No fitness cost for wheat's H gene-mediated resistance to Hessian fly (Diptera: Cecidomyiidae)

Resistance (R) genes have a proven record for protecting plants against biotic stress. A problem is parasite adaptation via Avirulence (Avr) mutations, which allows the parasite to colonize the R gene plant. Scientists hope to make R genes more durable by stacking them in a single cultivar. However, stacking assumes that R gene-mediated resistance has no fitness cost for the plant. We tested this assumption for wheat's resistance to Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae). Our study included ten plant fitness measures and four wheat genotypes, one susceptible, and three expressing either the H6, H9, or H13 resistance gene. Because R gene-mediated resistance has two components, we measured two types of costs: the cost of the constitutively-expressed H gene, which functions in plant surveillance, and the cost of the downstream induced responses, which were triggered by Hessian fly larvae rather than a chemical elicitor. For the constitutively expressed Hgene, some measures indicated costs, but a greater number of measures indicated benefits of simply expressing the H gene. For the induced resistance, instead of costs, resistant plants showed benefits of being attacked. Resistant plants were more likely to survive attack than susceptible plants, and surviving resistant plants produced higher yield and quality. We discuss why resistance to the Hessian fly has little or no cost and propose that tolerance is important, with compensatory growth occurring after H gene-mediated resistance kills the larva. We end with a caution: Given that plants were given good growing conditions, fitness costs may be found under conditions of greater biotic or abiotic stress.     

Plant–rhizobacteria interactions alleviate abiotic stress conditions

Root-colonizing non-pathogenic bacteria can increase plant resistance to biotic and abiotic stress factors. Bacterial inoculates have been applied as biofertilizers and can increase the effectiveness of phytoremediation. Inoculating plants with non-pathogenic bacteria can provide 'bioprotection' against biotic stresses, and some root-colonizing bacteria increase tolerance against abiotic stresses such as drought, salinity and metal toxicity. Systematic identification of bacterial strains providing cross-protection against multiple stressors would be highly valuable for agricultural production in changing environmental conditions. For bacterial cross-protection to be an effective tool, a better understanding of the underlying morphological, physiological and molecular mechanisms of bacterially mediated stress tolerance, and the phenomenon of cross-protection is critical. Beneficial bacteria-mediated plant gene expression studies under non-stress conditions or during pathogenic rhizobacteria–plant interactions are plentiful, but only few molecular studies on beneficial interactions under abiotic stress situations have been reported. Thus, here we attempt an overview of current knowledge on physiological impacts and modes of action of bacterial mitigation of abiotic stress symptoms in plants. Where available, molecular data will be provided to support physiological or morphological observations. We indicate further research avenues to enable better use of cross-protection capacities of root-colonizing non-pathogenic bacteria in agricultural production systems affected by a changing climate.     

The function of small RNAs in plant biotic stress response

Small RNAs(s RNAs) play essential roles in plants upon biotic stress. Plants utilize RNA silencing machinery to facilitate pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity to defend against pathogen attack or to facilitate defense against insect herbivores. Pathogens, on the other hand, are also able to generate effectors and s RNAs to counter the host immune response. The arms race between plants and pathogens/insect herbivores has triggered the evolution of s RNAs,RNA silencing machinery and pathogen effectors. A great number of studies have been performed to investigate the roles of s RNAs in plant defense, bringing in the opportunity to utilize s RNAs in plant protection. Transgenic plants with pathogen-derived resistance ability or transgenerational defense have been generated, which show promising potential as solutions for pathogen/insect herbivore problems in the field. Here we summarize the recent progress on the function of s RNAs in response to biotic stress, mainly in plant-pathogen/insect herbivore interaction,and the application of s RNAs in disease and insect herbivore control.     

Engineering Resistance Against Viruses in Field Crops Using CRISPR- Cas9

Food security is threatened by various biotic stresses that affect the growth and production of agricultural crops. Viral diseases have become a serious concern for crop plants as they incur huge yield losses. The enhancement of host resistance against plant viruses is a priority for the effective management of plant viral diseases. However, in the present context of the climate change scenario, plant viruses are rapidly evolving, resulting in the loss of the host resistance mechanism. Advances in genome editing techniques, such as CRISPR-Cas9 [clustered regularly interspaced palindromic repeats-CRISPR-associated 9], have been recognized as promising tools for the development of plant virus resistance. CRISPR-Cas9 genome editing tool is widely preferred due to high target specificity, simplicity, efficiency, and reproducibility. CRISPR-Cas9 based virus resistance in plants has been successfully achieved by gene targeting and cleaving the viral genome or altering the plant genome to enhance plant innate immunity. In this article, we have described the CRISPR-Cas9 system, mechanism of plant immunity against viruses and highlighted the use of the CRISPR-Cas9 system to engineer virus resistance in plants. We also discussed prospects and challenges on the use of CRISPR-Cas9-mediated plant virus resistance in crop improvement.     

