Phytophthora infestans Produces Oospores in Fruits and Seeds of Tomato

The Role of Silica Nanoparticles in Promoting the Germination of Tomato (Solanum lycopersicum) Seeds

The addition of nanoparticles has been reported to be an effective strategy for enhancing seed germination, but the underlying mechanisms whereby this occurs are unclear. In the present study, we added silica nanoparticles (SiNPs) to an aqueous growth medium in which tomato seeds were germinated. We examined the effects of SiNPs on growth and possible mechanisms of action. SiNPs had a diameter of 10-17 nm and 110-120 nm. SiNPs shortened the mean germination time from 5.24 ± 0.29 days to 4.64 ± 0.29 days. Seedling vigor, measured by criteria including length and weight, was also improved compared to the control condition. The presence of SiNPs in the seedlings was assessed using an X-ray fluorescence spectrometer. The nanoparticles may have promoted germination by enhancing water imbibition by the seeds or altering the external microenvironment. Scanning electron microscopy revealed changes in the seed coat during germination, many of which were only observed in the presence of nanoparticles. Soil bacteria affect germination; specifically, Bacillus sp. may promote germination. The number of Bacillus sp. changed in the germination medium with SiNPs compared to the control. This suggested that these bacteria could interact with SiNPs to promote germination.

Early detection of late blight in potato by whole-plant redox imaging

Late blight caused by the oomycete Phytophthora infestans is a most devastating disease of potatoes (Solanum tuberosum). Its early detection is crucial for suppressing disease spread. Necrotic lesions are normally seen in leaves at 4 days post-inoculation (dpi) when colonized cells are dead, but early detection of the initial biotrophic growth stage, when the pathogen feeds on living cells, is challenging. Here, the biotrophic growth phase of P. infestans was detected by whole-plant redox imaging of potato plants expressing chloroplast-targeted reduction-oxidation sensitive green fluorescent protein (chl-roGFP2). Clear spots on potato leaves with a lower chl-roGFP2 oxidation state were detected as early as 2 dpi, before any visual symptoms were recorded. These spots were particularly evident during light-to-dark transitions, and reflected the mislocalization of chl-roGFP2 outside the chloroplasts. Image analysis based on machine learning enabled systematic identification and quantification of spots, and unbiased classification of infected and uninfected leaves in inoculated plants. Comparing redox with chlorophyll fluorescence imaging showed that infected leaf areas that exhibit mislocalized chl-roGFP2 also showed reduced non-photochemical quenching and enhanced quantum PSII yield (ΦPSII) compared with the surrounding leaf areas. The data suggest that mislocalization of chloroplast-targeted proteins is an efficient marker of late blight infection, and demonstrate how it can be utilized for non-destructive monitoring of the disease biotrophic stage using whole-plant redox imaging.

Resistance Risk and Novel Resistance-Related Point Mutations in Target Protein PiORP1 of Fluoxapiprolin in Phytophthora infestans

Fluoxapiprolin is a new oxysterol binding protein inhibitor (OSBPI), which showed excellent inhibitory activity to plant pathogenic oomycetes. Its resistance risk and mechanism in Phytophthora infestans are unclear. In the current study, the sensitivities of 103 P. infestans isolates to fluoxapiprolin were investigated, and a unimodal distribution with a mean EC₅₀ value of 0.00035 μg/mL was observed. Four types of resistant mutants, with a resistance factor from 14 to more than 1000, and point mutations S768I+N837I, S768I+L860I, S768I, and I877F in PiORP1, were acquired using fungicide adaption. The fitness of the mutants was similar to or lower than that of the corresponding parental isolate. Positive cross-resistance was detected between fluoxapiprolin and oxathiapiprolin. The point mutations were verified in P. sojae homologue positions using the CRISPR/Cas9 genome editing system. Transformants containing S768I+N837I or S768I+L860I, showed high fluoxapiprolin resistance (RF > 1000). In conclusion, the risk of P. infestans resistance to fluoxapiprolin is moderate, and novel point mutation types S768I+N837I or S768I+L860I could cause high fluoxapiprolin resistance in P. infestans.

