Natural variation in Arabidopsis ISR1 affects iron localization and induced systemic resistance

Beneficial root-associated bacteria can induce systemic resistance (ISR) to foliar pathogens and there is known transcriptional and genetic overlap in the root response to iron deficiency and ISR. A previous study found that there is natural variation in ISR among Arabidopsis accessions. The Ws accession is deficient in ISR, and the responsible recessive genetic locus, named ISR1, was mapped to chromosome 3. To find candidate genes that may underlie ISR deficiency in Ws, we identified genes that are induced in response to the ISR-triggering bacterium Pseudomonas simiae WCS417 and to iron stress and that have non-synonymous mutations in the Ws genome with respect to the ISR-responsive Col-0. We identified a kelch-domain containing protein encoded by At3g07720 that has a genomic rearrangement in Ws. We found that overexpression of Col-0 At3g07720 restores ISR to Ws, indicating that At3g07720 encodes ISR1. Isr1 loss of function mutants do not affect plant growth under iron limiting conditions but have increased levels of apoplastic iron. We found that iron supplementation, P. simiae WCS417, or a loss of isr1 enhance ROS production in a non-additive manner, suggesting they work through the same mechanism to enhance resistance. Our findings show that ISR1 is required for iron localization, immunity, and ISR, and suggest that increased iron uptake induced by ISR-eliciting bacteria may directly contribute to immunity through increased reactive oxygen production.

Seasonal Dynamics of Fungi Associated with Healthy and Diseased Pinus sylvestris Needles in Northern Europe

The relationship between the ecological success of needle pathogens of forest trees and species richness of co-inhabiting endophytic fungi is poorly understood. One of the most dangerous foliar pathogens of pine is Dothistroma septosporum, which is a widely spread threat to northern European forests. We sampled two Pinus sylvestris sites in Estonia and two in Norway in order to analyse the relations between the abundance of D. septosporum and overall fungal richness, specific fungal species composition, time of season, needle age and position in the canopy. In both countries, the overall species richness of fungi was highest in autumn, showing a trend of increase with needle age. The overall species richness in the second-year needles in Estonia and third-year needles in Norway was similar, suggesting that a critical colonization threshold for needle shed in P. sylvestris is breached earlier in Estonia than in Norway. The fungal species richness in P. sylvestris needles was largely affected by Lophodermium conigenum. Especially in older needles, the relative abundance of L. conigenum was significantly higher in spring compared to summer or autumn. The timing of recruitment and colonization mechanisms of different foliage endophytes are shortly discussed.

Evaluation of Electrolysed Oxidizing Water for European Pear Rust Management on Pear (Pyrus cv. Conference)

Pear rust is a foliar pathogen of ornamental and fruiting pear trees. Unmanaged, yield and aesthetic losses can be severe. Over-reliance on synthetic fungicides means novel means of pathogen management are required. Field trials were conducted using pear (Pyrus cv. Conference) to assess the efficacy of electrolysed oxidizing water (EO water) as a rust protective compound. A synthetic fungicide (boscalid + pyraclostrobin) spray program used for pear rust management was included for comparison. Each treatment was applied 4 times prior to the visible appearance of rust (April through June, i.e., preventatively). Studies were conducted in 2017 and repeated in 2018. Efficacy of EO water as a rust protectant compound was confirmed (increased leaf chlorophyll content, reduced leaf rust incidence and severity). The degree of leaf rust severity protection conferred was not statistically different from a boscalid + pyraclostrobin spray program. Results suggest 4 spray applications of EO water provides a useful addition to existing methods of pear rust management under field conditions that may have applicability against other foliar pathogens.

