Ultrastructural study on interaction between a sterile,white endophytic fungus and eggplant roots

Eggplant roots colonized by a sterile, white mycelial endophyte (SWM) were previously found to become highly resistant to Verticillium wilt. SWM alone, however, caused no visible, disease symptoms, such as wilting or necrosis. The mechanism of the symptomless infection by SWM was investigated in this study. Electron microscopy revealed that hyphae of SWM were abundant on and inside the root epidermal cells 2 weeks after inoculation. Many terminal appressoria formed from apical tips of hyphae, and heavy degradation of the host cell walls was evident where hyphae accumulated. By 4 weeks following inoculation, penetration pegs easily breached epidermal cells, and the infection hyphae penetrated outer cortical cells. In response to the hyphal ingress, numerous tubule-like vesicles and membrane-bound, multivesicular bodies accumulated in cortical cytoplasm near the infection sites of the outer cortical cells, but no visible signs of the host reactions were seen in the epidermal cells. Papillae developed at the spaces between cell walls and plasma membranes at the infection sites. The penetration hyphae often grew out of the papillae, but further hyphal ingress was halted in the middle cortical cell layer. By 8 weeks following inoculation, papillae that developed in these cells contained larger amounts of highly electron-dense material and were reinforced by multilamellate, fibrous elements. Hyphae that entered such papillae were confined to them, and the hyphal cytoplasm degenerated. As the result of the activated resistance reactions, root vascular cylinders remained intact, and the host plants did not wilt.     

Initial Cellular Interactions Between Thielaviopsis basicola and Tobacco Root Hairs

ABSTRACT Cellular events that occur during the initial interactions between Thielaviopsis basicola and root hairs of tobacco (Nicotiana tabacum) were examined microscopically. Time-course documentation of the infection process indicated a dynamic interaction between T. basicola and the living host cell. Upon root hair contact and recognition, the vegetative apex of T. basicola rapidly differentiated to form infection structures, and the host cell responded cytologically. Penetration was achieved by threadlike hyphae that subsequently developed distal swellings, and intracellular hyphae of sickle-shaped morphology advanced from the distal swelling and colonized the cell. Streaming of the host cytoplasm became aggregated near the infection site prior to penetration and accumulated around the infecting hyphae as long as the host cell was viable. Substantial callose deposition, in the form of a bell-shaped collar around infection structures, resulted from the cytological activity at the infection site. Penetration of dead root hairs was common, but did not lead to the development of infection structures or to a sustained association with the host tissue; T. basicola exited dead root hairs and resumed vegetative growth. The establishment of the parasitic relationship by T. basicola was characteristic of hemibiotrophic fungi in that, initially, biotrophic infection led to tissue colonization, and host cell survival was limited under parasitism.     

禾顶囊壳小麦变种对小麦种子根的侵染过程

 光镜和电镜观察表明,禾顶囊壳小麦变种(Gaeumannomyces graminis var.tritici,小麦全蚀病菌)对小麦种子根的侵染过程可分为侵入前、侵入表皮层、进入皮层和进入中柱等4个连续阶段。麦根接菌后在15℃下培养,48 h后侵入表皮层细胞,60 h后进入皮层,120 h后进入中柱。病原菌主要以侵染菌丝直接侵入表皮层,表皮细胞间隙和根毛基细胞是主要侵入部位,少数由附着枝侵入。菌丝穿透细胞壁有明显的酶解作用特征,菌丝先端前方胞壁上还产生电子密物质。皮层细胞是病原菌定殖和发展的主要场所,病原菌还能离解胞间层,形成胞外空间,特别有利于菌丝和菌丝束的扩展。在侵入位点的寄主细胞壁和质膜之间,形成多种形状的木质管,其数量与侵入菌丝的数目相对应,但木质管不能阻止菌丝进入细胞。菌丝进入中柱后,可阻塞导管和筛管。小麦细胞发生退行性病变,尤以细胞壁膨大崩坏和早期质壁分离最明显,细胞间隙还产生性质不明的黄色物质。     

Histopathology of Infection and Colonization of Susceptible and Resistant Port-Orford-Cedar by Phytophthora lateralis

