Trafficking Processes and Secretion Pathways Underlying the Formation of Plant Cuticles

Cuticles are specialized cell wall structures that form at the surface of terrestrial plant organs. They are largely comprised lipidic compounds and are deposited in the apoplast, external to the polysaccharide-rich primary wall, creating a barrier to diffusion of water and solutes, as well as to environmental factors. The predominant cuticle component is cutin, a polyester that is assembled as a complex matrix, within and on the surface of which aliphatic and aromatic wax molecules accumulate, further modifying its properties. To reach the point of cuticle assembly the different acyl lipid-containing components are first exported from the cell across the plasma membrane and then traffic across the polysaccharide wall. The export of cutin precursors and waxes from the cell is known to involve plasma membrane-localized ATP-binding cassette (ABC) transporters; however, other secretion mechanisms may also contribute. Indeed, extracellular vesiculo-tubular structures have recently been reported in Arabidopsis thaliana (Arabidopsis) to be associated with the deposition of suberin, a polyester that is structurally closely related to cutin. Intriguingly, similar membranous structures have been observed in leaves and petals of Arabidopsis, although in lower numbers, but no close association with cutin formation has been identified. The possibility of multiple export mechanisms for cuticular components acting in parallel will be discussed, together with proposals for how cuticle precursors may traverse the polysaccharide cell wall before their assimilation into the cuticle macromolecular architecture.

The Complex Architecture of Plant Cuticles and Its Relation to Multiple Biological Functions

Terrestrialization of vascular plants, i.e., Angiosperm, is associated with the development of cuticular barriers that prevent biotic and abiotic stresses and support plant growth and development. To fulfill these multiple functions, cuticles have developed a unique supramolecular and dynamic assembly of molecules and macromolecules. Plant cuticles are not only an assembly of lipid compounds, i.e., waxes and cutin polyester, as generally presented in the literature, but also of polysaccharides and phenolic compounds, each fulfilling a role dependent on the presence of the others. This mini-review is focused on recent developments and hypotheses on cuticle architecture–function relationships through the prism of non-lipid components, i.e., cuticle-embedded polysaccharides and polyester-bound phenolics.

Investigations of plant subfossil cuticles at a Holocene raised bog complex, northern New Zealand

<p>Subfossil plant cuticles, the very resistant waxy layer covering vascular land plants, are a neglected source of information in peat studies, despite their high preservation and identification potential. A lack of standardised methods and reference material are major contributing factors. In this thesis, a new method is introduced to test if subfossil plant cuticles from Moanatuatua Bog in the northern North Island of New Zealand can give a robust reconstruction of local bog surface vegetation changes during the Holocene. The method was successfully established and applied at coarse sampling resolution to show vegetation changes across the full length of the core and at fine sampling resolution around charcoal layers to reconstruct the post-fire response pattern of the main plant species on the bog. Additionally, bulk density and organic matter analyses were carried out to provide further insight into these changes. At the core site, towards the southern margins of Moanatuatua Bog, swamp forest had developed by 15000 cal yr BP. Until ca. 10500 cal yr BP, the vegetation assemblage was sedge-dominated, indicating swamp and/or fen conditions. A significant increase in macroscopic charcoal particles coincided with the transition to a more diversified vegetation composition. At around 4500 cal yr BP, the vegetation became restiad-dominated, indicating full raised bog conditions. The coarse resolution cuticle results were further compared to a pollen record from the same sequence, which was established independently. This comparison showed that plant subfossil cuticles can provide additional information to pollen analysis in cases where pollen is hard to identify or poorly preserved. Specifically, restiad pollen is hard to differentiate, yet cuticles of Empodisma and Sporadanthus have very distinct features. Also, Cyperaceae pollen is very poorly preserved at Moanatuatua Bog and the Cyperaceae pollen curve shows a poor match with the Cyperaceae cuticle record. It is suggested therefore that Cyperaceae pollen at this site – and potentially other peat sites – is a less reliable indicator of local sedge communities than a Cyperaceae cuticle record. At fine resolution, results were blurred across a time interval that was marginal for reconstructing response patterns due to the constraints imposed by sampling resolution and peat accumulation rate of Moanatuatua Bog. Nevertheless, two out of three charcoal layers recorded a local fire on the bog surface, with one layer displaying the expected vegetation response. After the fire, Empodisma, as a mid-successional species, re-established on the bog surface before Sporadanthus, a late-successional species. The other layer was dominated by sedges and showed no response pattern, as is to be expected due to the very fast recovery of sedges. In general, sample preparation for cuticle analysis proved to be fast with relatively little equipment or chemicals needed. With detailed reference material, identification to species level is possible due to distinctive and pronounced cuticle features. Plant cuticle analysis is therefore proposed to be a reliable tool to reconstruct long-term and short-term vegetation changes from peat sequences.</p>

Investigations of plant subfossil cuticles at a Holocene raised bog complex, northern New Zealand

