Quantitative Imaging Reveals Distinct Contributions of SnRK2 and ABI3 in Plasmodesmatal Permeability in Physcomitrella patens

Abscisic acid signaling regulates primary plasmodesmata density for plant cell-to-cell communication

Cell-to-cell communication is essential for multicellular organisms. Plasmodesmata (PD) are plant-specific nanopore structures pivotal for cell-to-cell communication and plant survival. However, how PD form and their structure, regulation, and evolution remain largely unknown. Here, we demonstrate that the exogenous supply of abscisic acid (ABA), a well-conserved phytohormone in land plants, reduces primary PD density in the moss Physcomitrium patens. This regulation requires all core components of the ABA signaling pathway. Furthermore, we reveal that ABA-INSENSITIVE 5, a well-conserved transcription factor in the ABA signaling pathway of land plants, plays a pivotal role in PD density regulation, whereas ABA-INSENSITIVE 3 does not. Our findings show that the ABA-induced reduction in primary PD density is mediated by these ABA-responsive factors in P. patens. Considering previous reports on ABA-dependent PD regulation in both moss and angiosperms, we propose that the ABA-mediated control of PD biogenesis and permeability represents a conserved mechanism in land plants, with critical implications for cell-to-cell communication and stress adaptation.

Loss-of-functional mutation in ANGUSTIFOLIA3 causes leucine hypersensitivity and hypoxia response during Arabidopsis thaliana seedling growth

INTRODUCTION

The ANGUSTIFOLIA3 (AN3) gene encodes a transcriptional co-activator for cell proliferation in Arabidopsis thaliana leaves. We previously showed that Physcomitrium patens AN3 orthologs promote gametophore shoot formation through arginine metabolism.

OBJECTIVES

We analyzed the role of AN3 in Arabidopsis thaliana to understand how seedling growth is regulated by metabolic and physiological modulations.

METHODS

We first explored amino acids that affect the seedling growth of an3 mutants. Transcriptome and metabolome analyses were conducted to elucidate the metabolic and physiological roles of AN3 during seedling growth. Lastly, we examined the distribution of reactive oxygen species to corroborate our omics-based findings.

RESULTS

Our results indicated that an3 mutants were unable to establish seedlings when grown with leucine, but not arginine. Multi-omics analyses suggested that an3 mutants exhibit a hypoxia-like response. Abnormal oxidative status was confirmed by detecting an altered distribution of reactive oxygen species in the roots of an3 mutants.

CONCLUSION

AN3 helps maintain the leucine metabolism and oxidative balance during seedling growth in Arabidopsis thaliana. Future research is necessary to explore the interaction between these processes.

Infrared laser-induced gene expression in single cells characterized by quantitative imaging in Physcomitrium patens

A spatiotemporal understanding of gene function requires the precise control of gene expression in each cell. Here, we use an infrared laser-evoked gene operator (IR-LEGO) system to induce gene expression at the single-cell level in the moss Physcomitrium patens by heating a living cell with an IR laser and thereby activating the heat shock response. We identify the laser irradiation conditions that provide higher inducibility with lower invasiveness by changing the laser power and irradiation duration. Furthermore, we quantitatively characterize the induction profile of the heat shock response using a heat-induced fluorescence reporter system after the IR laser irradiation of single cells under different conditions. Our data indicate that IR laser irradiation with long duration leads to higher inducibility according to increase in the laser power but not vice versa, and that the higher laser power even without conferring apparent damage to the cells decelerates and/or delayed gene induction. We define the temporal shift in expression as a function of onset and duration according to laser power and irradiation duration. This study contributes to the versatile application of IR-LEGO in plants and improves our understanding of heat shock-induced gene expression.

Enhancement of salt tolerance of alfalfa: Physiological and molecular responses of transgenic alfalfa plants expressing Syntrichia caninervis-derived ScABI3

Alfalfa (Medicago sativa L.), a perennial forage plant, is a rich source of nutrients such as vitamins, minerals, and proteins. Salt stress, however, impedes its growth. The plant-specific transcription factor abscisic acid insensitive 3 (ABI3) has a critical contribution to the control of abscisic acid (ABA) signaling pathway and abiotic stress response. The gene ScABI3 from Syntrichia caninervis, a moss species tolerant to desiccation, could be considered a potential candidate gene to modify alfalfa's nutritional and growth aspects. However, it remains unclear how ScABI3 affects the salt stress response of transgenic alfalfa. Therefore, we elucidated the role and molecular mechanism of ScABI3 from S. caninervis as an ABA signaling factor in transgenic alfalfa. Our findings demonstrate that ScABI3 overexpression in transgenic alfalfa improves salt tolerance by promoting relative water content, antioxidant enzyme activity, and photosynthetic parameters. Furthermore, the key genes of plant hormone signaling and the classical salt tolerance pathway were activated in ScABI3 transgenic lines under salt stress. Based on these results, ScABI3 could be considered a potentially critical candidate gene to alleviate salt stress in alfalfa. The present study provides valuable insights for developing transgenic crop breeding strategies for saline-alkaline soils.

