Biological adhesion in seagrasses: The role of substrate roughness in Posidonia oceanica (L.) Delile seedling anchorage via adhesive root hairs

Natural nanofibers embedded in the seed mucilage envelope: composite hydrogels with specific adhesive and frictional properties

The increasing interests in natural, biodegradable, non-toxic materials that can find application in diverse industry branches, for example, food, pharmacy, medicine, or materials engineering, has steered the attention of many scientists to plants, which are a known source of natural hydrogels. Natural hydrogels share some features with synthetic hydrogels, but are more easy to obtain and recycle. One of the main sources of such hydrogels are mucilaginous seeds and fruits, which produce after hydration a gel-like, transparent capsule, the so-called mucilage envelope. Mucilage serves several important biological functions, such as supporting seed germination, protecting seeds against pathogens and predators, and allowing the seed to attach to diverse surfaces (e.g., soil or animals). The attachment properties of mucilage are thus responsible for seed dispersal. Mucilage represents a hydrophilic, three-dimensional network of polysaccharides (cellulose, pectins, and hemicelluloses) and is able to absorb large amounts of water. Depending on the water content, mucilage can behave as an efficient lubricant or as strong glue. The current work attempts to summarise the achievements in the research on the mucilage envelope, primarily in the context of its structure and physical properties, as well as biological functions associated with these properties.

Fruit encasing preserves the dispersal potential and viability of stranded Posidonia oceanica seeds

Posidonia oceanica meadows are the most productive coastal ecosystem in the Mediterranean. Posidonia oceanica seeds are enclosed in buoyant fleshy fruits that allow dispersal. Many fruits eventually strand on beaches, imposing a remarkable energy cost for the plant. This study aims to assess whether stranded seeds retain functional reproductive potential under a variety of environmental conditions. First, we measured the possibility that seeds could be returned to the sea, by tagging fruits and seeds. Second, we quantified the effect of air, sun and heat exposure on the viability and fitness of stranded fruits and naked seeds. The results showed that on average more than half of fruits and seeds are returned to the sea after stranding events and that fruits significantly protect from desiccation and loss of viability. Furthermore, in fruits exposed to the sun and in naked seeds, seedlings development was slower. This study indicates that a significant portion of stranded P. oceanica fruits have a second chance to recruit and develop into young seedlings, relieving the paradox of large energy investment in seed production and apparent low recruitment rate. Additionally, we provide practical indications for seed collection aimed at maximizing seedling production, useful in meadow restoration campaigns.

Smoothing out the misconceptions of the role of bark roughness in vascular epiphyte attachment

Vascular epiphytes represent c. 10% of all vascular plant species. In epiphytes, attachment is essential for survival throughout consecutive ontogenetic stages of their life, starting with: (1) initial propagule attachment to the host; followed by (2) the development of first root-substrate connections; and (3) maintenance of this attachment despite increased size and mechanical disturbances by rain, wind, or crossing animals. Although structural dependence on a host is a defining characteristic of an epiphyte, the fundamental mechanism(s) of how these plants initially attach and remain attached to their hosts remain poorly understood. Bark characteristics such as stability and roughness have been highlighted as keys to an understanding of this connection. Here, we stress that the understanding of how an epiphyte attaches itself to the substrate is central for a meaningful quantification and interpretation of bark roughness. Without explicit information on the attachment mechanism or the relative sizes of the attaching structures, simply linking a haphazardly chosen index of bark roughness to epiphyte establishment is flawed. This review introduces a conceptual framework to explain the mechanistic link between epiphytes and host in different ontogenetic stages and should guide future work designed to improve our understanding of this vital part of epiphyte ecology.

Building soil sustainability from root-soil interface traits

Great potential exists to harness plant traits at the root-soil interface, mainly rhizodeposition and root hairs, to 'build' soils with better structure that can trap more carbon and resources, resist climate stresses, and promote a healthy microbiome. These traits appear to have been preserved in modern crop varieties, but scope exists to improve them further because they vary considerably between genotypes and respond to environmental conditions. From emerging evidence, rhizodeposition can act as a disperser, aggregator, and/or hydrogel in soil, and root hairs expand rhizosheath size. Future research should explore impacts of selecting these traits on plants and soils concurrently, expanding from model plants to commercial genotypes, and observing whether impacts currently limited to glasshouse studies occur in the field.

Holding on or falling off: The attachment mechanism of epiphytic Anthurium obtusum changes with substrate roughness

PREMISE

For vascular epiphytes, secure attachment to their hosts is vital for survival. Yet studies detailing the adhesion mechanism of epiphytes to their substrate are scarce. Examination of the root hair-substrate interface is essential to understand the attachment mechanism of epiphytes to their substrate. This study also investigated how substrate microroughness relates to the root-substrate attachment strength and the underlying mechanism(s).

METHODS

Seeds of Anthurium obtusum were germinated, and seedlings were transferred onto substrates made of epoxy resin with different defined roughness. After 2 months of growth, roots that adhered to the resin tiles were subjected to anchorage tests, and root hair morphology at different roughness levels was analyzed using light and cryo scanning electron microscopy.

RESULTS

The highest maximum peeling force was recorded on the smooth surface (glass replica, 0 µm). Maximum peeling force was significantly higher on fine roughness (0, 0.3, 12 µm) than on coarse (162 µm). Root hair morphology varied according to the roughness of the substrate. On smoother surfaces, root hairs were flattened to achieve large surface contact with the substrate. Attachment was mainly by adhesion with the presence of a glue-like substance. On coarser surfaces, root hairs were tubular and conformed to spaces between the asperities on the surface. Attachment was mainly via mechanical interlocking of root hairs and substrate.