ABSTRACT Port-Orford-cedar (POC) root disease, caused by Phytophthora lateralis, continues to kill POC in landscape plantings and natural forests in western North America. POC trees resistant to P. lateralis have been identified and propagated. Cytological observations of P. lateralis in susceptible and resistant roots and stems were made with light and transmission electron microscopy to identify resistance mechanisms. No differences in infection pathway and initial colonization were observed between susceptible and resistant roots, although there were differences in the rate and extent of development. Germ tubes formed appressoria, and penetration hyphae grew either between or directly through epidermal cell walls; inter- and intracellular hyphae colonized the root cortex. In susceptible roots, hyphae penetrated into the vascular system within 48 h of inoculation. In contrast, hyphae in roots of resistant seedlings grew more slowly in cortical cells and were not observed to penetrate to the vascular tissues. In resistant roots, infection was marked by general thickening of cortical cell walls, wall appositions around penetrating hyphae, collapse of cortical cells, and accumulation of osmophillic granules around hyphae. In susceptible stems, hyphae grew inter- and intracellularly in all cells of the secondary phloem except fiber cells, but were concentrated in sieve and parenchyma cells in the functional phloem. The pattern of penetration and colonization of hyphae was similar in the resistant stems, except that hyphae were found in the fiber cells of the xylem. In resistant stems, there were fewer hyphae in the functional phloem, and cytological changes such as damaged nuclei and disintegrated cytoplasm were evident. Structural changes in resistant stems included collapsed cells, wall thickening, secretory bodies, apposition of electron dense materials, and crystals in cell walls.     

Association of Binucleate Rhizoctonia with Soybean and Mechanism of Biocontrol of Rhizoctonia solani

ABSTRACT The association of binucleate Rhizoctonia (BNR) AG-K with soybean and the interaction of BNR, R. solani AG-4, and soybean seedlings were investigated to elucidate the mechanism of biocontrol of R. solani by BNR. Sixty-hour-old seedlings were inoculated and incubated in a growth chamber at 24 degrees C; plants were examined with light microscopy and with scanning and transmission electron microscopy at various times following inoculation. BNR grew over hypocotyls, roots, and root hairs, but only colonized epidermal cells. Hyphae of BNR appeared to attach to the epidermis and, 5.5 h following inoculation, began penetrating cells by means of penetration pegs without forming distinct appressoria or infection cushions. There was evidence of cuticle degradation at the point of penetration. Infection hyphae moved to adjacent epidermal cells by direct penetration of epidermal radial walls. There were epidermal and cortical cell necrosis, beginning with the fragmentation of the tonoplast and followed by the disintegration of cytoplasm, organelles, and plasma membranes. Cell necrosis was also observed in adjacent cells where there was no evidence of BNR hyphae. Cell walls were not destroyed. After 144 h, there was noevidence of BNR hyphae in cortical cells. Attempted penetrations were observed, but papillae formed on the inside of cortical cell walls. Pre-inoculation of soybean seedlings with BNR 24 or 48 h before inoculation with R. solani (1 cm between inocula) affected the growth of R. solani on soybean tissue. There were fewer hyphae of R. solani, the hyphae branched sparingly, and infection cushions were rare when compared with hyphal growth on soybean inoculated only with R. solani. These effects were observed before the BNR hyphae began to intermingle with the hyphae of R. solani on the surface of the inoculated host. Preinoculation of soybean seedlings 24 h before inoculation with R. solani significantly (P = 0.05) reduced disease incidence and severity caused by R. solani AG-4. The lesions caused by R. solani always appeared distally, not proximally, to the BNR inoculum. The interactions of intermingling hyphae of BNR and R. solani were examined in vitro and on the surface of the host. There was no evidence of lysis, mycoparasitism, inhibition of growth, or any other form of antagonism between hyphae. The results of these studies strongly suggest that induced resistance is the mechanism of biocontrol of R. solani on soybean by BNR. The inhibition of hyphal growth of R. solani on the surface of soybean tissue preinoculated with BNR appears to be a novel characteristic of induced resistance.     

The process of antagonism of Sclerotium cepivorum in white rot affected onion roots by Trichoderma koningii

Trichoderma koningii (strain Tr5) grew in the epidermal mucilage of onion roots without entering healthy epidermal tissue. When placed on the epidermis of Sclerotium cepivorum -infected roots, T. koningii colonized epidermal passage cells, with little colonization of other epidermal tissues, then branched and spread throughout the root cortical tissues damaged by enzymes and toxins which diffused ahead of S. cepivorum hyphae, and impeded the path of the infection. When T. koningii colonized infected tissue, many S. cepivorum hyphae became detached at septa, cell walls dissolved and many hyphal apices burst. Contact between hyphae was not necessary for lysis to occur. T. koningii produced two endochitinases ( R _f 0·15 and 0·24) and two exo-acting chitinolytic enzymes ( R _f 0·46 and 0·62) during degradation of crabshell chitin and S. cepivorum cell walls. The R f 0·24 and 0·46 proteins were detected when T. koningii colonized S. cepivorum -infected roots and are likely to be a component of the antagonism process.     

