The life of phi: the development of phi thickenings in roots of the orchids of the genus Miltoniopsis

Molecular mechanism of vivipary as revealed by the genomes of viviparous mangroves and non-viviparous relatives

Vivipary is a prominent feature of mangroves, allowing seeds to complete germination while attached to the mother plant, and equips propagules to endure and flourish in challenging coastal intertidal wetlands. However, vivipary-associated genetic mechanisms remain largely elusive. Genomes of two viviparous mangrove species and a non-viviparous inland relative were sequenced and assembled at the chromosome level. Comparative genomic analyses between viviparous and non-viviparous genomes revealed that DELAY OF GERMINATION 1 (DOG1) family genes (DFGs), the proteins from which are crucial for seed dormancy, germination, and reserve accumulation, are either lost or dysfunctional in the entire lineage of true viviparous mangroves but are present and functional in their inland, non-viviparous relatives. Transcriptome dynamics at key stages of vivipary further highlighted the roles of phytohormonal homeostasis, proteins stored in mature seeds, and proanthocyanidins in vivipary under conditions lacking DFGs. Population genomic analyses elucidate dynamics of syntenic regions surrounding the missing DFGs. Our findings demonstrated the genetic foundation of constitutive vivipary in Rhizophoraceae mangroves.

Phi thickenings in Brassica oleracea roots are induced by osmotic stress and mechanical effects, both involving jasmonic acid

Phi thickenings are peculiar secondary cell wall thickenings found in radial walls of cortical cells in plant roots. However, while thickenings are widespread in the plant kingdom, research into their development has been lacking. Here, we describe a simple system for rapid induction of phi thickenings in primary roots of Brassica. Four-day-old seedlings were transferred from control agar plates to new plates containing increased levels of osmotica. Phi thickening development occurred within a narrow region of the differentiation zone proportional to osmolarity, with cellulose deposition and lignification starting after 12h and 15h, respectively. However, osmoprotectants not only failed to induce phi thickenings, but inhibited induction when tested in combination with thickening-inducing osmotica. An independent, biomechanical pathway exists regulating phi thickening induction, with root growth rates and substrate texture being important factors in determining thickening induction. Phi thickening development is also controlled by stress-related plant hormones, most notably jasmonic acid, but also abscisic acid. Our research not only provides the first understanding of the developmental pathways controlling phi thickening induction, but also provides tools with which the functions of these enigmatic structures might be clarified.

The Orchid Velamen: A Model System for Studying Patterned Secondary Cell Wall Development?

Understanding the mechanisms through which plants generate secondary cell walls is of more than academic interest: the physical properties of plant-derived materials, including timber and textiles, all depend upon secondary wall cellulose organization. Processes controlling cellulose in the secondary cell wall and their reliance on microtubules have been documented in recent decades, but this understanding is complicated, as secondary walls normally form in the plant’s interior where live cell imaging is more difficult. We investigated secondary wall formation in the orchid velamen, a multicellular epidermal layer found around orchid roots that consists of dead cells with lignified secondary cell walls. The patterns of cell wall ridges that form within the velamen vary between different orchid species, but immunolabelling demonstrated that wall deposition is controlled by microtubules. As these patterning events occur at the outer surface of the root, and as orchids are adaptable for tissue culture and genetic manipulation, we conclude that the orchid root velamen may indeed be a suitable model system for studying the organization of the plant cell wall. Notably, roots of the commonly grown orchid Laelia anceps appear ideally suited for developing this research.

Anatomical study of Orchidaceae epiphytes species occurring in indigenous territory in the Parque Estadual da Serra do Tabuleiro (P.E.S.T.), Santa Catarina, Brazil

Abstract Besides their ecological importance, epiphytic species of Orchidaceae play economic and social roles through their commercialization and some are at great risk of extinction. The objectives of this study were to characterize the leaf and root anatomy of fourteen epiphytic Orchidaceae species, which occur in indigenous territory in the Parque Estadual da Serra do Tabuleiro (P.E.S.T.), Santa Catarina, Brazil and to identify adaptive anatomical characteristics related to the epiphytic habit. The species are commercialized by the Guarani and were collected during interviews and guided tours with Guarani in the indigenous territory. The results reveal the species have many morphoanatomical structures that are useful during water shortages resulting from the epiphytic habit. Notable characteristics are related to reserving water (i.e., pseudobulbs and a hypodermis with water-storage cells) and resistance to desiccation in the leaf (i.e., conspicuous cuticle, suprastomatic chamber and extraxylary and pericyclic fibers) and root (i.e., tilosomes and/or exodermal thickening and cortex cells with phi thickenings or sclereids). Descriptions and the identification of adaptive characteristics of epiphytic plant species are useful for conservation and cultivation studies, especially for plants commercially used by the Guarani Indians.

