Jasmonate signaling drives defense responses against Alternaria alternata in chrysanthemum

Arabidopsis thaliana accumulates dehydroepiandrosterone after infection with phytopathogenic fungi - Effects on plants and fungi

Progestogens and androgens have been found in many plants, but little is known about their physiological function. We used a previously established UPLC-ESI-MS/MS method to analyze progestogen and androgen profiles in fungal infections. Here we show that dehydroepiandrosterone (DHEA), a C19 steroid, specifically accumulates in shoots of Arabidopsis thaliana (L.) HEYNH. infected with Alternaria brassicicola (SCHWEIN.) WILTSHIRE. Elevated DHEA levels in plants seem not to be product of fungal sterol/steroid precursor activity, but an intrinsic plant response to the infection. DHEA was applied exogenously to analyze the effects of the androgen on development and gene expression in A. thaliana. Our findings reveal that DHEA treatment downregulates membrane-associated, salicylic acid and abscisic acid-regulated, as well as stress-responsive genes. Notably, DHEA does not inhibit the isoprenoid or post-lanosterol pathway of the ergosterol biosynthesis. Moreover, A. brassicicola was also treated with DHEA to analyze the growth, sterol pattern and membrane-integrity. Our data suggest that DHEA enhances the permeability of plant and fungal biomembranes. We propose that DHEA accumulation is a plant defense response which reduces fungal growth in plant tissues.

16S rDNA sequencing combined with metabolomic probes to investigate the effects of Salmonella Pullorum on gut microbes and metabolites in broilers

Pullorum disease (PD) caused by Salmonella Pullorum (SP) results in high mortality in chicks and potential carriers in adult chickens, negatively affecting growth and egg production. This study identified SP infection in 100-day-old White Plymouth Rock hens by serum plate agglutination and fecal and anal swab polymerase chain reaction. SP-infected broilers were classified into positive (P) and negative (N) groups using hematoxylin-and-eosin staining, metabolome sequencing, and 16S rDNA to investigate the effects of SP infection on the metabolites and microorganisms in the cecum of broilers. Groups had different degrees of inflammatory cell infiltration in the cecum, spleen, liver, and lung tissues. The diversity of bacterial flora in the cecum of Groups P and N differed significantly (P < 0.05). o__Lactobacillales and o__Verrucomicrobiota were significantly higher in Group P than in Group N (P < 0.05). At the genus level, g__Akkermansia was significantly higher in Group N (P < 0.05). Metabolome sequencing of cecum contents in Groups P and N screened 77 differential metabolites at the secondary metabolite level. 11 metabolites, including 2,4-dimethylbenzaldehyde, 3a,6b,7b,12a-tetrahydroxy-5b-cholanoic acid, and LysoPG 19:1, were differentially expressed in Group P (P < 0.05). A combined analysis of 16S rDNA sequencing and cecal content metabolomics identified 28 genera significantly associated with 38 metabolites in the cecum (P < 0.05). Specific bacterial genera such as Corynebacterium and Roseobacter have particularly prominent effects on metabolites. These findings highlight the significant alterations in gut microbial composition and metabolic functions due to SP infection. The differential metabolites and bacterial taxa identified in this study may provide insights into the underlying mechanisms of PD pathogenesis and potential biomarkers for disease management.

Alternaria alternata effector AaAlta1 targets CmWD40 and participates in regulating disease resistance in Chrysanthemum morifolium

Black spot diseases caused by the necrotrophic fungal pathogen Alternaria alternata adversely affect the growth and yield of many plants worldwide. However, the molecular mechanisms underlying the virulence and pathogenicity of A. alternata remain largely unknown. In this study, we report the identification of a novel effector Alta1, secreted by A. alternata, which not only contributes to its virulence but also triggers the cell death and defense of the host plant. The expression of Alta1 in Chrysanthemum morifolium activated jasmonic acid (JA) signaling, which, in turn, enhanced plant resistance to A. alternata. Moreover, we found that Alta1 targeted the WD40-repeat protein of chrysanthemum (CmWD40) after entering host cells. Notably, the CmWD40 gene showed rhythmic basal expression, and the overexpression of CmWD40 increased the resistance of chrysanthemum leaves against A. alternata, whereas its loss of function led to a decrease in this resistance. The results of the comparative transcriptomics and JA content analyses indicated that CmWD40 is possibly involved in the accumulation and signaling of JA. The transcript levels of the MYC2 gene were significantly upregulated in lines overexpressing the CmWD40 gene compared with that in the wild type. Further, the results of the infection assay revealed that CmWD40 positively modulated Alta1-induced defense response by activating MYC2 transcription. Overall, the results obtained in this study demonstrate that identified effector Alta1, recognized by the circadian rhythm gene CmWD40, triggers JA-induced immune response and enhances disease resistance in chrysanthemum plants.

