Why are the inner and outer sides of many flower petals differently coloured?

The flower perianth has various, non-mutually exclusive functions, such as visual signalling to pollinators and protecting the reproductive organs from the elements and from florivores, but how different perianth structures and their different sides play a role in these functions is unclear. Intriguingly, in many species there is a clear colour difference between the different sides of the perianth, with colour patterns or pigmentation present on only one side. Any adaptive benefit from such colour asymmetry is unclear, as is how the asymmetry evolved. In this viewpoint paper, we address the phenomenon of flowers with differently coloured inner and outer perianth sides, focusing on petals of erect flowers. Guided by existing literature and our own observations, we delineate three non-mutually exclusive evolutionary hypotheses that may explain the factors underlying differently coloured perianth sides. The pollen-protection hypothesis predicts that the outer side of petals contributes to protect pollen against UV radiation, especially during the bud stage. The herbivore-avoidance hypothesis predicts that the outer side of petals reduces the flower's visibility to herbivores. The signalling-to-pollinators hypothesis predicts that flower colours evolve to increase conspicuousness to pollinators. The pollen-protection hypothesis, the herbivore-avoidance hypothesis, and the signalling-to-pollinators hypothesis generate largely but not entirely overlapping predictions about the colour of the inner and outer side of the petals. Field and laboratory research is necessary to disentangle the main drivers and adaptive significance of inner-outer petal side colour asymmetry.

Aristolochia mimics stink bugs to repel vertebrate herbivores via TRPA1 activation

Mimicry is the phenomenon in which one species (the mimic) closely resembles another (the model), enhancing its own fitness by deceiving a third party into interacting with it as if it were the model. In plants, mimicry is used primarily to gain fitness by withholding rewards from mutualists or deterring herbivores cost-effectively. While extensive work has been documented on putative defence mimicry, limited investigation has been conducted in the field of chemical mimicry. In this study, we used field experiments, chemical analyses, behavioural assays, and electrophysiology, to test the hypothesis that the birthwort Aristolochia delavayi employs chemical mimicry by releasing leaf scent that closely resembles stink bug defensive compounds and repels vertebrate herbivores. We show that A. delavayi leaf scent is chemically and functionally similar to the generalized defensive volatiles of stink bugs and that the scent effectively deters vertebrate herbivores, likely through the activation of TRPA1 channels via (E)-2-alkenal compounds. This study provides an unequivocal example of chemical mimicry in plants, revealing intricate dynamics between plants and vertebrate herbivores. Our study underscores the potency of chemical volatiles in countering vertebrate herbivory, urging further research to uncover their potentially underestimated importance.

Visual-, Olfactory-, and Nectar-Taste-Based Flower Aposematism

Florivory, i.e., flower herbivory, of various types is common and can strongly reduce plant fitness. Flowers suffer two very different types of herbivory: (1) the classic herbivory of consuming tissues and (2) nectar theft. Unlike the non-reversibility of consumed tissues, nectar theft, while potentially reducing a plant's fitness by lowering its attraction to pollinators, can, in various cases, be fixed quickly by the production of additional nectar. Therefore, various mechanisms to avoid or reduce florivory have evolved. Here, I focus on one of the flowers' defensive mechanisms, aposematism, i.e., warning signaling to avoid or at least reduce herbivory via the repelling of herbivores. While plant aposematism of various types was almost ignored until the year 2000, it is a common anti-herbivory defense mechanism in many plant taxa, operating visually, olfactorily, and, in the case of nectar, via a bitter taste. Flower aposematism has received only very little focused attention as such, and many of the relevant publications that actually demonstrated herbivore repellence and avoidance learning following flower signaling did not refer to repellence as aposematism. Here, I review what is known concerning visual-, olfactory-, and nectar-taste-based flower aposematism, including some relevant cases of mimicry, and suggest some lines for future research.

Volatile-mediated plant-plant communication and higher-level ecological dynamics

Volatile organic compounds (VOCs) in general and herbivory-induced plant volatiles (HIPVs) in particular are increasingly understood as major mediators of information transfer between plant tissues. Recent findings have moved the field of plant communication closer to a detailed understanding of how plants emit and perceive VOCs and seem to converge on a model that juxtaposes perception and emission mechanisms. These new mechanistic insights help to explain how plants can integrate different types of information and how environmental noise can affect the transmission of information. At the same time, ever-new functions of VOC-mediated plant-plant interactions are being revealed. Chemical information transfer between plants is now known to fundamentally affect plant organismal interactions and, additionally, population, community, and ecosystem dynamics. One of the most exciting new developments places plant-plant interactions along a behavioral continuum with an eavesdropping strategy at one end and mutually beneficial information-sharing among plants within a population at the other. Most importantly and based on recent findings as well as theoretical models, plant populations can be predicted to evolve different communication strategies depending on their interaction environment. We use recent studies from ecological model systems to illustrate this context dependency of plant communication. Moreover, we review recent key findings about the mechanisms and functions of HIPV-mediated information transfer and suggest conceptual links, such as to information theory and behavioral game theory, as valuable tools for a deeper understanding of how plant-plant communication affects ecological and evolutionary dynamics.

