Monitoring circadian rhythms of individual cells in plants

An endogenous basis for synchronisation characteristics of the circadian rhythm in proliferating Lemna minor plants

The circadian clock is a cell-autonomous system that functions through the coordination of time information in the plant body. Synchronisation of cellular clocks is based on coordination mechanisms; the synchronisation characteristics of proliferating plants remain unclear. The bioluminescence circadian rhythms of fronds (leaf-like plant units) of proliferating Lemna minor plants carrying a circadian bioluminescence reporter, AtCCA1:LUC, were spatiotemporally analysed at a cell-level resolution. We focused on spontaneous circadian organisation under constant light conditions for plants with light : dark treatment (LD grown) or without it (LL grown). Fronds developing even from an LL-grown parental frond showed coherent circadian rhythms among them. This allowed the maintenance of circadian rhythmicity in proliferating plants. Inside a frond, a centrifugal phase/period pattern was observed in LD-grown plants, whereas various phase patterns with travelling waves were formed in LL-grown plants. These patterns were model simulated by local coupling of heterogeneous cellular circadian oscillators with different initial synchronous states in fronds. Spatiotemporal analysis of the circadian rhythms in proliferating plants reveals spontaneous synchronisation manners that are associated with local cell-cell coupling, spatial phase patterns and developmental stages.

Return of the Lemnaceae: duckweed as a model plant system in the genomics and postgenomics era

The aquatic Lemnaceae family, commonly called duckweed, comprises some of the smallest and fastest growing angiosperms known on Earth. Their tiny size, rapid growth by clonal propagation, and facile uptake of labeled compounds from the media were attractive features that made them a well-known model for plant biology from 1950 to 1990. Interest in duckweed has steadily regained momentum over the past decade, driven in part by the growing need to identify alternative plants from traditional agricultural crops that can help tackle urgent societal challenges, such as climate change and rapid population expansion. Propelled by rapid advances in genomic technologies, recent studies with duckweed again highlight the potential of these small plants to enable discoveries in diverse fields from ecology to chronobiology. Building on established community resources, duckweed is reemerging as a platform to study plant processes at the systems level and to translate knowledge gained for field deployment to address some of society's pressing needs. This review details the anatomy, development, physiology, and molecular characteristics of the Lemnaceae to introduce them to the broader plant research community. We highlight recent research enabled by Lemnaceae to demonstrate how these plants can be used for quantitative studies of complex processes and for revealing potentially novel strategies in plant defense and genome maintenance.

Detection of Uncoupled Circadian Rhythms in Individual Cells of Lemna minor using a Dual-Color Bioluminescence Monitoring System

The plant circadian oscillation system is based on the circadian clock of individual cells. Circadian behavior of cells has been observed by monitoring the circadian reporter activity, such as bioluminescence of AtCCA1::LUC+. To deeply analyze different circadian behaviors in individual cells, we developed the dual-color bioluminescence monitoring system that automatically measured the luminescence of two luciferase reporters simultaneously at a single-cell level. We selected a yellow-green-emitting firefly luciferase (LUC+) and a red-emitting luciferase (PtRLUC) that is a mutant form of Brazilian click beetle ELUC. We used AtCCA1::LUC+ and CaMV35S::PtRLUC. CaMV35S::LUC+ was previously reported as a circadian reporter with a low-amplitude rhythm. These bioluminescent reporters were introduced into the cells of a duckweed, Lemna minor, by particle bombardment. Time series of the bioluminescence of individual cells in a frond were obtained using a dual-color bioluminescence monitoring system with a green-pass- and red-pass filter. Luminescence intensities from the LUC+ and PtRLUC of each cell were calculated from the filtered luminescence intensities. We succeeded in reconstructing the bioluminescence behaviors of AtCCA1::LUC+ and CaMV35S::PtRLUC in the same cells. Under prolonged constant light conditions, AtCCA1::LUC+ showed a robust circadian rhythm in individual cells in an asynchronous state in the frond, as previously reported. By contrast, CaMV35S::PtRLUC stochastically showed circadian rhythms in a synchronous state. These results strongly suggested the uncoupling of cellular behavior between these circadian reporters. This dual-color bioluminescence monitoring system is a powerful tool to analyze various stochastic phenomena accompanying large cell-to-cell variation in gene expression.

Plant Defence Mechanisms Are Modulated by the Circadian System

Simple Summary

The circadian clock is an endogenous time keeping mechanism found in living organisms and their respective pathogens. Numerous studies demonstrate that rhythms generated by this internal biological oscillator regulate and modulate most of the physiological, developmental, and biochemical processes of plants. Importantly, plant defence responses have also been shown to be modulated by the host circadian clock and vice versa. In this review we discuss the current understanding of the interactions between plant immunity and the circadian system. We also describe the possibility of pathogens directly or indirectly influencing plants’ circadian rhythms and suggest that these interactions could help us devise better disease management strategies for plants. Our review raises further research questions and we conclude that experimentation should be completed to unravel the complex mechanisms underlying interactions between plant defence and the circadian system.