Evaluation of Inducing Agents and Synthetic Fungicide Combinations for Management of Foliar Pathogens of Urban Trees

Unmanaged, foliar pathogens of urban trees can be detrimental to tree health and aesthetics. Overreliance on synthetic fungicides increasingly means alternative means of pathogen management are now required. The purpose of these studies was to investigate the efficacy of 3 commercially available agents, harpin protein, salicylic acid derivative, and liquid chitosan, which can initiate induced resistance (IR) in plants. IR agents were applied independently and in combination with a synthetic fungicide (boscalid + pyraclostrobin) against 2 foliar pathogens (Venturia pirina and Guignardia aesculi) under field conditions with Pyrus communis ‘Williams’ Bon Chrétien’ and horse chestnut (Aesculus hippocastanum) acting as tree hosts. These agents were tested over 3 consecutive years. In 4 of 5 field studies, the use of an IR agent alone reduced pathogen symptom severity, increased fruit/seed yield, and enhanced leaf chlorophyll content. In virtually all studies, application of boscalid + pyraclostrobin at 2/3 strength plus an IR agent provided the same degree of pathogen control as boscalid + pyraclostrobin at full strength. Application of boscalid + pyraclostrobin at 1/3 strength plus an IR agent provided a reasonable degree of foliar pathogen control. Results showed that a combined mix of an IR agent with a 1/3 reduced dose of boscalid + pyraclostrobin was as effective at reducing symptom severity of 2 foliar pathogens as boscalid + pyraclostrobin applied at full strength, provided at least 4 sprays were applied during a growing season.

Mapping and Validation of Major Quantitative Trait Loci for Resistance to Northern Corn Leaf Blight Along With the Determination of the Relationship Between Resistances to Multiple Foliar Pathogens of Maize (Zea mays L.)

Among various foliar diseases affecting maize yields worldwide, northern corn leaf blight (NCLB) is economically important. The genetics of resistance was worked out to be quantitative in nature thereby suggesting the need for the detection of quantitative trait loci (QTL) to initiate effective marker-aided breeding strategies. From the cross CML153 (susceptible) × SKV50 (resistant), 344 F2:3 progenies were derived and screened for their reaction to NCLB during the rainy season of 2013 and 2014. The identification of QTL affecting resistance to NCLB was carried out using the genetic linkage map constructed with 194 polymorphic SNPs and the disease data recorded on F2:3 progeny families. Three QTL for NCLB resistance were detected on chromosomes 2, 5, and 8 with the QTL qNCLB-8-2 explaining the highest phenotypic variation of 16.34% followed by qNCLB-5 with 10.24%. QTL for resistance to sorghum downy mildew (SDM) and southern corn rust (SCR) were also identified from one season phenotypic data, and the co-location of QTL for resistance to three foliar diseases was investigated. QTL present in chromosome bins 8.03, 5.03, 5.04, and 3.04 for resistance to NCLB, SDM, and SCR were co-localized, indicating their usefulness for the pyramiding of quantitative resistance to multiple foliar pathogens. Marker-assisted selection was practiced in the crosses CM212 × SKV50, HKI162 × SKV50, and CML153 × SKV50 employing markers linked to major QTL on chromosomes 8, 2, and 10 for NCLB, SDM, and SCR resistance, respectively. The populations were advanced to F6 stage to derive multiple disease-resistant inbred lines. Out of the 125 lines developed, 77 lines were tested for their combining ability and 39 inbred lines exhibited high general combining ability with an acceptable level of resistance to major diseases.

Amplicon Sequencing Reveals Extensive Coinfections of Foliar Pathogens in Soybean

Soybean (Glycine max) is one of the most economically important crops grown in North America and in other regions worldwide. However, the plant is susceptible to a variety of foliar pathogenic microorganisms, some of which are a significant threat to production. Several molecular and serological approaches are currently available to diagnose plant pathogens, but all have limitations including their capability to accurately detect coinfections of individual plants. We therefore used 16S and internal transcribed spacer amplicon sequencing to identify the suite of bacterial and fungal organisms infecting 96 soybean leaf samples collected throughout southern Manitoba, Canada, at two growth stages (V2/3 and R6). We were able to confirm the presence of pathogens previously known to our sampling regions, such as Septoria glycines, Alternaria alternata, and Pseudomonas spp. Importantly, we found that most of plants were infected by more than one putative pathogen, with 64% of V2/3 and 26% of R6 plants infected by three or more pathogens. Amplicon sequencing also indicated the presence of residual pathogens that infect crops other than soybean, as well as nonfoliar pathogens and nonpathogenic microorganisms. We discuss some of the benefits and drawbacks of using amplicon sequencing to detect foliar pathogens of soybean.