Novel Regulators and Their Epistatic Networks in Arabidopsis' Defence Responses to Alternaria alternata Infection

Necrotrophic pathogens cause serious threats to agricultural crops, and understanding the resistance genes and their genetic networks is key to breeding new plant cultivars with better resistance traits. Although Alternaria alternata causes black spot in important leafy brassica vegetables, and leads to significant loss of yield and food quality, little is known about plant-A. alternata interactions. In this study, we used a unique and large collection of single, double and triple mutant lines of defence metabolite regulators in Arabidopsis to explore how these transcription factors and their epistatic networks may influence A. alternata infections. This identified nine novel regulators and 20 pairs of epistatic interactions that modulate Arabidopsis plants' defence responses to A. alternata infection. We further showed that the glucosinolate 4-methoxy-indol-3-ylmethyl is the only glucosinolate consistently responsive to A. alternata infection in Col-0 ecotype. With the further exploration of the regulators and the genetic networks on modulating the accumulation of glucosinolates under A. alternata infection, an inverted triangle regulatory model was proposed for Arabidopsis plants' defence responses at a metabolic level and a phenotypic level.

The fungal effector AaAlta1 inhibits PATHOGENESIS-RELATED PROTEIN10-2-mediated callose deposition and defense responses in apple

Apple leaf spot, caused by Alternaria alternata f. sp mali (ALT), poses a substantial threat to the global apple (Malus × domestica Borkh.) industry. Fungal effectors promote pathogen infestation and survival by interfering with plant immune responses. In our study, we investigated the secretion of effector proteins by the virulent ALT7 strain. Using mass spectrometry, we identified the effector AaAlta1, which belongs to the Alt a 1 protein family (AA1s). Further analysis confirmed that ALT7 secretes AaAlta1. AaAlta1 knockdown mutants displayed reduced pathogenicity in apple tissue culture seedlings, while overexpression strains exhibited enhanced pathogenicity compared to the wild-type ALT7 strain. Using immunoprecipitation followed by mass spectrometry, we isolated pathogenesis-related protein 10-2 (PR10-2) as an interaction partner of AaAlta1 in apple. Knockdown mutants of AaAlta1 showed increased PR10-2-mediated callose deposition in apple, a critical plant defense response. The enhanced defense responses in apple substantially reduced their susceptibility to infection by these ALT7 mutants. Our findings delineate an infection strategy whereby ALT7 secretes AaAlta1 to suppress PR10-2, thereby circumventing the apple defense system.

Evaluation of Five Chrysanthemum morifolium Cultivars against Leaf Blight Disease Caused by Alternaria alternata at Rooting and Seedling Growth Stages

During the winter of 2018, leaf blight on florist's daisy (Chrysanthemum morifolium L.) was noticed in Egypt. The disease, which was identified as caused by Alternaria alternata, was widely spread and led to serious damage for the exportation sector of this crop. Therefore, a study was conducted to better understand what can be conducted to minimize the problem in the future. Isolates were gathered and evaluated on five chrysanthemum cultivars (i.e., 'Feeling Green Dark', 'Talitha', 'Chrystal Regan', 'Arctic queen', and 'Podolsk Purple') grown in a greenhouse. The objectives were to isolate and identify the phytopathogen and detect the resistant degree of these cultivars with emphasis on the early growth stages of the crop. The results showed that 'Podolsk Purple' was the most resistant cultivar against the different isolates during the rooting and seedling growth stages. 'Chrystal Regan' was very susceptible to the different isolates. In addition, the isolate from 'Feeling Green Dark' was the strongest, which negatively affected the chlorophyll content and its fluorescence parameters besides other measured vegetative and anatomical features. The findings indicated that the best anatomical characters of the stem and leaf, like the thickness of cuticle and cortex, stem diameter, xylem vessel diameter, and thickness of epidermis as well as lamina thickness were recorded in the 'Podolsk Purple' cultivar. This study highlighted that by using the right cultivars, chrysanthemum can be cultivated during the winter season under Egyptian conditions. These results can be a part of solution to overcome the leaf blight caused by A. alternata on chrysanthemum during the early growing stages.

Plant disease resistance outputs regulated by AP2/ERF transcription factor family

Plants have evolved a complex and elaborate signaling network to respond appropriately to the pathogen invasion by regulating expression of defensive genes through certain transcription factors. The APETALA2/ethylene response factor (AP2/ERF) family members have been determined as key regulators in growth, development, and stress responses in plants. Moreover, a growing body of evidence has demonstrated the critical roles of AP2/ERFs in plant disease resistance. In this review, we describe recent advances for the function of AP2/ERFs in defense responses against microbial pathogens. We summarize that AP2/ERFs are involved in plant disease resistance by acting downstream of mitogen activated protein kinase (MAPK) cascades, and regulating expression of genes associated with hormonal signaling pathways, biosynthesis of secondary metabolites, and formation of physical barriers in an MAPK-dependent or -independent manner. The present review provides a multidimensional perspective on the functions of AP2/ERFs in plant disease resistance, which will facilitate the understanding and future investigation on the roles of AP2/ERFs in plant immunity.