Abstract

Plant health is an important aspect of food security, with pathogens, pests, and herbivores all contributing to yield losses in crops. Plants’ defence against pathogens is complex and utilises several metabolic processes, including the circadian system, to coordinate their response. In this review, we examine how plants’ circadian rhythms contribute to defence mechanisms, particularly in response to bacterial pathogen attack. Circadian rhythms contribute to many aspects of the plant–pathogen interaction, although significant gaps in our understanding remain to be explored. We conclude that if these relationships are explored further, better disease management strategies could be revealed.

Crosstalk in the darkness: bulb vernalization activates meristem transition via circadian rhythm and photoperiodic pathway

BACKGROUND

Geophytes possess specialized storage organs - bulbs, tubers, corms or rhizomes, which allow their survival during unfovarable periods and provide energy support for sprouting and sexual and vegetative reproduction. Bulbing and flowering of the geophyte depend on the combined effects of the internal and external factors, especially temperature and photoperiod. Many geophytes are extensively used in agriculture, but mechanisms of regulation of their flowering and bulbing are still unclear.

RESULTS

Comparative morpho-physiological and transcriptome analyses and quantitative validation of gene expression shed light on the molecular regulation of the responses to vernalization in garlic, a typical bulbous plant. Long dark cold exposure of bulbs is a major cue for flowering and bulbing, and its interactions with the genetic makeup of the individual plant dictate the phenotypic expression during growth stage. Photoperiod signal is not involved in the initial nuclear and metabolic processes, but might play role in the later stages of development, flower stem elongation and bulbing. Vernalization for 12 weeks at 4 °C and planting in November resulted in flower initiation under short photoperiod in December-January, and early blooming and bulbing. In contrast, non-vernalized plants did not undergo meristem transition. Comparisons between vernalized and non-vernalized bulbs revealed ~ 14,000 differentially expressed genes.

CONCLUSIONS

Low temperatures stimulate a large cascades of molecular mechanisms in garlic, and a variety of flowering pathways operate together for the benefit of meristem transition, annual life cycle and viable reproduction results.The circadian clock appears to play a central role in the transition of the meristem from vegetative to reproductive stage in bulbous plant, serving as integrator of the low-temperature signals and the expression of the genes associated with vernalization, photoperiod and meristem transition. The reserved photoperiodic pathway is integrated at an upstream point, possibly by the same receptors. Therefore, in bulb, low temperatures stimulate cascades of developmental mechanisms, and several genetic flowering pathways intermix to achieve successful sexual and vegetative reproduction.

Use of a duckweed species, Wolffiella hyalina, for whole-plant observation of physiological behavior at the single-cell level

We developed a new model system to analyze physiological behavior at the single-cell level in whole plants. Wolffiella hyalina is a species of rootless duckweed, which has a thin and very small structure and can grow rapidly on the surface of culture medium. Epidermal and mesophyll cells were transfected with a reporter gene using particle bombardment and were observed at the single-cell level in the whole living plant. An EM-CCD camera system with a macro zoom microscope was used to capture time-lapse images of bioluminescence, and we successfully detected circadian rhythms in individual cells that expressed a luciferase gene under the control of a circadian promoter. We also detected individual S-phase cells in meristematic tissues of intact W. hyalina plants by using a 5-ethynyl-2'-deoxyuridine (EdU)-labeling assay. Our observations indicated that low-molecular-weight compounds could access the inside of the plant body. Thus, W. hyalina showed the experimental characteristics suitable for single-cell analyses that could be combined with whole-plant observations and/or pharmacological analyses/chemical biology.

Long-term monitoring of bioluminescence circadian rhythms of cells in a transgenic Arabidopsis mesophyll protoplast culture

The circadian system of plants is based on the cell-autonomously oscillating circadian clock. In the plant body, these cellular clocks are associated with each other, but their basic and intrinsic properties are still largely unknown. Here we report a method that enables long-term monitoring of bioluminescence circadian rhythms of a protoplast culture in a complete synthetic medium. From the leaves of Arabidopsis transgenic plants carrying the luciferase gene under a clock-gene promoter, mesophyll protoplasts were isolated and their bioluminescence was automatically measured every 20 min for more than one week. Decreasing luminescence intensities were observed in protoplasts when they were cultured in a Murashige and Skoog-based medium and also in W5 solution. This decrease was dramatically improved by adding the phytohormones auxin and cytokinin to the MS-based medium; robust circadian rhythms were successfully monitored. Interestingly, the period lengths of bioluminescence circadian rhythms of protoplasts under constant conditions were larger than those of detached leaves, suggesting that the period lengths of mesophyll cells in leaves were modulated from their intrinsic properties by the influence of other tissues/cells. The entrainability of protoplasts to light/dark signals was clearly demonstrated by using this monitoring system. By analyzing the circadian behavior of isolated protoplasts, the basic circadian system of plant cells may be better understood.