Ultrastructure of the Infection of Sorghum bicolor by Colletotrichum sublineolum

ABSTRACT Ultrastructural studies of the infection of susceptible and resistant cultivars of Sorghum bicolor by Colletotrichum sublineolum were conducted. Initial penetration events were the same on both susceptible and resistant cultivars. Germ tubes originating from germinated conidia formed globose, melanized appressoria, that penetrated host epidermal cells directly. Appressoria did not produce appressorial cones, but each penetration pore was surrounded by an annular wall thickening. Inward deformation of the cuticle and localized changes in staining properties of the host cell wall around the infection peg suggests that penetration involves both mechanical force and enzymic dissolution. In compatible interactions, penetration was followed by formation of biotrophic globular infection vesicles in epidermal cells. Filamentous primary hyphae developed from the vesicles and went on to colonize many other host cells as an intracellular mycelium. Host cells initially survived penetration. The host plasma membrane invaginated around infection vesicles and primary hyphae and was appressed tightly to the fungal cell wall, with no detectable matrix layer at the interface. Necrotrophic secondary hyphae appeared after 66 h and ramified through host tissue both intercellularly and intracellularly, forming hypostromatic acervuli by 114 h. Production of secondary hyphae was accompanied by the appearance of electron-opaque material within infected cells. This was thought to represent the host phytoalexin response. In incompatible interactions, infection vesicles and primary hyphae were formed in epidermal cells by 42 h. However, they were encrusted with electron-opaque material and appeared dead. These observations are discussed in relation to the infection processes of other Colletotrichum spp. and the host phytoalexin response.     

Microscopy of invasion of red clover roots by Trichocladium basicola, and effects of benomyl and prochloraz

Roots of red clover seedlings grown on plates of water agar, or water agar containing benomyl or prochloraz, were inoculated with conidia of Trichocladium basicola and examined by light and transmission electron microscopy. Penetration of host epidermal cells occurred from about 16 h after inoculation of untreated or fungicide-treated seedlings. Intracellular hyphae were constricted at septa and had a beaded appearance. They invaginated the host plasmalemma, but had no obvious deleterious effect on the cytoplasm until they had grown to fill much of the lumen, when host cells degenerated and died. As colonization of the cortex progressed, straight, unconstricted hyphae were formed and from these reproductive hyphae developed, which produced endoconidiophores and chlamydospores on the root surface. Penetration of host cell walls appeared to involve localized action of fungal enzymes. Papillae were often found at sites of penetration, but these rarely obstructed fungal development. Seedlings treated with prochloraz had fewer sites of fungal penetration, and fewer cells in the beaded hyphae than untreated seedlings or those treated with benomyl. Both fungicides caused abnormalities in fungal ultrastructure. Hyphae treated with benomyl were often found to contain lomasomes, while those treated with prochloraz had thickened, fragmented walls, and disorganized cell contents.     

Fine Structure of the Early Interaction of Lily Roots with Fusarium oxysporum f.sp. lilii

The early interaction of lily roots with the cortical rot pathogen Fusarium oxysporum f.sp. lilii was studied using roots of lily bulblets grown in Hoagland's solution, inoculated with the pathogen, and sampled up to 48h later. Conidia produced germ tubes within 6h, which extended towards and into the mucilage covering the root elongation zone, and along and into the anticlinal grooves and middle lamellae of epidermal cells. By 24–48h, infecting hyphae had reached the periclinal walls and intercellular spaces between the epidermis and the outermost cells of the cortex. Penetration of intercellularly growing hyphae directly across host cell walls was not observed; invasion of the cell lumen only occurred by gradual infringing of hyphae upon successive primary wall layers. Non-cellulosic wall appositions rich in vesicles and covered by a cellulosic protective-like layer were formed in response to approaching hyphae in resistant cv.Connecticut King, but rarely in susceptible cv. Esther which seemed more susceptible to plasmolysis and rot. Finger-like projections of the appositions into the host cell cytoplasm likely represent early stages of transfer cell formation.     

