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Recent extreme wildfire seasons in several regions have been associated with exceptionally hot, dry conditions, made more probable by climate change. Much research has focused on extreme fire weather and its drivers, but natural wildfire regimes—and their interactions with human activities—are far from being comprehensively understood. There is a lack of clarity about the ‘causes’ of wildfire, and about how ecosystems could be managed for the co-existence of wildfire and people. We present evidence supporting an ecosystem-centred framework for improved understanding and modelling of wildfire. Wildfire has a long geological history and is a pervasive natural process in contemporary plant communities. In some biomes, wildfire would be more frequent without human settlement; in others they would be unchanged or less frequent. A world without fire would have greater forest cover, especially in present-day savannas. Many species would be missing, because fire regimes have co-evolved with plant traits that resist, adapt to or promote wildfire. Certain plant traits are favoured by different fire frequencies, and may be missing in ecosystems that are normally fire-free. For example, post-fire resprouting is more common among woody plants in high-frequency fire regimes than where fire is infrequent. The impact of habitat fragmentation on wildfire crucially depends on whether the ecosystem is fire-adapted. In normally fire-free ecosystems, fragmentation facilitates wildfire starts and is detrimental to biodiversity. In fire-adapted ecosystems, fragmentation inhibits fires from spreading and fire suppression is detrimental to biodiversity. This interpretation explains observed, counterintuitive patterns of spatial correlation between wildfire and potential ignition sources. Lightning correlates positively with burnt area only in open ecosystems with frequent fire. Human population correlates positively with burnt area only in densely forested regions. Models for vegetation-fire interactions must be informed by insights from fire ecology to make credible future projections in a changing climate.

The encapsulation of seeds in hard coats and fruit walls (pericarp layers) fulfils protective and dispersal functions in many plant families. In angiosperms, packaging structures possess a remarkable range of different morphologies and functionalities, as illustrated by thermo and hygro-responsive seed pods and appendages, as well as mechanically strong and water-impermeable shells. Key to these different functionalities are characteristic structural arrangements and chemical modifications of the underlying sclerenchymatous tissues. Although many ecological aspects of hard seed encapsulation have been well documented, a detailed understanding of the relationship between tissue structure and function only recently started to emerge, especially in the context of environmentally driven fruit opening and seed dispersal (responsive encapsulations) and the outstanding durability of some seed coats and indehiscent fruits (static encapsulations). In this review, we focus on the tissue properties of these two systems, with particular consideration of water interactions, mechanical resistance, and force generation. Common principles, as well as unique adaptations, are discussed in different plant species. Understanding how plants integrate a broad range of functions and properties for seed protection during storage and dispersal plays a central role for seed conservation, population dynamics, and plant-based material developments.

The outer protective shells of nuts can have remarkable toughness and strength, which are typically achieved by a layered arrangement of sclerenchyma cells and fibers with a polygonal form. Here, the tissue structure of walnut shells is analyzed in depth, revealing that the shells consist of a single, never reported cell type: the polylobate sclereid cells. These irregularly lobed cells with concave and convex parts are on average interlocked with 14 neighboring cells. The result is an intricate arrangement that cannot be disassembled when conceived as a 3D puzzle. Mechanical testing reveals a significantly higher ultimate tensile strength of the interlocked walnut cell tissue compared to the sclerenchyma tissue of a pine seed coat lacking the lobed cell structure. The higher strength value of the walnut shell is explained by the observation that the crack cannot simply detach intact cells but has to cut through the lobes due to the interlocking. Understanding the identified nutshell structure and its development will inspire biomimetic material design and packaging concepts. Furthermore, these unique unit cells might be of special interest for utilizing nutshells in terms of food waste valorization, considering that walnuts are the most widespread tree nuts in the world. Walnut shells consist of a single, previously unreported, cell type—the polylobate 3D sclereid cells—which intricately interlock all neighboring cells. This novel cell type is described and discussed in detail by considering the structural changes during nutshell maturation and the mechanical implications that arise from their 3D puzzle shape.