Bacillus subtilis: A plant-growth promoting rhizobacterium that also impacts biotic stress

Plants encounter many biotic agents, such as viruses, bacteria, nematodes, weeds, and arachnids. These entities induce biotic stress in their hosts by disrupting normal metabolism, and as a result, limit plant growth and/or are the cause of plant mortality. Some biotic agents, however, interact symbiotically or synergistically with their host plants. Some microbes can be beneficial to plants and perform the same role as chemical fertilizers and pesticides, acting as a biofertilizer and/or biopesticide. Plant growth promoting rhizobacteria (PGPR) can significantly enhance plant growth and represent a mutually helpful plant-microbe interaction. Bacillus species are a major type of rhizobacteria that can form spores that can survive in the soil for long period of time under harsh environmental conditions. Plant growth is enhanced by PGPR through the induction of systemic resistance, antibiosis, and competitive omission. Thus, the application of microbes can be used to induce systemic resistance in plants against biotic agents and enhance environmental stress tolerance. Bacillus subtilis exhibits both a direct and indirect biocontrol mechanism to suppress disease caused by pathogens. The direct mechanism includes the synthesis of many secondary metabolites, hormones, cell-wall-degrading enzymes, and antioxidants that assist the plant in its defense against pathogen attack. The indirect mechanism includes the stimulation of plant growth and the induction of acquired systemic resistance. Bacillus subtilis can also solubilize soil P, enhance nitrogen fixation, and produce siderophores that promote its growth and suppresses the growth of pathogens. Bacillus subtilis enhances stress tolerance in their plant hosts by inducing the expression of stress-response genes, phytohormones, and stress-related metabolites. The present review discusses the activity of B. subtilis in the rhizosphere, its role as a root colonizer, its biocontrol potential, the associated mechanisms of biocontrol and the ability of B. subtilis to increase crop productivity under conditions of biotic and abiotic stress.     

The Chitinase A from the baculovirus AcMNPV enhances resistance to both fungi and herbivorous pests in tobacco

Biotechnology has allowed the development of novel strategies to obtain plants that are more resistant to pests, fungal pathogens and other agents of biotic stress. The obvious advantages of having genotypes with multiple beneficial traits have recently fostered the development of gene pyramiding strategies, but less attention has been given to the study of genes that can increase resistance to different types of harmful organisms. Here we report that a recombinant Chitinase A protein of the Autographa californica nuclear polyhedrosis virus (AcMNPV) has both antifungal and insecticide properties in vitro. Transgenic tobacco plants expressing an active ChiA protein showed reduced damages caused by fungal pathogens and lepidopteran larvae, while did not have an effect on aphid populations. To our knowledge, this is the first report on the characterisation and expression in plants of a single gene that increases resistance against herbivorous pests and fungal pathogens and not affecting non-target insects. The implications and the potential of the ChiA gene for plant molecular breeding and biotechnology are discussed.     

Effects of experimental inbreeding on herbivore resistance and plant fitness: the role of history of inbreeding, herbivory and abiotic factors

Inbreeding is common in plant populations and can affect plant fitness and resistance against herbivores. These effects are likely to depend on population history. In a greenhouse experiment with plants from 17 populations of Lychnis flos-cuculi, we studied the effects of experimental inbreeding on resistance and plant fitness. Depending on the levels of past herbivory and abiotic factors at the site of plant origin, we found either inbreeding or outbreeding depression in herbivore resistance. Furthermore, when not damaged experimentally by snail herbivores, plants from populations with higher heterozygosity suffered from inbreeding depression and those from populations with lower heterozygosity suffered from outbreeding depression. These effects of inbreeding and outbreeding were not apparent under experimental snail herbivory. We conclude that inbreeding effects on resistance and plant fitness depend on population history. Moreover, herbivory can mask inbreeding effects on plant fitness. Thus, understanding inbreeding effects on plant fitness requires studying multiple populations and considering population history and biotic interactions.     