Subcuticular-Intracellular Hemibiotrophic and Intercellular Necrotrophic Development of Colletotrichum acutatum on Almond

ABSTRACT The early infection and colonization processes of Colletotrichum acutatum on leaves and petals of two almond cultivars with different susceptibility to anthracnose (i.e., cvs. Carmel and Nonpareil) were examined using digital image analysis of light micrographs and histological techniques. Inoculated tissue surfaces were evaluated at selected times after inoculation and incubation at 20 degrees C. Depth maps and line profiles of the digital image analysis allowed rapid depth quantification of fungal colonization in numerous tissue samples. The results showed that the early development of C. acutatum on petals was different from that on leaf tissue. On petals, conidia germinated more rapidly, germ tubes were longer, and fewer appressoria developed than on leaves. On both tissues, penetration by the pathogen occurred from appressoria and host colonization was first subcuticular and then intracellular. On petals, colonizing hyphae were first observed 24 h after inoculation and incubation at 20 degrees C, whereas on leaves they were seen 48 to 72 h after inoculation. Intercellular hyphae were formed before host cells became necrotic and macroscopic lesions developed on petals >/=48 h and on leaves >/=96 h after inoculation. Histological studies complemented data obtained by digital image analysis and showed that the fungus produced infection vesicles and broad hyphae below the cuticle and in epidermal cells. In both tissues, during the first 24 to 48 h after penetration fungal colonization was biotrophic based on the presence of healthy host cells adjacent to fungal hyphae. Later, during intercellular growth, the host-pathogen interaction became necrotrophic with collapsed host cells. Quantitative differences in appressorium formation and host colonization were found between the two almond cultivars studied. Thus, on the less susceptible cv. Nonpareil fewer appressoria developed and host colonization was reduced compared with that on cv. Carmel.     

禾谷镰刀菌在小麦穗部侵染过程的细胞学研究

 采用扫描和透射电镜技术系统地观察了禾谷镰刀菌(Fusarium graminearum)在小麦穗部的侵染过程。接种后6~12 h,分生孢子在小麦穗部的任何部位均可萌发,每个孢子可产生1至多个芽管,新产生的芽管并不立即入侵寄主组织,而是在寄主体表生长扩展;接种后36~48 h观察,小穗颖片、外稃、内稃的内侧和子房的表面形成了密集的菌丝网,然而在小麦穗轴表面、颖片和内稃的外表面,菌丝生长缓慢、分布稀疏,但颖片外表边缘的菌丝可跨越边缘扩展到颖片的内表皮上;接种后36 h,寄主体表菌丝产生入侵菌丝,以直接入侵方式由颖片、外稃、内稃的内侧及子房的顶部侵入寄主组织体内,随后,菌丝以胞间和胞内生长的方式向下扩展;接种后4~5 d,菌丝由上述组织扩展到达穗轴后,在穗轴内沿微管束组织和皮层组织向上和向下扩展,延伸到相邻小花,随菌丝在小麦穗部组织内不断地生长扩展,使得寄主细胞坏死、解体,并最终导致整个麦穗的枯死。     

Infection of resistant and susceptible cultivars of wheat by Pyrenophora tritici-repentis

Conidial germination, appressorial formation. penetration of epidermal walls, formation of intracellular vesicles and growth of intracellular hyphae in epidermal cells occurred within 12 h of inoculation. Hyphae then grew slowly between mesophyll cells for the next 12 h. Some papillae formed beneath appressoria and most infected epidermal cells retained stain by 24 h after inoculation, indicating major changes in cellular physiology. Slight differences between cultivars in some of these events were not related to resistance.
On the second day. intercellular hyphae emerged more extensively from the infection sites into the mesophyll of the susceptible cultivar Banks, and formed significantly larger mycelia than in the resistant cultivar BH1146 by 3-5 days from inoculation. Rapid intercellular growth then continued in the susceptible cultivar but not in the resistant cultivar. Necrotic lesions expanded faster in the susceptible cultivar from day 3. By day 10. most lesions in this cultivar were large and light brown with a conspicuous chlorotic margin but those in the resistant cultivar were small and dark brown with inconspicuous chlorosis.     