Wildfires are a natural component in many terrestrial ecosystems and often play a crucial role in maintaining biodiversity, particularly in the fire-prone regions of Australia. A prime example of plants that are able to persist in these regions is the genus . Most species that occur in fire-prone regions produce woody seed pods (follicles), which open during or soon after fire to release seeds into the post-fire environment. For population persistence, many species depend on recruitment from these canopy-stored seeds. Therefore, it is critical that their seeds are protected from heat and rapid oxidation during fire. Here, we show how different species of protect their seeds inside follicles while simultaneously opening up when experiencing fire. The ability of the follicles to protect seeds from heat is demonstrated by intense 180 s experimental burns, in which the maximum temperatures near the seeds ranged from ∼75°C for to ∼90°C for and ∼95°C for , contrasting with the mean surface temperature of ∼450°C. Many seeds of native Australian plants, including those of , are able to survive these temperatures. Structural analysis of individual follicles from these three species demonstrates that all of them rely on a multicomponent system, consisting of two valves, a porous separator and a thin layer of air surrounding the seeds. The particular geometric arrangement of these components determines the rate of heat transfer more than the tissue properties alone, revealing that a strong embedment into the central rachis can compensate for thin follicle valves. Furthermore, we highlight the role of the separator as an important thermal insulator. Our study suggests that the genus employs a variety of combinations in terms of follicle size, valve thickness, composition and geometric arrangement to effectively protect canopy-stored seeds during fire.

Main conclusionHigh symplastic connectivity via pits was linked to the lignification of the developing walnut shell. With maturation, this network lessened, whereas apoplastic intercellular space remained and became relevant for shell drying.The shell of the walnut (Juglans regia) sclerifies within several weeks. This fast secondary cell wall thickening and lignification of the shell tissue might need metabolites from the supporting husk tissue. To reveal the transport capacity of the walnut shell tissue and its connection to the husk, we visualised the symplastic and apoplastic transport routes during shell development by serial block face-SEM and 3D reconstruction. We found an extensive network of pit channels connecting the cells within the shell tissue, but even more towards the husk tissue. Each pit channel ended in a pit field, which was occupied by multiple plasmodesmata passing through the middle lamella. During shell development, secondary cell wall formation progressed towards the interior of the cell, leaving active pit channels open. In contrast, pit channels, which had no plasmodesmata connection to a neighbouring cell, got filled by cellulose layers from the inner cell wall lamellae. A comparison with other nut species showed that an extended network during sclerification seemed to be linked to high cell wall lignification and that the connectivity between cells got reduced with maturation. In contrast, intercellular spaces between cells remained unchanged during the entire sclerification process, allowing air and water to flow through the walnut shell tissue when mature. The connectivity between inner tissue and environment was essential during shell drying in the last month of nut development to avoid mould formation. The findings highlight how connectivity and transport work in developing walnut shell tissue and how finally in the mature state these structures influence shell mechanics, permeability, conservation and germination.

Heat‐triggered fruit opening and delayed release of mature seeds are widespread among plants in fire‐prone ecosystems. Here, the material characteristics of the seed‐containing follicles of Banksia attenuata (Proteaceae), which open in response to heat frequently caused by fire, are investigated. Material analysis reveals that long‐term dimensional stability and opening temperatures of follicles collected across an environmental gradient increase as habitats become drier, hotter, and more fire prone. A gradual increase in the biaxial curvature of the hygroscopic valves provides the follicles in the driest region with the highest flexural rigidity. The irreversible deformation of the valves for opening is enabled via a temperature‐dependent reduction of the elastic modulus of the innermost tissue layer, which then allows releasing the stresses previously generated by shrinkage of the fiber bundles in the adjacent layer during follicle drying. These findings illustrate the level of sophistication by which this species optimizes its fruit opening mechanism over a large distribution range with varying environmental conditions, and may not only have great relevance for developing biomimetic actuators, but also for elucidating the species' capacity to cope with climatic changes. Seed pods of Banksia attenuata open through cycles of humidity following an initial heat stimulus. The heat‐triggered, initial opening process is regulated by an interplay of geometry, temperature, and mechanics. Gradual changes in curvature result in distinct opening temperatures along a pronounced climatic gradient in Western Australia with varying fire risk.

In article number 1900644, Notburga Gierlinger and co‐workers discover a new cell type in walnut shells. The polylobate sclereid cell is visualized during development and, based on X‐ray tomography, the 3D‐puzzle shape confirmed. The intricate interlocking between the puzzle cells is proven to result in higher ultimate tensile strength.

