Do strigolactones contribute to plant defence?

Strigolactones are multifunctional molecules involved in several processes outside and within the plant. As signalling molecules in the rhizosphere, they favour the establishment of arbuscular mycorrhizal symbiosis, but they also act as host detection cues for root parasitic plants. As phytohormones, they are involved in the regulation of plant architecture, adventitious rooting, secondary growth and reproductive development, and novel roles are emerging continuously. In the present study, the possible involvement of strigolactones in plant defence responses was investigated. For this purpose, the resistance/susceptibility of the strigolactone‐deficient tomato mutant Slccd8 against the foliar fungal pathogens Botrytis cinerea and Alternaria alternata was assessed. Slccd8 was more susceptible to both pathogens, pointing to a new role for strigolactones in plant defence. A reduction in the content of the defence‐related hormones jasmonic acid, salicylic acid and abscisic acid was detected by high‐performance liquid chromatography coupled to tandem mass spectrometry in the Slccd8 mutant, suggesting that hormone homeostasis is altered in the mutant. Moreover, the expression level of the jasmonate‐dependent gene PinII, involved in the resistance of tomato to B. cinerea, was lower than in the corresponding wild‐type. We propose here that strigolactones play a role in the regulation of plant defences through their interaction with other defence‐related hormones, especially with the jasmonic acid signalling pathway.     

Plant biology: do DELLAs do defence?

DELLA proteins repress plant growth and developmental processes. Recent data suggest that DELLAs improve survival by imposing growth restraint during plant stress, enabling limited resources to be diverted to pathogen defence.     

Molluscan defence mechanisms: immunity or population biology?

In recent studies of associations between host and parasite, there has been a tendency to consider the internal defence mechanisms of the mollusc as pivotal in the relationship. However, evolution is a population-based phenomenon, and it is by considering populations rather than individuals that the success of these associations can best be appreciated. Among susceptible host species within the biocoenosis, truly resistant strains appear to be remarkably rare. Clive Shiff here examines mechanisms of defence or avoidance among molluscan hosts of tremotodes from both the internal and ecological perspective.     

Jasmonate in plant defence: sentinel or double agent?

Plants and their biotic enemies, such as microbial pathogens and herbivorous insects, are engaged in a desperate battle which would determine their survival–death fate. Plants have evolved efficient and sophisticated systems to defend against such attackers. In recent years, significant progress has been made towards a comprehensive understanding of inducible defence system mediated by jasmonate (JA), a vital plant hormone essential for plant defence responses and developmental processes. This review presents an overview of JA action in plant defences and discusses how microbial pathogens evade plant defence system through hijacking the JA pathway.     

Leaf domatia: carbon‐limited indirect defence?

Plant‐based defence mutualisms utilize plant morphology to reduce the performance of plant parasites through their natural enemies. Leaf domatia primarily occur in the axials of secondary veins and are often inhabited by microbivorous and predaceous mites which often increase plant growth rates and reproductive success by controlling plant pests. Our study investigated if domatia investment is limited by plant primary productivity. To our knowledge no studies have tested if foliar domatia are resource‐limited. We tested our hypothesis using the genus Coprosma (Rubiaceae), conducting correlative field surveys and manipulative experiments measuring domatia production in new leaves along temperature, nutrient and irradiance gradients. Field surveys indicated a strong positive association between leaf area, the number of secondary veins, and domatia per leaf. The number of potential sites for domatia is underutilised, with leaves on selected Coprosma species having on average 47 to 72% of the ‘maximum’ number of available sites where domatia could occur. Foliar carbon was positively associated with domatia investment. Coprosma plants held under elevated night‐time temperatures showed a 34–91% decrease in daily carbon gain, a 38% decrease in domatia per leaf mass, and a positive relationship between domatia investment and integrated daily carbon gain. Under irradiance and nutrient stress, our data indicated evidence of a positive relationship between domatia investment and foliar carbon. We found a significant negative association between relative investment in domatia produced and investment in new leaf biomass. Our findings suggest investment in foliar domatia is limited by primary productivity. We propose that domatia are discretionary goods and not intrinsic structures produced automatically on leaves that mites utilize. We suggest that plants have the ability to regulate domatia formation during leaf ontogeny, with investment controlled by resource availability and some intrinsic allocation mechanism to defence.     

Are there general patterns in plant defence against megaherbivores?