Infection and colonization of strawberry by <Emphasis Type="Italic">Gnomonia fragariae</Emphasis> strain expressing green fluorescent protein

Gnomonia fragariae is a poorly studied ascomycete, which was recently demonstrated to be a cause of severe root rot and petiole blight of strawberry. The pathogen was genetically transformed with the GFP as a vital marker and hygromycin resistance gene. Several stable transformants were obtained, which did not differ in their phenotype from the wild type isolate. Using one of the GFP-tagged isolates the infection process and colonization of roots and petioles of host plant by the pathogen were studied. Fluorescence microscopy examinations of the inoculated plants at different time points showed that plant infection occurs 24 h after inoculation and intensively continues during first 3 days. The specific penetration sites on epidermal cells and preferences in colonization for certain root and petiole tissues were observed. The pathogen intensively colonized and destroyed cortex of roots and petioles and spread rapidly longitudinally within intercellular spaces. The petioles were colonized by the hyphae, which grew mostly in the intracellular spaces of the cortical cells while in the roots the intracellular growth of hyphae occurred only in the later stages of infection. The fungus was also capable to infect the vascular tissues of petioles although these were not the primary tissues colonized by the pathogen. The mature ascomata were formed on the infected petiole bases several weeks after the inoculation. This study presents a genetic transformation method for Gnomonia fragariae and it demonstrates details on infection process and colonization of root, crown and petiole tissues of strawberry by the pathogen.     

Histology of Sclerotium cepivorum infection of onion roots and the spatial relationships of pectinases in the infection process

Sclerotium cepivorum (isolate Sc4) hyphae penetrated the epidermis and hypodermis of onion roots and grew into the cortex. Immediately following penetration only the cells through which S. cepivorum grew were lysed, but subsequently cells were killed and cell walls disintegrated ahead of the infection hyphae. Sclerotium cepivorum produced two polygalacturonases (PG) and two pectinesterases (PE) in culture. These isozymes were also found in infected onion root tissues and another PG and a PE were occasionally detected. Two isozymes of PG and three isozymes of PE diffused ahead of the infection hyphae. The spatial distribution of these enzymes was associated with cell death and cell wall degradation. The epidermis, hypodermis, endodermis and vascular tissues were more resistant to hydrolysis than the cortex, but only the endodermis and cells within it retained nuclei following hydrolysis of the surrounding cortical tissues. The cavity within the root cortex became filled with swollen, vacuolate S. cepivorum hyphae.     

荸荠茎点霉秆枯病菌侵染过程的超微观察

<正>荸荠(Eleocharis dulcis),又称马蹄,为莎草科多年生草本植物,是一种具有食用和药用价值的水生蔬菜。近年来,随着荸荠在我国种植面积的不断扩大,病害发生也呈逐年上升趋势。荸荠茎点霉秆枯病是2009年在湖北省荸荠产区发现的一种新病害,由Phoma bellidis侵染引起,该病在湖北省团风地区发生尤为严重,对荸荠的产量和品质造成严重影响;病害一般在8~12月发生,发病初期在荸荠茎秆上产生圆形或梭形红褐色小斑,随后病斑沿茎     

大豆疫霉菌对大豆下胚轴侵染过程的细胞学研究

 接种后1.5~24h,用光镜和电镜研究了2个大豆品种与大豆疫霉菌Ps411的亲和性和非亲和性互作。观察结果表明,大豆疫霉菌对大豆下胚轴的侵染过程可分为侵入前、侵入、皮层组织中的扩展和进入维管束组织4个连续阶段。大豆下胚轴接种后在25℃保湿培养,1.5h后游动孢子即形成休止孢并萌发产生附着孢,3h后侵入表皮细胞,6h后进入皮层组织,24h后进入维管束组织。病原菌主要以侵染菌丝直接侵入表皮,表皮细胞间隙是主要侵入部位。皮层细胞是病原菌定殖和发展的主要场所,胞间菌丝侵入皮层细胞并形成吸器。在菌丝与寄主细胞接触部位的寄主细胞壁与质膜之间常有胞壁沉积物的形成。在抗病品种上病菌的侵染事件与感病品种基本一致,但不能形成正常的吸器,胞壁沉积物明显多于感病品种,菌丝在寄主组织内的扩展明显受到抑制。利用β-1,3-葡聚糖免疫金标记单克隆抗体进行的免疫细胞化学的研究表明,胞壁沉积物内含有大量的β-1,3-葡聚糖,在大豆疫霉菌菌丝壁中也存在β-1,3-葡聚糖。以上结果表明,病原菌的侵染可诱导抗病寄主细胞内β-1,3-葡聚糖迅速的合成与积累、并形成胞壁沉积物,以抵御病菌的侵染与扩展。     

Histological Comparison of Fibrous Root Infection of Disease-Tolerant and Susceptible Citrus Hosts by Phytophthora nicotianae and P. palmivora

