Comparative Hessian Fly Larval Transcriptomics Provides Novel Insight into Host and Nonhost Resistance

The Hessian fly is a destructive pest of wheat. Employing additional molecular strategies can complement wheat’s native insect resistance. However, this requires functional characterization of Hessian-fly-responsive genes, which is challenging because of wheat genome complexity. The diploid Brachypodium distachyon (Bd) exhibits nonhost resistance to Hessian fly and displays phenotypic/molecular responses intermediate between resistant and susceptible host wheat, offering a surrogate genome for gene characterization. Here, we compared the transcriptomes of Biotype L larvae residing on resistant/susceptible wheat, and nonhost Bd plants. Larvae from susceptible wheat and nonhost Bd plants revealed similar molecular responses that were distinct from avirulent larval responses on resistant wheat. Secreted salivary gland proteins were strongly up-regulated in all larvae. Genes from various biological pathways and molecular processes were up-regulated in larvae from both susceptible wheat and nonhost Bd plants. However, Bd larval expression levels were intermediate between larvae from susceptible and resistant wheat. Most genes were down-regulated or unchanged in avirulent larvae, correlating with their inability to establish feeding sites and dying within 4–5 days after egg-hatch. Decreased gene expression in Bd larvae, compared to ones on susceptible wheat, potentially led to developmentally delayed 2nd-instars, followed by eventually succumbing to nonhost resistance defense mechanisms.

A Homolog of the Arabidopsis TIME FOR COFFEE Gene Is Involved in Nonhost Resistance to Wheat Stem Rust in Brachypodium distachyon

Plants resist infection by pathogens using both preexisting barriers and inducible defense responses. Inducible responses are governed in a complex manner by various hormone signaling pathways. The relative contribution of hormone signaling pathways to nonhost resistance to pathogens is not well understood. In this study, we examined the molecular basis of disrupted nonhost resistance to the fungal species Puccinia graminis, which causes stem rust of wheat, in an induced mutant of the model grass Brachypodium distachyon. Through bioinformatic analysis, a 1 base pair deletion in the mutant genotype was identified that introduces a premature stop codon in the gene Bradi1g24100, which is a homolog of the Arabidopsis thaliana gene TIME FOR COFFEE (TIC). In Arabidopsis, TIC is central to the regulation of the circadian clock and plays a crucial role in jasmonate signaling by attenuating levels of the transcription factor protein MYC2, and its mutational disruption results in enhanced susceptibility to the hemi-biotroph Pseudomonas syringae. Our similar finding for an obligate biotroph suggests that the biochemical role of TIC in mediating disease resistance to biotrophs is conserved in grasses, and that the correct modulation of jasmonate signaling during infection by Puccinia graminis may be essential for nonhost resistance to wheat stem rust in B. distachyon.

Maize and common bean seed exudates mediate part of nonhost resistance to Phytophthora sojae prior to infection

Phytophthora sojae does not infect nonhost maize (Zea mays) but infects nonhost common bean (Phaseolus vulgaris) under inoculation. Soybean seed exudates participate in mediating host resistance to P. sojae prior to infection. This study aims to elucidate the role of seed exudates in mediating the nonhost resistance to P. sojae prior to infection. The behaviors of P. sojae zoospores in response to the seed exudates were determined using an assay chamber and a concave slide. The proteomes of P. sojae zoospores in response to the seed exudates were analyzed with the tandem mass tag (TMT) method. The key proteins were quantitatively verified by parallel reaction monitoring (PRM). Maize seed exudates exerted a repellent effect on zoospores. This result explains why zoospores sense repelling signaling molecules that weaken and strongly inhibit chemotaxis signals in the phosphatidylinositol signaling pathway and arachidonic acid metabolism pathway. Common bean seed exudates did not exhibit any attraction to the zoospores because the G protein signaling pathway, had no significant change. The proteins protecting the cell membrane structure were significantly downregulated, and the early apoptosis signal glutathione was enhanced in zoospores responding to common bean seed exudates, which resulted in dissolution of the cysts. Maize and common bean seed exudates mediate part of the nonhost resistance to P. sojae via different mechanisms prior to infection. The immunity of maize to P. sojae is due to the repellent effect of maize seed exudates on zoospores. Common bean seed exudates participate in mediating nonhost resistance by dissolving cysts.