Abstract Walnut (Juglans regia) kernels are protected by a tough shell consisting of polylobate sclereids that interlock into a 3D puzzle. The shape transformations from isodiametric to lobed cells is well documented for 2D pavement cells, but not for 3D puzzle sclereids. Here, we tackle the morphogenesis of these cells by using a combination of different imaging techniques. Serial face-microtomy enabled us to reconstruct tissue growth of whole walnut fruits in 3D and serial block face-scanning electron microscopy exposed cell shapes and their transformation in 3D during shell tissue development. In combination with Raman and fluorescence microscopy we revealed multiple loops of cellulosic thickenings in cell walls, acting as stiff restrictions during cell expansion and leading to the lobed cell shape. Our findings contribute to a better understanding of the 3D shape transformation of polylobate sclereids and the role of pectin and cellulose within this process. Competing Interest Statement The authors have declared no competing interest.

Cacti develop spines instead of conventional leaves, which often serve as mechanical defence against herbivores. However, some cactus species grow porous and flexible spines, suggesting fundamentally different functions. Here we demonstrate the mechanism of fog harvesting in the porous spines of Turbinicarpus alonsoi, a cactus native to central Mexico. Surprisingly, we discovered that the spines are highly hygroscopic and straighten when exposed to fog, leading to increased fog water collection rates. Experiments and numerical simulations confirm that straightening is driven by swelling-induced pressure in the cell walls of the spine tissue. Swelling results from capillary imbibition of fog water and predominantly generates expansion in the transverse plane, which causes the pre-curved spines to straighten. Despite their porosity and hygroscopicity, the spines prevent direct absorption of fog water into the living cortex due to the presence of a suberin-rich tissue layer at the spine base that instead promotes surface runoff towards the roots. Our work suggests that hygro-morphing emerges from distinct structural, biochemical and geometric adaptations of cactus spines, and enables a fine modulation of the flow dynamics on the surface of spines. We conclude that increasing plant water supply from fog by shape morphing may provide an adaptive advantage for survival of the species in a hot, semi-arid region with frequent fog formation. In many arid regions of the world, fog is a critical source of fresh water. This has shaped plant evolution in striking ways. For cacti with porous spines, a widely accepted belief is that they collect fog water directly by capillary imbibition. However, here we demonstrate that, in Turbinicarpus alonsoi, water-impermeable tissue at the spine base prevents direct transport of water into the living cortex. Instead, a dual process increases fog water collection: the curved spines initially imbibe fog water, which causes them to swell and straighten, and a thin liquid film then forms on the spine and runs off along the plant surface down to the roots. Hygro-morphing spines therefore enhance the capacity of cacti to collect fog water.

Although behavioral mechanisms in the association among depression, anxiety, and cancer are plausible, few studies have empirically studied mediation by health behaviors. We aimed to examine the mediating role of several health behaviors in the associations among depression, anxiety, and the incidence of various cancer types (overall, breast, prostate, lung, colorectal, smoking-related, and alcohol-related cancers). Two-stage individual participant data meta-analyses were performed based on 18 cohorts within the Psychosocial Factors and Cancer Incidence consortium that had a measure of depression or anxiety ( = 319 613, cancer incidence = 25 803). Health behaviors included smoking, physical inactivity, alcohol use, body mass index (BMI), sedentary behavior, and sleep duration and quality. In stage one, path-specific regression estimates were obtained in each cohort. In stage two, cohort-specific estimates were pooled using random-effects multivariate meta-analysis, and natural indirect effects (i.e. mediating effects) were calculated as hazard ratios (HRs). Smoking (HRs range 1.04-1.10) and physical inactivity (HRs range 1.01-1.02) significantly mediated the associations among depression, anxiety, and lung cancer. Smoking was also a mediator for smoking-related cancers (HRs range 1.03-1.06). There was mediation by health behaviors, especially smoking, physical inactivity, alcohol use, and a higher BMI, in the associations among depression, anxiety, and overall cancer or other types of cancer, but effects were small (HRs generally below 1.01). Smoking constitutes a mediating pathway linking depression and anxiety to lung cancer and smoking-related cancers. Our findings underline the importance of smoking cessation interventions for persons with depression or anxiety.