Tomato Domestication Attenuated Responsiveness to a Beneficial Soil Microbe for Plant Growth Promotion and Induction of Systemic Resistance to Foliar Pathogens

Crop domestication events followed by targeted breeding practices have been pivotal for improvement of desirable traits and to adapt cultivars to local environments. Domestication also resulted in a strong reduction in genetic diversity among modern cultivars compared to their wild relatives, though the effect this could have on tripartite relationships between plants, belowground beneficial microbes and aboveground pathogens remains undetermined. We quantified plant growth performance, basal resistance and induced systemic resistance (ISR) by Trichoderma harzianum, a beneficial soil microbe against Botrytis cinerea, a necrotrophic fungus and Phytophthora infestans, a hemi-biotrophic oomycete, in 25 diverse tomato genotypes. Wild tomato related species, tomato landraces and modern commercial cultivars that were conventionally or organically bred, together, representing a domestication gradient were evaluated. Relationships between basal and ISR, plant physiological status and phenolic compounds were quantified to identify potential mechanisms. Trichoderma enhanced shoot and root biomass and ISR to both pathogens in a genotype specific manner. Moreover, improvements in plant performance in response to Trichoderma gradually decreased along the domestication gradient. Wild relatives and landraces were more responsive to Trichoderma, resulting in greater suppression of foliar pathogens than modern cultivars. Photosynthetic rate and stomatal conductance of some tomato genotypes were improved by Trichoderma treatment whereas leaf nitrogen status of the majority of tomato genotypes were not altered. There was a negative relationship between basal resistance and induced resistance for both diseases, and a positive correlation between Trichoderma-ISR to B. cinerea and enhanced total flavonoid contents. These findings suggest that domestication and breeding practices have altered plant responsiveness to beneficial soil microbes. Further studies are needed to decipher the molecular mechanisms underlying the differential promotion of plant growth and resistance among genotypes, and identify molecular markers to integrate selection for responsiveness into future breeding programs.

Siliceous Natural Nanomaterials as Biorationals—Plant Protectants and Plant Health Strengtheners

This review aims to evaluate the scientific evidences for siliceous natural nanomaterials (SNNMs), natural zeolites, and diatomaceous earth, as biorationals. Both SNNMs are multifaceted agricultural inputs—plant protectants, plant biostimulants/plant strengtheners, soil improvers. The effects depend on the plant parts, where such siliceous natural nanomaterials (SNNMs) are applied. For stored grains, SNNMs act as plant protectants. Foliar applied SNNMs protect plants against biotic and abiotic stress—plant protectant and plant strengtheners. When applied to soil/roots, SNNMs stimulate root development and improve soil characteristics. These effects are related to the composition and porous (nano)structure of SNNMs. The large active siliceous surfaces of SNNMs are involved in: desiccation of the insects damaging stored grains, fungistatic effects against mycotoxigenic fungi and adsorption of their mycotoxins, desiccation of foliar pathogens and pests, stimulation of photosynthesis, release of soluble silicon species, improved soil characteristics. Similar to other biorationals from the category of basic substances with low risk, the SNNMs efficacy as plant protectants and plant health strengtheners is rather low. Complementary active ingredients should be used to enhance the effects of SNNMs on treated plants. For SNNMs applied as protectants of stored seeds, such strategy, of using complementary biorationals/low risk substances, proved to be highly effective.