<p>Subfossil plant cuticles, the very resistant waxy layer covering vascular land plants, are a neglected source of information in peat studies, despite their high preservation and identification potential. A lack of standardised methods and reference material are major contributing factors. In this thesis, a new method is introduced to test if subfossil plant cuticles from Moanatuatua Bog in the northern North Island of New Zealand can give a robust reconstruction of local bog surface vegetation changes during the Holocene. The method was successfully established and applied at coarse sampling resolution to show vegetation changes across the full length of the core and at fine sampling resolution around charcoal layers to reconstruct the post-fire response pattern of the main plant species on the bog. Additionally, bulk density and organic matter analyses were carried out to provide further insight into these changes. At the core site, towards the southern margins of Moanatuatua Bog, swamp forest had developed by 15000 cal yr BP. Until ca. 10500 cal yr BP, the vegetation assemblage was sedge-dominated, indicating swamp and/or fen conditions. A significant increase in macroscopic charcoal particles coincided with the transition to a more diversified vegetation composition. At around 4500 cal yr BP, the vegetation became restiad-dominated, indicating full raised bog conditions. The coarse resolution cuticle results were further compared to a pollen record from the same sequence, which was established independently. This comparison showed that plant subfossil cuticles can provide additional information to pollen analysis in cases where pollen is hard to identify or poorly preserved. Specifically, restiad pollen is hard to differentiate, yet cuticles of Empodisma and Sporadanthus have very distinct features. Also, Cyperaceae pollen is very poorly preserved at Moanatuatua Bog and the Cyperaceae pollen curve shows a poor match with the Cyperaceae cuticle record. It is suggested therefore that Cyperaceae pollen at this site – and potentially other peat sites – is a less reliable indicator of local sedge communities than a Cyperaceae cuticle record. At fine resolution, results were blurred across a time interval that was marginal for reconstructing response patterns due to the constraints imposed by sampling resolution and peat accumulation rate of Moanatuatua Bog. Nevertheless, two out of three charcoal layers recorded a local fire on the bog surface, with one layer displaying the expected vegetation response. After the fire, Empodisma, as a mid-successional species, re-established on the bog surface before Sporadanthus, a late-successional species. The other layer was dominated by sedges and showed no response pattern, as is to be expected due to the very fast recovery of sedges. In general, sample preparation for cuticle analysis proved to be fast with relatively little equipment or chemicals needed. With detailed reference material, identification to species level is possible due to distinctive and pronounced cuticle features. Plant cuticle analysis is therefore proposed to be a reliable tool to reconstruct long-term and short-term vegetation changes from peat sequences.</p>

Dynamics of moisture diffusion and adsorption in plant cuticles including the role of cellulose

Food production must increase significantly to sustain a growing global population. Reducing plant water loss may help achieve this goal and is especially relevant in a time of climate change. The plant cuticle defends leaves against drought, and so understanding water movement through the cuticle could help future proof our crops and better understand native ecology. Here, via mathematical modelling, we identify mechanistic properties of water movement in cuticles. We model water sorption in astomatous isolated cuticles, utilising three separate pathways of cellulose, aqueous pores and lipophilic. The model compares well to data both over time and humidity gradients. Sensitivity analysis shows that the grouping of parameters influencing plant species variations has the largest effect on sorption, those influencing cellulose are very influential, and aqueous pores less so but still relevant. Cellulose plays a significant role in diffusion and adsorption in the cuticle and the cuticle surfaces.

The Plant Cuticle: An Ancient Guardian Barrier Set Against Long-Standing Rivals

The aerial surfaces of plants are covered by a protective barrier formed by the cutin polyester and waxes, collectively referred to as the cuticle. Plant cuticles prevent the loss of water, regulate transpiration, and facilitate the transport of gases and solutes. As the cuticle covers the outermost epidermal cell layer, it also acts as the first line of defense against environmental cues and biotic stresses triggered by a large array of pathogens and pests, such as fungi, bacteria, and insects. Numerous studies highlight the cuticle interface as the site of complex molecular interactions between plants and pathogens. Here, we outline the multidimensional roles of cuticle-derived components, namely, epicuticular waxes and cutin monomers, during plant interactions with pathogenic fungi. We describe how certain wax components affect various pre-penetration and infection processes of fungi with different lifestyles, and then shift our focus to the roles played by the cutin monomers that are released from the cuticle owing to the activity of fungal cutinases during the early stages of infection. We discuss how cutin monomers can activate fungal cutinases and initiate the formation of infection organs, the significant impacts of cuticle defects on the nature of plant–fungal interactions, along with the possible mechanisms raised thus far in the debate on how host plants perceive cutin monomers and/or cuticle defects to elicit defense responses.

Unravelling the role of alcohol copper radical oxidases in fungal plant pathogens

Copper radical oxidases (CRO) form a class of enzymes with a longstanding history encompassing diverse substrate specificities. While the biological function of most CROs remains unknown, we observed that CROs active on aliphatic alcohols are found only in fungal plant pathogens. Here, we unveil the role of these CROs and the identity of their natural redox partner, a haem-iron peroxidase. Combining multiscale approaches, we report that Colletotrichum and Magnaporthe appressoria (specialized cells that puncture the plant cuticles) co-secrete this pair of metalloenzymes early during penetration. We show in vivo that mutant appressoria lacking either or both enzymes have impaired penetration ability and pathogenicity. We reveal in vitro a finely-tuned enzyme interplay is responsible for the oxidation of plant cuticular long-chain alcohols into aldehyde products, suggested to act as key molecular signals in the fungal infection machinery. Our results open new avenues to design oxidase-specific inhibitors as anti-penetrants for crop protection.