Involvement and interaction of various signaling compounds on the plant metabolic events during defense response,resistance to stress factors,formation of secondary metabolites and their molecular aspects

Plants are a nearly unlimited source of phytochemicals. The plants produce various secondary metabolites, which are useful in its interaction with the environment, various stress factors and development of resistance against pathogen attack. A wide array of external stimuli are capable of triggering changes in the plant cell which leads to a cascade of reactions, ultimately resulting in the formation and accumulation of secondary metabolites which helps the plant to overcome the stress factors. The biotic and abiotic elicitors can result in an enhancement of the secondary metabolite production. The stimuli are perceived by receptors, which then result in the activation of the secondary messengers. These then transmit the signals into the cell through the signal transduction pathways leading to gene expression and biochemical changes. There is interplay of the signaling molecules also which regulates the entire pathway. This review is oriented towards the factors, which influence signal transduction pathway(s) with special reference to polyamines, calcium, jasmonates, salicylates, nitric oxide and ethylene. The interplay of these components to elicit a defense response is discussed. Molecular aspects of disease resistance and regulation of plant secondary metabolism has also been presented.     

Climate change and invasion by intracontinental range-expanding exotic plants: the role of biotic interactions

Background and Aims

In this Botanical Briefing we describe how the interactions between plants and their biotic environment can change during range-expansion within a continent and how this may influence plant invasiveness.

Scope

We address how mechanisms explaining intercontinental plant invasions by exotics (such as release from enemies) may also apply to climate-warming-induced range-expanding exotics within the same continent. We focus on above-ground and below-ground interactions of plants, enemies and symbionts, on plant defences, and on nutrient cycling.

Conclusions

Range-expansion by plants may result in above-ground and below-ground enemy release. This enemy release can be due to the higher dispersal capacity of plants than of natural enemies. Moreover, lower-latitudinal plants can have higher defence levels than plants from temperate regions, making them better defended against herbivory. In a world that contains fewer enemies, exotic plants will experience less selection pressure to maintain high levels of defensive secondary metabolites. Range-expanders potentially affect ecosystem processes, such as nutrient cycling. These features are quite comparable with what is known of intercontinental invasive exotic plants. However, intracontinental range-expanding plants will have ongoing gene-flow between the newly established populations and the populations in the native range. This is a major difference from intercontinental invasive exotic plants, which become more severely disconnected from their source populations.     

The molecular bases of plant resistance and defense responses to aphid feeding: current status

Plant genes participating in the recognition of aphid herbivory in concert with plant genes involved in defense against herbivores mediate plant resistance to aphids. Several such genes involved in plant disease and nematode resistance have been characterized in detail, but their existence has only recently begun to be determined for arthropod resistance. Hundreds of different genes are typically involved and the disruption of plant cell wall tissues during aphid feeding has been shown to induce defense responses in Arabidopsis, Triticum, Sorghum, and Nicotiana species. Mi‐1.2, a tomato gene for resistance to the potato aphid, Macrosiphum euphorbiae (Thomas), is a member of the nucleotide‐binding site and leucine‐rich region Class II family of disease, nematode, and arthropod resistance genes. Recent studies into the differential expression of Pto‐ and Pti1‐like kinase genes in wheat plants resistant to the Russian wheat aphid, Diuraphis noxia (Mordvilko), provide evidence of the involvement of the Pto class of resistance genes in arthropod resistance. An analysis of available data suggests that aphid feeding may trigger multiple signaling pathways in plants. Early signaling includes gene‐for‐gene recognition and defense signaling in aphid‐resistant plants, and recognition of aphid‐inflicted cell damage in both resistant and susceptible plants. Furthermore, signaling is mediated by several compounds, including jasmonic acid, salicylic acid, ethylene, abscisic acid, giberellic acid, nitric oxide, and auxin. These signals lead to the development of direct chemical defenses against aphids and general stress‐related responses that are well characterized for a number of abiotic and biotic stresses. In spite of major plant taxonomic differences, similarities exist in the types of plant genes expressed in response to feeding by different species of aphids. However, numerous differences in plant signaling and defense responses unique to specific aphid–plant interactions have been identified and warrant further investigation.     

Priming of antiherbivore defensive responses in plants

Defense priming is defined as increased readiness of defense induction. A growing body of literature indicates that plants (or intact parts of a plant) are primed in anticipation of impending environmental stresses, both biotic and abiotic, and upon the following stimulus, induce defenses more quickly and strongly. For instance, some plants previously exposed to herbivore‐inducible plant volatiles (HIPVs) from neighboring plants under herbivore attack show faster or stronger defense activation and enhanced insect resistance when challenged with secondary insect feeding. Research on priming of antiherbivore defense has been limited to the HIPV‐mediated mechanism until recently, but significant advances were made in the past three years, including non‐HIPV‐mediated defense priming, epigenetic modifications as the molecular mechanism of priming, and others. It is timely to consider the advances in research on defense priming in the plant–insect interactions.     