CRISPR/Cas9-Mediated SlMYBS2 Mutagenesis Reduces Tomato Resistance to Phytophthora infestans

Phytophthora infestans (P. infestans) recently caused epidemics of tomato late blight. Our study aimed to identify the function of the SlMYBS2 gene in response to tomato late blight. To further investigate the function of SlMYBS2 in tomato resistance to P. infestans, we studied the effects of SlMYBS2 gene knock out. The SlMYBS2 gene was knocked out by CRISPR-Cas9, and the resulting plants (SlMYBS2 gene knockout, slmybs2-c) showed reduced resistance to P. infestans, accompanied by increases in the number of necrotic cells, lesion sizes, and disease index. Furthermore, after P. infestans infection, the expression levels of pathogenesis-related (PR) genes in slmybs2-c plants were significantly lower than those in wild-type (AC) plants, while the number of necrotic cells and the accumulation of reactive oxygen species (ROS) were higher than those in wild-type plants. Taken together, these results indicate that SlMYBS2 acts as a positive regulator of tomato resistance to P. infestans infection by regulating the ROS level and the expression level of PR genes.

Seed transmission of Pseudoperonospora cubensis

Pseudoperonospora cubensis, an obligate biotrophic oomycete causing devastating foliar disease in species of the Cucurbitaceae family, was never reported in seeds or transmitted by seeds. We now show that P. cubensis occurs in fruits and seeds of downy mildew-infected plants but not in fruits or seeds of healthy plants. About 6.7% of the fruits collected during 2012-2014 have developed downy mildew when homogenized and inoculated onto detached leaves and 0.9% of the seeds collected developed downy mildew when grown to the seedling stage. This is the first report showing that P. cubensis has become seed-transmitted in cucurbits. Species-specific PCR assays showed that P. cubensis occurs in ovaries, fruit seed cavity and seed embryos of cucurbits. We propose that international trade of fruits or seeds of cucurbits might be associated with the recent global change in the population structure of P. cubensis.

Pathogenic Fitness of Oosporic Progeny Isolates of Phytophthora infestans on Late-Blight-Resistant Tomato Lines

Four A1 field isolates and one A2 field isolate of Phytophthora infestans were crossed to produce oospores in tomato leaves. The oospores were extracted and mixed with perlite and water, and healthy tomato leaves were used as bait for oospore-progeny infection. Twenty-nine lesions were obtained from the four crosses and 283 single-sporangium isolates were recovered and tested on four tomato differential lines carrying different major genes (Ph-0, Ph-1, Ph-2, and 3707) for late blight resistance. The pathogenic fitness (number of sporangia per unit leaf area) of parental and progeny isolates was strongly dependent on the host genotype; it decreased in the order Ph-0 > Ph-1 > Ph-2 > 3707. The A2 parent had a higher pathogenic fitness than the A1 parents on Ph-0 and Ph-1 but similar, lower fitness on Ph-2. Different levels of pathogenic fitness were observed across all isolates on Ph-0, although Ph-0 lacks resistance genes. Pathogenic fitness on one tomato genotype was not related to the pathogenic fitness on the other tomato genotypes. Some isolates exhibited reduced pathogenic fitness relative to the respective A1 parent, whereas others demonstrated a higher pathogenic fitness compared with the A2 parent. The tomato genotype Solanum pimpinellifolium L3707/5 was resistant to all five parental isolates of P. infestans. However, 37 of the 283 progeny isolates from 11 different lesions had compatible reactions with this line, producing up to 31 × 10³ sporangia/cm². Overall, reduced fitness was more frequent among the progeny isolates than increased fitness.

Reproductive strategies of field Phytophthora infestans populations on potato and tomato in some regions of Russia

The occurrence of zoosporangia and oospores is analyzed for 88 field populations of Phytophthora infestans (Mont.) de Bary in Moscow region and other regions of Russia from 1997 to 2006. For estimation of zoosporangia and oospore frequencies in the populations, the indexes IZ (for zoosporangia) and IO (for oospores) are suggested. The combinations of these indexes enabled a reveal of 4 strategies for P. infestans reproduction, which were found in the field populations of the pathogen. These strategies were more numerous in P. infestans field populations on tomato than on potato (4 versus 2) in spite of the fact that potato populations in Moscow region strongly predominated. It was connected with rareness or absence of oospores in potato and their presence in tomato, especially in fruit of many populations. The knowledge about strategies of reproduction can be applied for verification of efficiency of new methods for crop protection against late blight, and for investigations of P. infestans populations.

