Plant stem cell research is uncovering the secrets of longevity and persistent growth

Plant stem cells have several extraordinary features: they are generated de novo during development and regeneration, maintain their pluripotency, and produce another stem cell niche in an orderly manner. This enables plants to survive for an extended period and to continuously make new organs, representing a clear difference in their developmental program from animals. To uncover regulatory principles governing plant stem cell characteristics, our research project ‘Principles of pluripotent stem cells underlying plant vitality’ was launched in 2017, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Japanese government. Through a collaboration involving 28 research groups, we aim to identify key factors that trigger epigenetic reprogramming and global changes in gene networks, and thereby contribute to stem cell generation. Pluripotent stem cells in the shoot apical meristem are controlled by cytokinin and auxin, which also play a crucial role in terminating stem cell activity in the floral meristem; therefore, we are focusing on biosynthesis, metabolism, transport, perception, and signaling of these hormones. Besides, we are uncovering the mechanisms of asymmetric cell division and of stem cell death and replenishment under DNA stress, which will illuminate plant-specific features in preserving stemness. Our technology support groups expand single-cell omics to describe stem cell behavior in a spatiotemporal context, and provide correlative light and electron microscopic technology to enable live imaging of cell and subcellular dynamics at high spatiotemporal resolution. In this perspective, we discuss future directions of our ongoing projects and related research fields.     

MAPKKK-Related protein kinase NPK1: Regulation of the M phase of plant cell cycle

The tobaccoNPK1 gene encodes a homolog of mitogenactivated protein kinase kinase kinases. We have recently identified tobacco kinesin-like proteins (NACK1/2) as activators for NPK1. Immunochemical analyses of NPK1 and NACK1 proteins suggest that NPK1 is involved in the regulation of some process in the M phase of the plant cell cycle. The extended abstract of a paper presented at the 13th International Symposium in Conjugation with Award of the International Prize for Biology “Frontier of Plant Biology”     

New insights into the phylogenetic distribution and evolutionary origins of the septins

Until recently, it had appeared that the septin family of proteins was restricted to the opisthokont eukaryotes (the fungi and animals and their close relatives the microsporidia and choanoflagellates). It has now become apparent that septins are also present in several other widely divergent eukaryotic lineages (chlorophyte algae, brown algae, and ciliates). This distribution and the details of the non-opisthokont septin sequences appear to require major revisions to hypotheses about the origins and early evolution of the septins.     

Regulation of mitochondrial morphology and cell survival by Mitogenin I and mitochondrial single-stranded DNA binding protein

We found that a mouse homolog of human DNA polymerase delta interacting protein 38, referred to as Mitogenin I in this paper, and mitochondrial single-stranded DNA-binding protein (mtSSB), identified as upregulated genes in the heart of mice with juvenile visceral steatosis, play a role in the regulation of mitochondrial morphology. We demonstrated that overexpression of Mitogenin I or mtSSB increased elongated or fragmented mitochondria in mouse C2C12 myoblast cells, respectively. On the other hand, the silencing of Mitogenin I or mtSSB by RNA interference led to an increase in fragmented or elongated mitochondria in the cells, respectively, suggesting that Mitogenin I and mtSSB are involved in the processes of mitochondrial fusion and fission, respectively. In addition, we showed that the silencing of Mitogenin I resulted in an increase in the number of trypan blue-positive cells and the silencing of mtSSB resulted in an enhancement of the sensitivity of the cells to apoptotic stimulation by etoposide. The present results demonstrated that these proteins play a role in cell survival.     

Plant Homologues of Components of MAPK (Mitogen-Activated Protein Kinase) Signal Pathways in Yeast and Animal Cells

As they respond to numerous extracellular and intracellularstimuli, plants develop various morphological features and thecapacity for a large variety of physiological processes duringtheir growth. If we are to understand the molecular basis ofsuch developments, we must elucidate the way in which signalsgenerated by such stimuli can be transduced into plant cellsand transmitted by cellular components to induce the appropriateterminal events. In yeast and animal systems, signal pathwaysthat are known collectively as MAPK (mitogen-activated proteinkinase) cascades have been shown to play a central role in thetransmission of various signals. The components of these pathwaysinclude the MAPK family, the activator kinases of the MAPK family(the MAPKK family) and the activator kinases of the MAPKK family(the MAPKKK family). The members of each respective family arestructurally conserved and signals are transmitted by similarphosphotransfer reactions at corresponding steps that are mediatedby a specific member of each family in turn. Both cDNAs andgenes that encode putative homologues of these components haverecently been isolated from plant sources. Some of them havebeen shown to be related not only structurally but also functionallyto members of the MAPK cascades of other organisms. These findingssuggest that plants have signal pathways that are analogousto the MAPK cascades in yeast and animal cells but it remainsto be proven that plant homologues do in fact constitute kinasecascades. Given the presence of so many homologues of MAPKsand MAPKKKs in a single plant species, namely, Arabidopsis thaliana,we can be fairly confident that the putative MAPK cascades areinvolved in various physiological processes in plants. (Received March 28, 1995; )     

Progress in Studies of Plant Homologs of Mitogen-Activated Protein (MAP) Kinase and Potential Upstream Components in Kinase Cascades

Mitogen-activated protein (MAP) kinase cascades were originally identified as protein phosphorylation systems that control the division and the growth of yeast and animal cells. Such cascades consist of MAP kinases, MAP-kinase kinases, and MAP-kinase-kinase kinases. In addition, these organisms have been also shown to have structurally related but functionally different MAP kinase cascades, which are involved in various cellular processes such as a response to osmotic stress and apoptosis. Plants also have been shown to have a number of members of each kinase family. Although physiological and genetic functions of most plant members have yet to be established, some of members have been shown to be responsible for the cellular transmission of signals generated by wounding or a mechanical stress, which predicts that MAP kinase cascades may function in a variety of physiological processes in the plant cells. In the present review, we summarize recent progresses of researches on plant members of each kinase family as well as those of analyses of the cascades in other organisms.