Field surveys were conducted to test whether plants deploy structural defences in ways that match the distribution of megaherbivores. In Western Australian scrublands, where adult plants are within the reach of megaherbivores, structural defences increased vertically and were deployed preferentially by adult plants. Conversely, in woodlands of Eastern Australia and California, where adult plants grow above the reach of megaherbivores, structural defences decreased vertically. Populations of closely‐related taxa on offshore islands exhibited significant reductions in defence in the absence of megaherbivores. The results also demonstrate that island plant taxa can evolve vertical changes in defence after colonizing continents, where they are exposed to megaherbivores. Overall, the results of the present study illustrate a complex array of spatial patterns in plant defence that match the distribution and foraging behaviour of large mammals. When interpreted alongside previous work demonstrating similar spatial patterns in other types of plant defence, the results may help to unify our understanding of how megaherbivores have shaped the evolution of plant form and function. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 38–48.     

Erythrophagocytosis in malaria: Host defence or menace to the macrophage?

Macrophages in the host's bloodstream and tissue serve as a first line of defence during infection with Plasmodium. While the killing effect of these cells on parasites has been investigated extensively, relatively little is known about the phagocytosis of infected red blood cells. In this article, Paolo Arese and Franca Turrini have joined Hagai Ginsburg to address the perplexing relationships between the macrophage and the malaria-infected red blood cell. They suggest that the same molecular mechanisms that normally operate to remove senescent or damaged red blood cells also operate during malaria, although the parasite may indirectly cause the destruction of macrophages.     

When do altricial birds reach maximum of their brood defence intensity?

It has been suggested that the brood defence by parents of altricial birds should increase during the breeding attempt until the young depart from the nest. The two proximate hypotheses provide alternative predictions about the peak of brood defence intensity: (1) the vulnerability hypothesis predicts a rapid rise in brood defence after hatching of the chicks, with maximum defence intensity just before fledging and strong decline afterwards; (2) the feedback hypothesis predicts that brood defence intensity will, after a rapid rise, reach a plateau at the end of the nestling period and early after fledging and then slowly decline. I compared brood defence behaviour of altricial meadow pipit (Anthus pratensis) breeding in the Czech Republic during the late nestling stage and during the fledging time. A stuffed stoat (Mustela erminea) was placed 5 m from a meadow pipit nest and the defence behaviour of parents was recorded for 10 min from a hide. Brood defence intensity was higher during the fledgling time than during the late nestling stage, and this trend was more evident in males than in females. Regardless of the proportion of already fledged chicks and those still present in the nest, brood defence did not significantly decrease during the fledgling time in males or females. The results do not agree with the predictions of the vulnerability hypothesis and support the predictions of the feedback hypothesis.     

Seedling–herbivore interactions: insights into plant defence and regeneration patterns

Background

Herbivores have the power to shape plant evolutionary trajectories, influence the structure and function of vegetation, devastate entire crops, or halt the spread of invasive weeds, and as a consequence, research into plant–herbivore interactions is pivotal to our understanding of plant ecology and evolution. However, the causes and consequences of seedling herbivory have received remarkably little attention, despite the fact that plants tend to be most susceptible to herbivory during establishment, and this damage can alter community composition and structure.

Scope

In this Viewpoint article we review why herbivory during early plant ontogeny is important and in so doing introduce an Annals of Botany Special Issue that draws together the latest work on the topic. In a synthesis of the existing literature and a collection of new studies, we examine several linked issues. These include the development and expression of seedling defences and patterns of selection by herbivores, and how seedling selection affects plant establishment and community structure. We then examine how disruption of the seedling–herbivore interaction might affect normal patterns of plant community establishment and discuss how an understanding of patterns of seedling herbivory can aid our attempts to restore semi-natural vegetation. We finish by outlining a number of areas where more research is required. These include a need for a deeper consideration of how endogenous and exogenous factors determine investment in seedling defence, particularly for the very youngest plants, and a better understanding of the phylogenetic and biogeographical patterns of seedling defence. There is also much still be to be done on the mechanisms of seedling selection by herbivores, particularly with respect to the possible involvement of volatile cues. These inter-related issues together inform our understanding of how seedling herbivory affects plant regeneration at a time when anthropogenic change is likely to disrupt this long-established, but all-too-often ignored interaction.     

Activation of defence reactions in Solanaceae: where is the specificity?