Combresomyces cornifer gen. sp. nov., an endophytic peronosporomycete in Lepidodendron from the Carboniferous of central France

Structurally preserved periderm of the lycophyte Lepidodendron rhodumnense from the Visean (Mississippian) of central France contains a peronosporomycete (Combresomyces cornifer gen. sp. nov.) that occurs in the form of pyriform to subglobose terminal oogonia. On the surface is a conspicuous ornamentation, which may have formed through condensation of a mucilaginous extra-oogonial wall secretion. Some oogonia contain thin-walled spherules, which may represent (walled) oospheres or spores of an endoparasitic fungus (?chytrid), whereas single, large spheres in the interior are interpreted as oospores. Antheridia adpressed to several of the specimens are clavate and paragynous. This discovery sheds light on the morphology and biology of peronosporomycetes in a terrestrial ecosystem some 330My ago. Although the organism occurs exclusively in the periderm of L. rhodumnense, it is not known whether it represents a symptomless endophyte, pathogen, or saprotroph.

An Improved Method for Infecting Tomato Leaves or Seedlings with Oospores of Phytophthora infestans Used to Investigate F1 Progeny

An improved, soil-free laboratory method was developed to induce late blight infection in detached tomato leaves or tomato seedlings with in vivo-produced oospores of Phytophthora infestans. Oospores were produced in detached tomato leaves infected with an inoculum mixture of A1 and A2 isolates. The infected leaves were homogenized in distilled water. The homogenates were exposed to two cycles of drying and wetting to kill sporangia, zoospores, and mycelia and, thereafter, mixed with perlite and water. Tomato leaves or seed were floated on the perlite mixture in petri dishes and incubated for up to 30 days. Results from 13 experiments showed that, in dishes containing oospores, 96 of 820 leaflets (11.7%) developed late blight lesions within 6 to 30 days; whereas, in 3 experiments with seedlings, 12 of 1,400 plants (0.9%) were blighted within 17 to 26 days. No late blight developed in leaflets or seedlings maintained in dishes containing homogenates of the control leaves infected with either A1 or A2 inoculum. In all, 234 single-sporangium F1 isolates recovered from 24 leaf lesions produced by four crosses and 27 isolates recovered from 12 seedlings infected with two crosses were examined for sensitivity to metalaxyl and mating type. Results showed that, although the parent isolates were either S (sensitive) or R (resistant) to metalaxyl, most F1 progeny isolates exhibited various levels of intermediate resistance to metalaxyl. Most isolates belonged to either the A1 or the A2 mating type, depending on the cross and lesion. However, some isolates belonged to the unusual mating type A1A2 or were sterile. Current experiments are intended to elucidate the virulence and aggressiveness of these F1 progenies to tomato. This improved method may facilitate our understanding of the role of in vivo-produced oospores in the epidemiology of late blight in tomato.

Phytophthora infestans Populations from Tomato and Potato in North Carolina Differ in Genetic Diversity and Structure

ABSTRACT Phytophthora infestans causes a destructive disease on tomato and potato. In North Carolina (NC) potatoes are mostly grown in the east, whereas tomatoes are grown in the mountainous areas in the western part of the state. Five genotypes of P. infestans were identified from 93 and 157 isolates collected from tomato and potato over a 5 year period between 1993 and 1998. All isolates collected from potato in eastern NC were the US-8 genotype, whereas only a single isolate was the US-1 genotype. Tuber blight was found on immature daughter tubers in a single field in 1997, however infection on mature tubers was not observed. Within potato fields, a range of sensitivity to metalaxyl was observed among isolates but all were either intermediate or highly resistant to the fungicide. In contrast, isolates from tomatoes included previously reported US-7 and US-8 genotypes and two new genotypes called US-18 and US-19 (A2 mating type, allozyme genotype Gpi 100/100 and Pep 92/100). These genotypes had unique restriction fragment length polymorphism banding patterns, were sensitive to metalaxyl, and have not been reported elsewhere. All genotypes, with the exception of the US-1, were the Ia mitochondrial haplotype. Thus, isolates of P. infestans from tomato were more genetically diverse over time in NC than those from potato and include two new genotypes that are sensitive to metalaxyl.