Recent advances in bio‐chemical,molecular and physiological aspects of membrane lipid derivatives in plant pathology

Plant pathogens pose a significant threat to the food industry and food security accounting for 10–40% crop losses annually on a global scale. Economic losses from plant diseases are estimated at $300B for major food crops and are associated with reduced food availability and accessibility and also high food costs. Although strategies exist to reduce the impact of diseases in plants, many of these introduce harmful chemicals to our food chain. Therefore, it is important to understand and utilize plants' immune systems to control plant pathogens to enable more sustainable agriculture. Lipids are core components of cell membranes and as such are part of the first line of defense against pathogen attack. Recent developments in omics technologies have advanced our understanding of how plant membrane lipid biosynthesis, remodelling and/or signalling modulate plant responses to infection. Currently, there is limited information available in the scientific literature concerning lipid signalling targets and their biochemical and physiological consequences in response to plant pathogens. This review focusses on the functions of membrane lipid derivatives and their involvement in plant responses to pathogens as biotic stressors. We describe major plant defense systems including systemic‐acquired resistance, basal resistance, hypersensitivity and the gene‐for‐gene concept in this context.     

Synergies and trade-offs between insect and pathogen resistance in maize leaves and roots

Determining links between plant defence strategies is important to understand plant evolution and to optimize crop breeding strategies. Although several examples of synergies and trade-offs between defence traits are known for plants that are under attack by multiple organisms, few studies have attempted to measure correlations of defensive strategies using specific single attackers. Such links are hard to detect in natural populations because they are inherently confounded by the evolutionary history of different ecotypes. We therefore used a range of 20 maize inbred lines with considerable differences in resistance traits to determine if correlations exist between leaf and root resistance against pathogens and insects. Aboveground resistance against insects was positively correlated with the plant's capacity to produce volatiles in response to insect attack. Resistance to herbivores and resistance to a pathogen, on the other hand, were negatively correlated. Our results also give first insights into the intraspecific variability of root volatiles release in maize and its positive correlation with leaf volatile production. We show that the breeding history of the different genotypes (dent versus flint) has influenced several defensive parameters. Taken together, our study demonstrates the importance of genetically determined synergies and trade-offs for plant resistance against insects and pathogens.     

Plant immunization

Plant immunization is the process of activating natural defense system present in plant induced by biotic or abiotic factors. Plants are pre-treated with inducing agents stimulate plant defense responses that form chemical or physical barriers that are used against the pathogen invasion. Inducers used usually give the signals to rouse the plant defense genes ultimately resulting into induced systemic resistance. In many plant-pathogen interactions, R-Avr gene interactions results in localized acquired resistance or hypersensitive response and at distal ends of plant, a broad spectrum resistance is induced known as systemic acquired resistance (SAR). Various biotic or abiotic factors induce systemic resistance in plants that is phenotypically similar to pathogen-induced systemic acquired resistance (SAR). Some of the biotic or abiotic determinants induce systemic resistance in plants through salicylic acid (SA) dependent SAR pathway, others require jasmonic acid (JA) or ethylene. Host plant remains in induced condition for a period of time, and upon challenge inoculation, resistance responses are accelerated and enhanced. Induced systemic resistance (ISR) is effective under field conditions and offers a natural mechanism for biological control of plant disease.     

Effects of plant quality and ant defence on herbivory rates in a neotropical ant‐plant

1. Understanding the degree to which populations and communities are limited by both bottom‐up and top‐down effects is still a major challenge for ecologists, and manipulation of plant quality, for example, can alter herbivory rates in plants. In addition, biotic defence by ants can directly influence the populations of herbivores, as demonstrated by increased rates of herbivory or increased herbivore density after ant exclusion. The aim of this study was to evaluate bottom‐up and top‐down effects on herbivory rates in a mutualistic ant‐plant. 2. In this study, the role of Azteca alfari ants as biotic defence in individuals of Cecropia pachystachya was investigated experimentally with a simultaneous manipulation of both bottom‐up (fertilisation) and top‐down (ant exclusion) factors. Four treatments were used in a fully factorial design, with 15 replicates for each treatment: (i) control plants, without manipulation; (ii) fertilised plants, ants not manipulated; (iii) unfertilised plants and excluded ants and (iv) fertilised plants and ants excluded. 3. Fertilisation increased the availability of foliar nitrogen in C. pachystachya, and herbivory rates by chewing insects were significantly higher in fertilised plants with ants excluded. 4. Herbivory, however, was more influenced by bottom‐up effects – such as the quality of the host plant – than by top‐down effects caused by ants as biotic defences, reinforcing the crucial role of leaf nutritional quality for herbivory levels experienced by plants. Conditionality in ant defence under increased nutritional quality of leaves through fertilisation might explain increased levels of herbivory in plants with higher leaf nitrogen.     