Phenotypic and transcriptomic analyses reveal major differences between apple and pear scab nonhost resistance

Background: Nonhost resistance is the outcome of most plant/pathogen interactions, but it has rarely been described in Rosaceous fruit species. Apple (Malus x domestica Borkh.) is a nonhost for Venturia pyrina, the scab species attacking European pear (Pyrus communis L.). Reciprocally, P. communis is a nonhost for Venturia inaequalis, the scab species attacking apple. The major objective of our study was to compare the scab nonhost resistance in apple and in European pear, at the phenotypic and transcriptomic levels. Results: Macro- and microscopic observations after reciprocal scab inoculations indicated that, after a similar germination step, nonhost apple/V. pyrina interaction remained nearly symptomless, whereas hypersensitive reactions were observed during nonhost pear/V. inaequalis interaction. Comparative transcriptomic analyses of apple and pear nonhost interactions with V. pyrina and V. inaequalis, respectively, revealed considerable differences. Very few differentially expressed genes were detected during apple/V. pyrina interaction, which is consistent with a symptomless type I nonhost resistance. On the contrary, numerous genes were differentially expressed during pear/V. inaequalis interaction, as expected in a type II nonhost resistance involving visible hypersensitive reaction. Pre-invasive defense, such as stomatal closure, was detected, as well as several post-invasive defense mechanisms (apoplastic reactive oxygen species accumulation, phytoalexin production and alterations of the epidermis composition). In addition, a comparative analysis between pear scab host and nonhost interactions indicated that, although specificities were observed, two major defense lines were shared in these resistances: cell wall and cuticle modifications and phenylpropanoid pathway induction. Conclusion: This first deciphering of the molecular mechanisms underlying a nonhost scab resistance in pear offers new possibilities for the genetic engineering of sustainable scab resistance in this species.

Epidermal chloroplasts are defense-related motile organelles equipped with plant immune components

In addition to conspicuous large mesophyll chloroplasts, where most photosynthesis occurs, small epidermal chloroplasts have also been observed in plant leaves. However, the functional significance of this small organelle remains unclear. Here, we present evidence that Arabidopsis epidermal chloroplasts control the entry of fungal pathogens. In entry trials, specialized fungal cells called appressoria triggered dynamic movement of epidermal chloroplasts. This movement is controlled by common regulators of mesophyll chloroplast photorelocation movement, designated as the epidermal chloroplast response (ECR). The ECR occurs when the PEN2 myrosinase-related higher-layer antifungal system becomes ineffective, and blockage of the distinct steps of the ECR commonly decreases preinvasive nonhost resistance against fungi. Furthermore, immune components were preferentially localized to epidermal chloroplasts, contributing to antifungal nonhost resistance in the pen2 background. Our findings reveal that atypical small chloroplasts act as defense-related motile organelles by specifically positioning immune components in the plant epidermis, which is the first site of contact between the plant and pathogens. Thus, this work deepens our understanding of the functions of epidermal chloroplasts.

Maize and Common Bean Seed Exudates Mediate Part of Nonhost Resistance to Phytophthora Sojae before Infection

Purpose: Phytophthora sojae does not infect the nonhost maize (Zea mays L.) but could infect the nonhost common bean (Phaseolus vulgaris L.). Soybean seed exudates participate in mediating host resistance to P. sojae before infection. This study aimed to elucidate the role of nonhost seed exudates in mediating nonhost resistance to P. sojae before infection. Methods: The response behavior of P. sojae zoospores to the seed exudates was determined using an assay chamber and a concave slide, and the proteomes of P. sojae zoospores treated with the seed exudates were analysed with the tandem mass tag (TMT) method. The key proteins were quantified by parallel reaction monitoring (PRM).Results: Maize seed exudates exerted a repellent effect on zoospores, whereas common bean seed exudates did not exhibit any attraction to zoospores but could dissolve the cysts. The key proteins related to zoospores chemotaxis showed no significant changes in response to maize seed exudates, but the key proteins in arachidonic acid pathway were downregulated and controlled the repellent behavior of zoospores. Proteins protecting the cell membrane structure were significantly downregulated in zoospores responding to common bean seed exudates, which confirmed the bacteriolytic effect of common bean seed exudates on cysts. Conclusion: Maize and common bean seed exudates mediate part of the nonhost resistance via different mechanisms prior to P. sojae infection. The immune of maize to P. sojae is due to the repellent effect of maize seed exudates on zoospores. Common bean seed exudates participate in mediating nonhost resistance by dissolving cysts.