Eco-Evo-Devo of petal pigmentation patterning

Colourful spots, stripes and rings decorate the corolla of most flowering plants and fulfil important biotic and abiotic functions. Spatial differences in the pigmentation of epidermal cells can create these patterns. The last few years have yielded new data that have started to illuminate the mechanisms controlling the function, formation and evolution of petal patterns. These advances have broad impacts beyond the immediate field as pigmentation patterns are wonderful systems to explore multiscale biological problems: from understanding how cells make decisions at the microscale to examining the roots of biodiversity at the macroscale. These new results also reveal there is more to petal patterning than meets the eye, opening up a brand new area of investigation. In this mini-review, we summarise our current knowledge on the Eco-Evo-Devo of petal pigmentation patterns and discuss some of the most exciting yet unanswered questions that represent avenues for future research.

Evolution and Ecology of Parasite Avoidance

Parasite avoidance is a host defense that reduces the contact rate with parasites. We investigate avoidance as a primary driver of variation among individuals in the risk of parasitism and the evolution of host-parasite interactions. To bridge mechanistic and taxonomic divides, we define and categorize avoidance by its function and position in the sequence of host defenses. We also examine the role of avoidance in limiting epidemics and evaluate evidence for the processes that drive its evolution. Throughout, we highlight important directions to advance our conceptual and theoretical understanding of the role of avoidance in host-parasite interactions. We emphasize the need to test assumptions and quantify the effect of avoidance independent of other defenses. Importantly, many open questions may be most tractable in host systems that have not been the focus of traditional behavioral avoidance research, such as plants and invertebrates.

A new conceptual and quantitative approach to exploring and defining potential open-access olfactory information

All organisms emit odour, providing 'open-access' olfactory information for any receiver with the right sensory apparatus. Characterizing open-access information emitted by groups of organisms, such as plant species, provides the means to answer significant questions about ecological interactions and their evolution. We present a new conceptual framework defining information reliability and a practical method to characterize and recover information from amongst olfactory noise. We quantified odour emissions from two tree species, one focal group and one outgroup, to demonstrate our approach using two new R statistical functions. We explore the consequences of relaxing or tightening criteria defining information and, from thousands of odour combinations, we identify and quantify those few likely to be informative. Our method uses core general principles characterizing information while incorporating knowledge of how receivers detect and discriminate odours. We can now map information in consistency-precision reliability space, explore the concept of information, and test information-noise boundaries, and between cues and signals.

The phenomenon of red and yellow autumn leaves: Hypotheses, agreements and disagreements

Yellow and red autumn leaves are typical of many temperate/boreal woody plants. Since the 19^th century, it has been either considered the non-functional outcome of chlorophyll degradation that unmasks the pre-existing yellow and red pigments or that the de novo synthesis of red anthocyanins in autumn leaves indicated that it should have a physiological function, although it was commonly ignored. Defending free amino acids and various other resources released especially following the breakdown of the photosynthetic system, and mobilizing them for storage in other organs before leaf fall, is the cornerstone of both the physiological and anti-herbivory hypotheses about the functions of yellow and red autumn leaf colouration. The complicated phenomenon of conspicuous autumn leaf colouration has received significant attention since the year 2000, especially because ecologists started paying attention to its anti-herbivory potential. The obvious imperfection of the hypotheses put forth in several papers stimulated many other scientists. Hot debates among physiologists, among ecologists, and between physiologists and ecologists have been common since the year 2000, first because the various functions of yellow and red autumn leaf colouration are non-exclusive, and second because many scientists were trained to focus on a single subject. Here, I will review the debates, especially between the photoprotective and the anti-herbivory hypotheses, and describe both the progress in their understanding and the required progress.

Responses of Adult Hypera rumicis L. to Synthetic Plant Volatile Blends

The behavioral responses of Hypera rumicis L. adults to varying blends of synthetic plant volatiles (SPVs) at various concentrations in lieu of single compounds are reported for the first time. For this study, Rumex confertus plants were treated with two blends of SPVs at different quantities that act as either attractants or repellents to insects. Blend 1 (B1) consisted of five green leaf volatiles (GLVs), namely (Z)-3-hexenal, (E)-2-hexenal, (Z)-3-hexenol, (E)-2-hexenol, and (Z)-3-hexen-1-yl acetate. Blend 2 (B2) contained six plant volatiles, namely (Z)-ocimene, linalool, benzyl acetate, methyl salicylate, β-caryophyllene, and (E)-β-farnesene. Each blend was made available in four different amounts of volatiles, corresponding to each compound being added to 50 µL of hexane in amounts of 1, 5, 25 and 125 ng. The effects of the two blends at the different concentrations on the insects were evaluated using a Y-tube olfactometer. Both sexes of the insects were found to be significantly repelled by the highest volatile levels of B1 and by two levels of B2 (25 and 125 ng). Females were also observed to be repelled using B2 with 5 ng of each volatile. Attraction was observed for both sexes only for B1 at the three lower volatile levels (1, 5 and 25 ng). In additional experiments, using only attractants, unmated females were found to be attracted to males, whereas mated females were only attracted to B1. Both unmated and mated males (previously observed in copula) were attracted only to females.

Scale-dependent trends in the investment of leaf domatia

Theory predicts that plants invest in defences proportional to the value or amount of tissue at risk. Domatia-bearing plants house predatory arthropods that defend against insect and fungal attack. Though leaf domatia represent a direct investment in the defence of leaf tissues, it remains unknown whether domatia production scales with amount of tissue at risk. I investigated how domatia investment scales with leaf size in 20 species of trees and shrubs from the south-west Pacific. Large-leaved species produced more domatia than smaller leaved species. However, domatia production did not consistently scale with leaf area among individuals of the same species, illustrating that trends in domatia investment are scale-dependent. Overall results suggest the processes modulating the allocation of resources to defence at the interspecific level are distinct from those operating at the intraspecific level.