CONCLUSIONS

This study demonstrates for the first time that the attachment mechanism of epiphytes varies depending on substrate microtopography, which is important for understanding epiphyte attachment on natural substrates varying in roughness.

Significance of root hairs in developing stress-resilient plants for sustainable crop production

Root hairs represent a beneficial agronomic trait to potentially reduce fertilizer and irrigation inputs. Over the past decades, research in the plant model Arabidopsis thaliana has provided insights into root hair development, the underlying genetic framework and the integration of environmental cues within this framework. Recent years have seen a paradigm shift, where studies are now highlighting conservation and diversification of root hair developmental programs in other plant species and the agronomic relevance of root hairs in a wider ecological context. In this review, we specifically discuss the molecular evolution of the RSL (RHD Six-Like) pathway that controls root hair development and growth in land plants. We also discuss how root hairs contribute to plant performance as an active physiological rooting structure by performing resource acquisition, providing anchorage and constructing the rhizosphere with desirable physical, chemical and biological properties. Finally, we outline future research directions that can help achieve the potential of root hairs in developing sustainable agroecosystems.

Rice Root Hair Phenotypes Imaged by Cryo-SEM

Cryo-scanning electron microscopy (cryo-SEM) was first introduced for scientific use in the 1980s. Since then, cryo-SEM has become a routine technique for studying the surfaces and internal structures of biological samples with a high water content. In contrast to traditional SEM, cryo-SEM requires no sample pretreatment processes; thus, we can obtain the most authentic images of the sample shape and structure. Cryo-SEM has two main steps: cryoprocessing of samples and scanning electron microscopy (SEM) observation. The cryoprocessing step includes preparation of the cooled slushing station, cooling of the preparation chamber, sample preparation, and sputtering. The sample is then transferred to an SEM cold stage for observation. We used cryo-SEM to study rice root hair tissues, but the methods and protocols can be applied to other root systems. This protocol optimizes the two key operation steps of reducing the humidity in the growth chamber and previewing the samples before sputtering and can more quickly obtain high-quality images.

Recovery and Community Succession of the Zostera marina Rhizobiome after Transplantation

Seagrasses can form mutualisms with their microbiomes that facilitate the exchange of energy sources, nutrients, and hormones and ultimately impact plant stress resistance. Little is known about community succession within the belowground seagrass microbiome after disturbance and its potential role in the plant's recovery after transplantation. We transplanted Zostera marina shoots with and without an intact rhizosphere and cultivated plants for 4 weeks while characterizing microbiome recovery and effects on plant traits. Rhizosphere and root microbiomes were compositionally distinct, likely representing discrete microbial niches. Furthermore, microbiomes of washed transplants were initially different from those of sod transplants and recovered to resemble an undisturbed state within 14 days. Conspicuously, changes in the microbial communities of washed transplants corresponded with changes in the rhizosphere sediment mass and root biomass, highlighting the strength and responsive nature of the relationship between plants, their microbiome, and the environment. Potential mutualistic microbes that were enriched over time include those that function in the cycling and turnover of sulfur, nitrogen, and plant-derived carbon in the rhizosphere environment. These findings highlight the importance and resilience of the seagrass microbiome after disturbance. Consideration of the microbiome will have meaningful implications for habitat restoration practices.IMPORTANCE Seagrasses are important coastal species that are declining globally, and transplantation can be used to combat these declines. However, the bacterial communities associated with seagrass rhizospheres and roots (the microbiome) are often disturbed or removed completely prior to transplantation. The seagrass microbiome benefits seagrasses through metabolite, nutrient, and phytohormone exchange and contributes to the ecosystem services of seagrass meadows by cycling sulfur, nitrogen, and carbon. This experiment aimed to characterize the importance and resilience of the seagrass belowground microbiome by transplanting Zostera marina with and without intact rhizospheres and tracking microbiome and plant morphological recovery over 4 weeks. We found the seagrass microbiome to be resilient to transplantation disturbance, recovering after 14 days. Additionally, microbiome recovery was linked with seagrass morphology, coinciding with increases in the rhizosphere sediment mass and root biomass. The results of this study can be used to include microbiome responses in informing future restoration work.

Letter to the editor regarding the article "Taking advantage of seagrass recovery potential to develop novel and effective meadow rehabilitation methods" by Alagna et al., published in Marine Pollution Bulletin, 149: 2019 (110578)

Alagna et al. (2019) suggest new transplantation methods for Posidonia oceanica (Linnaeus) Delile, inspired by its natural recovery process after disturbance due to dredging operations for gas-pipelines. They observe that P. oceanica vegetative fragments naturally settled only on loose calcareous stones deployed to fill the trenches of the gas-pipeline. No recovery was noted on dead matte, sand and large calcarenitic boulders. Following a new pilot restoration project currently ongoing in the same area, we demonstrate that natural recovery also occurs on dead matte. After examining other alternative transplantation methods for P. oceanica, the Authors suggest using their "habitat enhancement units" method for the restoration of seagrasses, not only on rocky bottom but also on sand and other bare substrate requiring general environmental restoration. Here we express disagreement on certain issues reported in the paper.