Genetic contributions to agricultural sustainability

The current tools of enquiry into the structure and operation of the plant genome have provided us with an understanding of plant development and function far beyond the state of knowledge that we had previously. We know about key genetic controls repressing or stimulating the cascades of gene expression that move a plant through stages in its life cycle, facilitating the morphogenesis of vegetative and reproductive tissues and organs. The new technologies are enabling the identification of key gene activity responses to the range of biotic and abiotic challenges experienced by plants. In the past, plant breeders produced new varieties with changes in the phases of development, modifications of plant architecture and improved levels of tolerance and resistance to environmental and biotic challenges by identifying the required phenotypes in a few plants among the large numbers of plants in a breeding population. Now our increased knowledge and powerful gene sequence-based diagnostics provide plant breeders with more precise selection objectives and assays to operate in rationally planned crop improvement programmes. We can expect yield potential to increase and harvested product quality portfolios to better fit an increasing diversity of market requirements. The new genetics will connect agriculture to sectors beyond the food, feed and fibre industries; agri-business will contribute to public health and will provide high-value products to the pharmaceutical industry as well as to industries previously based on petroleum feedstocks and chemical modification processes.     

植物内生菌及其防治植物病害的研究进展

综述了植物内生菌及其防治植物病害的研究进展.植物内生菌分布广,种类多,几乎存在于所有目前已研究过的陆生及水生植物中,目前全世界至少已在80个属290多种禾本科植物中发现有内生真菌,在各种农作物及经济作物中发现的内生细菌已超过120种.感染内生菌的植物宿主往往具有生长快速、抗逆境、抗病害、抗动物危害等优势,比未感染内生菌的植株更具生存竞争力.植物内生菌的防病机理主要表现在通过产生抗生素类,水解酶类,植物生长调节剂和生物碱类物质,与病原菌竞争营养物质,增强宿主植物的抵抗力以及诱导植物产生系统抗性等途径抑制病原菌生长.另外,对植物内生真菌和内生细菌的分离、筛选和检测方法;利用植物内生菌控制植物病害的途径如人工接种内生菌,利用内生菌代谢产生的抗生素以及将内生菌作为基因工程的载体菌等进行了综述.同时,对植物内生菌作为生物防治因子未来发展前景及存在的问题进行了讨论.利用植物内生菌作为生物防治因子进行大田防病,需要考虑它的病理学、生态学和形态学等方面的影响.     

Parents lend a helping hand to their offspring in plant defence

Plants under attack by pathogens and pests can mount a range of inducible defences, encompassing both chemical and structural changes. Although few reports exist, it appears that plants responding to pathogen or herbivore attack, or chemical defence elicitors, may produce progeny that are better able to defend themselves against attack, compared with progeny from unthreatened or untreated plants. To date, all research on transgenerational effects of biotic stress has been conducted on dicotyledenous plants. We examined the possibility that resistance induced by application of chemical defence elicitors to the monocot plant barley, could be passed on to the progeny. Plants were treated with acibenzolar-S-methyl (ASM) or saccharin, and grain harvested at maturity. Germination was unaffected in seed collected from plants treated with saccharin, while germination was reduced significantly in seed collected from ASM-treated plants. The subsequent growth of the seedlings was not significantly different in any of the treatments. However, plants from parents treated with both ASM or saccharin exhibited significantly enhanced resistance to infection by Rhynchosporium commune, despite not being treated with elicitor themselves. These data hint at the possibility of producing disease-resistant plants by exposing parent plants to chemical elicitors.     

Molecular defense responses in roots and the rhizosphere against Fusarium oxysporum

Plants face many different concurrent and consecutive abiotic and biotic stresses during their lifetime. Roots can be infected by numerous pathogens and parasitic organisms. Unlike foliar pathogens, root pathogens have not been explored enough to fully understand root-pathogen interactions and the underlying mechanism of defense and resistance. PR gene expression, structural responses, secondary metabolite and root exudate production, as well as the recruitment of plant defense–assisting “soldier” rhizosphere microbes all assist in root defense against pathogens and herbivores. With new high-throughput molecular tools becoming available and more affordable, now is the opportune time to take a deep look below the ground. In this addendum, we focus on soil-borne Fusarium oxysporum as a pathogen and the options plants have to defend themselves against these hard-to-control pathogens.