The ABI3 Transcription Factor Interaction and Antagonism with Ubiquitin E3 Ligase ScPRT1 in Syntrichia caninervis

The ubiquitination pathway has been found to regulate plant responses to environmental stress. However, the role of E3 ubiquitin ligase in desiccation tolerant moss has not yet been elucidated. Previous research has shown that the abscisic acid (ABA) signaling factor ScABI3 can significantly increase desiccation tolerance and reduce ABA sensitivity in the desert moss Syntrichia caninervis. In this study, we identified a RING-type E3 ubiquitin ligase, ScPRT1, and showed that ScABI3 can directly interact with ScPRT1 in vitro and in vivo. Furthermore, we found that the high expression of ScPRT1 can interfere with the transcription of ScABI3 under ABA treatment. Therefore, we speculate that ScPRT1 may degrade ScABI3 through the ubiquitin-26S proteasome system and participate in ABA-dependent signaling in response to ABA-insensitivity or desiccation tolerance in S. caninervis. The findings from our study may enrich our knowledge of the role of E3 ubiquitin ligase in desiccation tolerance and lay a theoretical foundation for an in-depth study of the relationship between ubiquitination modification and ABA signal transduction under environmental stress.

Tracking Intercellular Movement of Fluorescent Proteins in Bryophytes

An important approach to investigate intercellular connectivity via plasmodesmata is to visualize and track the movement of fluorescent proteins between cells. The intercellular connectivity is largely controlled by the size exclusion limit of the pores. Over the past few decades, the technique to observe and analyze intercellular movement of a fluorescent protein has been developed mainly in angiosperms such as Arabidopsis thaliana. We recently applied the corresponding system to track the intercellular movement of the fluorescent protein Dendra2 in the moss Physcomitrium (Physcomitrella) patens. The protonemal tissues are particularly suited for observation of the intercellular movement due to the simple organization. Here, we describe a protocol suitable for the analysis of Dendra2 movement between cells in P. patens.

Callose Detection and Quantification at Plasmodesmata in Bryophytes

In bryophytes (i.e., mosses, liverworts, and hornworts), extant representatives of early land plants, plasmodesmata have been described in a wide range of tissues. Although their contribution to bryophyte morphogenesis remains largely unexplored, several recent studies have suggested that the deposition of callose around plasmodesmata might regulate developmental and physiological responses in mosses. In this chapter, we provide a protocol to image and quantify callose levels in the filamentous body of the model moss Physcomitrium (Physcomitrella) patens and discuss possible alternatives and pitfalls. More generally, this protocol establishes a framework to explore the distribution of callose in other bryophytes.

Phytohormone biosynthesis and signaling pathways of mosses

Most known phytohormones regulate moss development. We present a comprehensive view of the synthesis and signaling pathways for the most investigated of these compounds in mosses, focusing on the model Physcomitrium patens. The last 50 years of research have shown that most of the known phytohormones are synthesized by the model moss Physcomitrium patens (formerly Physcomitrella patens) and regulate its development, in interaction with responses to biotic and abiotic stresses. Biosynthesis and signaling pathways are best described in P. patens for the three classical hormones auxins, cytokinins and abscisic acid. Furthermore, their roles in almost all steps of development, from early filament growth to gametophore development and sexual reproduction, have been the focus of much research effort over the years. Evidence of hormonal roles exist for ethylene and for CLE signaling peptides, as well as for salicylic acid, although their possible effects on development remain unclear. Production of brassinosteroids by P. patens is still debated, and modes of action for these compounds are even less known. Gibberellin biosynthesis and signaling may have been lost in P. patens, while gibberellin precursors such as ent-kaurene derivatives could be used as signals in a yet to discover pathway. As for jasmonic acid, it is not used per se as a hormone in P. patens, but its precursor OPDA appears to play a corresponding role in defense against abiotic stress. We have tried to gather a comprehensive view of the biosynthesis and signaling pathways for all these compounds in mosses, without forgetting strigolactones, the last class of plant hormones to be reported. Study of the strigolactone response in P. patens points to a novel signaling compound, the KAI2-ligand, which was likely employed as a hormone prior to land plant emergence.

Metabolic Control of Gametophore Shoot Formation through Arginine in the Moss Physcomitrium patens

Shoot formation is accompanied by active cell proliferation and expansion, requiring that metabolic state adapts to developmental control. Despite the importance of such metabolic reprogramming, it remains unclear how development and metabolism are integrated. Here, we show that disruption of ANGUSTIFOLIA3 orthologs (PpAN3s) compromises gametophore shoot formation in the moss Physcomitrium patens due to defective cell proliferation and expansion. Trans-omics analysis reveals that the downstream activity of PpAN3 is linked to arginine metabolism. Elevating arginine level by chemical treatment leads to stunted gametophores and causes Ppan3 mutant-like transcriptional changes in the wild-type plant. Furthermore, ectopic expression of AtAN3 from Arabidopsis thaliana ameliorates the defective arginine metabolism and promotes gametophore formation in Ppan3 mutants. Together, these findings indicate that arginine metabolism is a key pathway associated with gametophore formation and provide evolutionary insights into the establishment of the shoot system in land plants through the integration of developmental and metabolic processes.

The evolving role of abscisic acid in cell function and plant development over geological time

Abscisic acid (ABA) is found in a wide diversity of organisms, yet we know most about the hormonal action of this compound in the ecologically dominant and economically important angiosperms. In angiosperms, ABA regulates a suite of critical responses from desiccation tolerance through to seed dormancy and stomatal closure. Work exploring the function of key genes in the ABA signalling pathway of angiosperms has revealed that this signal transduction pathway is ancient, yet considerable change in the physiological roles of this hormone have occurred over geological time. With recent advances in our capacity to characterise gene function in non-angiosperms we are on the cusp of revealing the origins of this critical hormonal signalling pathway in plants, and understanding how a simple hormone may have shaped land plant diversity, ecology and adaptation over the past 500 million years.