When a potential pathogen attempts to infect a plant, biochemical and molecular communication takes place and leads to the induction of plant defence mechanisms. In the case of efficient defence, visible symptoms are restricted and the pathogen does not multiply (incompatible interaction); when defence is inefficient, the plant becomes rapidly infected (compatible interaction). During the last 30 years, a growing body of knowledge on plant-pathogen interactions has been gathered, and a large number of studies investigate the induction of various plant defence reactions by pathogens or by pathogen-derived compounds. However, as most papers focus on incompatible interactions, there is still a lack of understanding about the similarities and differences between compatible and incompatible situations. This review targets the question of specificity in Solanaceae-pathogen interactions, by comparing defence patterns in plants challenged with virulent or avirulent pathogens (or with pathogen-associated molecular patterns from these). A special emphasis is made on analysing whether defence reactions in Solanaceae depend primarily on the type of elicitor, on the plant genotype/species, or on the type of interaction (compatible or incompatible).     

How effective were mangroves as a defence against the recent tsunami?

Whether or not mangroves function as buffers against tsunamis is the subject of in-depth research, the importance of which has been neglected or underestimated before the recent killer tsunami struck. Our preliminary post-tsunami surveys of Sri Lankan mangrove sites with different degrees of degradation indicate that human activity exacerbated the damage inflicted on the coastal zone by the tsunami.     

Do metal-rich plants deter herbivores? A field test of the defence hypothesis

Some plant species growing on metalliferous soils are able to accumulate heavy metals in their shoots up to very high concentrations, but the selective advantage of this behaviour is still unknown. The most popular hypothesis, that metals protect plants against herbivores, has been tested several times in laboratory conditions, with contradictory results. We carried out the first large-scale test of the defence hypothesis in eight natural populations of the model Zn hyperaccumulator Thlaspi caerulescens J. and C. Presl (Brassicaceae). In two climatic regions (temperate, Belgium–Luxembourg, and Mediterranean, southern France), we worked in metalliferous and in normal, uncontaminated environments, with plants spanning a wide range of Zn concentrations. We also examined the importance of glucosinolates (main secondary metabolites of Brassicaceae) as antiherbivore defences. When exposed to natural herbivore populations, T. caerulescens suffered lower herbivory pressures in metal-enriched soils than in normal soils, both in Belgium–Luxembourg and in southern France. The trapping of gastropods shows an overall lower population density in metalliferous compared to normal environments, which suggests that herbivory pressure from gastropods is lower on metalliferous soils. In addition, foliar concentration of glucosinolates was constitutively lower in all populations from metal-enriched soils, suggesting that these have evolved towards lower investment in organic defences in response to lower herbivory pressure. The Zn concentration of plants had a protective role only for Belgian metallicolous plants when transplanted in normal soils of Luxembourg. These results do not support the hypothesis that Zn plays a key role in the protection of T. caerulescens against enemies. In contrast, glucosinolates appear to be directly involved in the defence of this hyperaccumulator against herbivores.     

Contrasting consequences of different defence strategies in a natural multihost–parasite system

Hosts counteract infections using two distinct defence strategies, resistance (reduction in pathogen fitness) and tolerance (limitation of infection damage). These strategies have been minimally investigated in multi-host systems, where they may vary across host species, entailing consequences both for hosts (virulence) and parasites (transmission). Comprehending the interplay among resistance, tolerance, virulence and parasite success is highly relevant for our understanding of the ecology and evolution of infectious and parasitic diseases. Our work investigated the interaction between an insect parasite and its most common bird host species, focusing on two relevant questions: (i) are defence strategies different between main and alternative hosts and, (ii) what are the consequences (virulence and parasite success) of different defence strategies? We conducted a matched field experiment and longitudinal studies at the host and the parasite levels under natural conditions, using a system comprising Philornis torquans flies and three bird hosts – the main host and two of the most frequently used alternative hosts. We found that main and alternative hosts have contrasting defence strategies, which gave rise in turn to contrasting virulence and parasite success. In the main bird host, minor loss of fitness, no detectable immune response, and high parasite success suggest a strategy of high tolerance and negligible resistance. Alternative hosts, on the contrary, resisted by mounting inflammatory responses, although with very different efficiency, which resulted in highly dissimilar parasite success and virulence. These results show clearly distinct defence strategies between main and alternative hosts in a natural multi-host system. They also highlight the importance of defence strategies in determining virulence and infection dynamics, and hint that defence efficiency is a crucial intervening element in these processes.