A new insight on structural and some functional aspects of peri-endodermal thickenings, a specific layer in Noccaea caerulescens roots

BACKGROUND AND AIMS

Cell walls of the peri-endodermis, a layer adjacent to the endodermis in alpine pennycress (Noccaea caerulescens) roots, form C-shaped peri-endodermal thickenings (PETs). Despite its specific position close to the endodermis, the assumed similarity of PETs to phi thickenings in many other species, and the fact that N. caerulescens is a well-studied heavy-metal-hyperaccumulating plant, the PET as a root trait is still not understood.

METHODS

Here, we characterized PET cell walls by histochemical techniques, Raman spectroscopy, immunolabelling and electron microscopy. Moreover, a role of PETs in solute transport was tested and compared with Arabidopsis thaliana plants, which do not form PETs in roots.

KEY RESULTS

Cell walls with PETs have a structured relief mainly composed of cellulose and lignin. Suberin, typical of endodermal cells, is missing but pectins are present on the inner surface of the PET. Penetrating dyes are not able to cross PETs either by the apoplasmic or the symplasmic pathway, and a significantly higher content of metals is found in root tissues outside of PETs than in innermost tissues.

CONCLUSIONS

Based on their development and chemical composition, PETs are different from the endodermis and closely resemble phi thickenings. Contrarily, the different structure and dye impermeability of PETs, not known in the case of phi thickenings, point to an additional barrier function which makes the peri-endodermis with PETs a unique and rare layer.

The Induction and Roles Played by Phi Thickenings in Orchid Roots

Phi thickenings are specialised secondary wall thickenings present in the root cortex of many plant species, including both angiosperms and gymnosperms. While environmental stresses induce phi thickenings, their role(s) in the root remain unclear. Suggested functions include regulation of transport through the apoplast in a manner similar to the Casparian strip, limiting fungal infections, and providing mechanical support to the root. We investigated phi thickening induction and function in Miltoniopsis sp., an epiphytic orchid. As movement of a fluorescent tracer through the apoplast was not blocked by phi thickenings, and as phi thickenings developed in the roots of sterile cultures in the absence of fungus and did not prevent fungal colonisation of cortical cells, the phi thickenings in Miltoniopsis did not function as a barrier. Phi thickenings, absent in roots grown on agar, remained absent when plants were transplanted to moist soil, but were induced when plants were transplanted to well-drained media, and by the application of water stress. We suggest that it is likely that phi thickenings stabilise to the root during water stress. Nevertheless, the varied phi thickening induction responses present in different plant species suggest that the phi thickenings may play multiple adaptive roles depending on species.

Phi thickenings in roots: novel secondary wall structures responsive to biotic and abiotic stresses

Phi thickenings are specialized secondary walls found in root cortical cells. Despite their widespread occurrence throughout the plant kingdom, these specialized thickenings remain poorly understood. First identified by Van Tieghem in 1871, phi thickenings are a lignified and thickened cell wall band that is deposited inside the primary wall, as a ring around the cells' radial walls. Phi thickenings can, however, display structural variations including a fine, reticulate network of wall thickenings extending laterally from the central lignified band. While phi thickenings have been proposed to mechanically strengthen roots, act as a permeability barrier to modulate solute movement, and regulate fungal interactions, these possibilities remain to be experimentally confirmed. Furthermore, since temporal and spatial development of phi thickenings varies widely between species, thickenings may perform diverse roles in different species. Phi thickenings can be induced by abiotic stresses in different species; they can, for example, be induced by heavy metals in the Zn/Cd hyperaccumulator Thlaspi caerulescens, and in a cultivar-specific manner by water stress in Brassica. This latter observation provides an experimental platform to probe phi thickening function, and to identify genetic pathways responsible for their formation. These pathways might be expected to differ from those involved in secondary wall formation in xylem, since phi thickening deposition in not linked to programmed cell death.

