which contains a single phytochrome gene, the phytochrome regulates re-entry into the cell cycle and cell shape in newly regenerating tissues. Our morphological and cytological observations revealed that S-phase entry of G1-arrested epidermal cells around the midrib on the ventral surface of thallus explants was greatly retarded in the dark or under phytochrome-inactive R/FR cycle irradiation conditions, where, nevertheless, small, laterally narrow regenerants were eventually formed. Thus, consistent with earlier descriptions published over a century ago, light is not essential for, but exerts profound effects on regeneration in M. polymorpha. Ventral cells in regenerants grown under R/FR cycle conditions were longer and narrower than those under R cycle. Expression of a constitutively active mutant of M. polymorpha phytochrome allowed regeneration of well grown, widely expanded thalli even in the dark when sugar was supplied, further demonstrating that the phytochrome signal promotes cell proliferation, which is rate-limited by sucrose availability. Similar effects of R and FR irradiation on cell division and elongation were observed in sporelings as well. Thus, besides activation of photosynthesis, major roles of R in regeneration of M. polymorpha are to facilitate proliferation of rounder cells through the phytochrome by mechanisms that are likely to operate in the sporeling.