The origin and evolution of mycorrhizal symbioses: from palaeomycology to phylogenomics

Contents Summary 1012 I. Introduction 1013 II. The mycorrhizal symbiosis at the dawn and rise of the land flora 1014 III. From early land plants to early trees: the origin of roots and true mycorrhizas 1016 IV. The diversification of the AM symbiosis 1019 V. The ECM symbiosis 1021 VI. The recently evolved ericoid and orchid mycorrhizas 1023 VII. Limits of paleontological vs genetic approaches and perspectives 1023 Acknowledgements 1025 References 1025 SUMMARY: The ability of fungi to form mycorrhizas with plants is one of the most remarkable and enduring adaptations to life on land. The occurrence of mycorrhizas is now well established in c. 85% of extant plants, yet the geological record of these associations is sparse. Fossils preserved under exceptional conditions provide tantalizing glimpses into the evolutionary history of mycorrhizas, showing the extent of their occurrence and aspects of their evolution in extinct plants. The fossil record has important roles to play in establishing a chronology of when key fungal associations evolved and in understanding their importance in ecosystems through time. Together with calibrated phylogenetic trees, these approaches extend our understanding of when and how groups evolved in the context of major environmental change on a global scale. Phylogenomics furthers this understanding into the evolution of different types of mycorrhizal associations, and genomic studies of both plants and fungi are shedding light on how the complex set of symbiotic traits evolved. Here we present a review of the main phases of the evolution of mycorrhizal interactions from palaeontological, phylogenetic and genomic perspectives, with the aim of highlighting the potential of fossil material and a geological perspective in a cross-disciplinary approach.

Developmental Biology and Induction of Phi Thickenings by Abiotic Stress in Roots of the Brassicaceae

Phi thickenings are specialized bands of secondary wall deposited around radial walls of root cortical cells. These structures have been reported in various species from the Brassicaceae, including Brassica oleracea, where previous reports using hydroponics indicated that they can be induced by exposure to salt. Using roots grown on agar plates, we show that both salt and sucrose can induce the formation of phi thickenings in a diverse range of species within the Brassicaceae. Within the genus Brassica, both B. oleracea and B. napus demonstrated the formation of phi thickenings, but in a strongly cultivar-specific manner. Confocal microscopy of phi thickenings showed that they form a complex network of reinforcement surrounding the inner root cortex, and that a delicate, reticulate network of secondary wall deposition can also variously form on the inner face of the cortical cell layer with phi thickenings adjacent to the endodermal layer. Results presented here indicate that phi thickenings can be induced in response to salt and water stress and that wide variation occurs in these responses even within the same species.

Optimisation approaches for concurrent transmitted light imaging during confocal microscopy

BACKGROUND

The transmitted light detectors present on most modern confocal microscopes are an under-utilised tool for the live imaging of plant cells. As the light forming the image in this detector is not passed through a pinhole, out-of-focus light is not removed. It is this extended focus that allows the transmitted light image to provide cellular and organismal context for fluorescence optical sections generated confocally. More importantly, the transmitted light detector provides images that have spatial and temporal registration with the fluorescence images, unlike images taken with a separately-mounted camera.

RESULTS

Because plants often provide difficulties for taking transmitted light images, with the presence of pigments and air pockets in leaves, this study documents several approaches to improving transmitted light images beginning with ensuring that the light paths through the microscope are correctly aligned (Köhler illumination). Pigmented samples can be imaged in real colour using sequential scanning with red, green and blue lasers. The resulting transmitted light images can be optimised and merged in ImageJ to generate colour images that maintain registration with concurrent fluorescence images. For faster imaging of pigmented samples, transmitted light images can be formed with non-absorbed wavelengths. Transmitted light images of Arabidopsis leaves expressing GFP can be improved by concurrent illumination with green and blue light. If the blue light used for YFP excitation is blocked from the transmitted light detector with a cheap, coloured glass filters, the non-absorbed green light will form an improved transmitted light image. Changes in sample colour can be quantified by transmitted light imaging. This has been documented in red onion epidermal cells where changes in vacuolar pH triggered by the weak base methylamine result in measurable colour changes in the vacuolar anthocyanin.

CONCLUSIONS

Many plant cells contain visible levels of pigment. The transmitted light detector provides a useful tool for documenting and measuring changes in these pigments while maintaining registration with confocal imaging.