ABSTRACT Phytophthora nicotianae and P. palmivora infect and cause rot of fibrous roots of susceptible and tolerant citrus rootstocks in Florida orchards. The infection and colonization by the two Phytophthora spp. of a susceptible citrus host, sour orange (Citrus aurantium), and a tolerant host, trifoliate orange (Poncirus trifoliata), were compared using light and electron microscopy. Penetration by both Phytophthora spp. occurred within 1 h after inoculation, regardless of the host species. No differences were observed in mode of penetration of the hypodermis or the hosts' response to infection. After 24 h, P. palmivora had a significantly higher colonization of cortical cells in susceptible sour orange than in tolerant trifoliate orange. Intracellular hyphae of both Phytophthora spp. were observed in the cortex of sour orange, and cortical cells adjacent to intercellular hyphae of P. palmivora were disrupted. In contrast, the cortical cells of sour orange and trifoliate orange adjacent to P. nicotianae hyphae and the cortical cells of trifoliate orange adjacent to P. palmivora were still intact. After 48 h, the cortical cells of both hosts adjacent to either Phytophthora spp. were disrupted. After 48 and 72 h, P. palmivora hyphae colonized the cortex of sour orange more extensively than the cortex of trifoliate orange; P. palmivora also colonized both hosts more extensively than P. nicotianae. A higher rate of electrolyte leakage among host-pathogen combinations reflected the combined effects of greater cell disruption by P. palmivora than by P. nicotianae, and the higher concentration of electrolytes in healthy roots of trifoliate orange than of sour orange. Although cellular responses unique to the tolerant host were not observed, reduced hyphal colonization by both pathogens in the cortex of trifoliate orange compared with sour orange is evidence for a putative resistance factor(s) in the trifoliate orange roots that inhibits the growth of Phytophthora spp.     

Ultrastructural Characterization of Infection and Colonization of Maize Leaves by Colletotrichum graminicola, and by a C. graminicola Pathogenicity Mutant

ABSTRACT Observations were made of the ultrastructure of infection and colonization of leaves of a susceptible maize inbred by Colletotrichum graminicola and by a C. graminicola pathogenicity mutant. The mutant causes no symptoms on either maize leaves or stalks. Prior evidence suggested that it is deficient in production of signal peptidase, responsible for cleavage of signal peptides from proteins destined for transport through the endoplasmic reticulum. There was no significant difference in the process of infection or colonization by the mutant and wild-type strains up to 48 h after inoculation. Both the mutant and the wild type produced globose, melanized appressoria within 24 h after inoculation on the host surface. By 36 h, both strains had penetrated the host epidermal cells directly. The host cells frequently formed papillae in response to appressoria, but these were not usually successful in preventing fungal ingress in either case. Penetration was followed by formation of irregularly shaped, swollen infection hyphae. Infection hyphae of both strains grew biotrophically for a relatively short time (less than 12 h). One or more hyphal branches was produced from each infection hypha, and these invaded adjacent mesophyll cells. Both strains of the fungus grew cell-to-cell, setting up new biotrophic interactions in each cell, between 36 and 48 h after inoculation. Papillae were frequently formed by the mesophyll cells, but these were not successful in preventing fungal ingress. The first noticeable difference between the mutant and the wild type was related to their interaction with mesophyll cells. Cells invaded by the wild type died relatively quickly, whereas those infected by the mutant appeared to survive longer. The most dramatic difference between the mutant and wild type occurred when the mutant completely failed to make a transition to necrotrophic growth, while the wild type made that switch at 48 to 72 h after inoculation. The mutant may be unable to secrete sufficient quantities of one or more proteins that are necessary to support the switch between biotrophy and necrotrophy.     

Light and Scanning Electron Microscopy Studies on the Infection of Oriental Lily Leaves By Botrytis Elliptica

Light, scanning electron and fluorescent microscopy were used to observe the infection process of Botrytis elliptica on leaves of oriental lily (cv. Star Gazer). At 20 °C and 100% relative humidity, conidia germinated on both adaxial and abaxial foliar surfaces, but germ tubes failed to invade epidermal cells on the adaxial surface. On abaxial surfaces, short (< 20 m) swollen germ tube appressoria penetrated through stomatal openings (19%), through the epidermis near guard cells (52%), or directly through epidermal cells (29%). Esterase activity was detected on germ tubes and conidia after 6 h of incubation, and deformation of the cuticle on abaxial surfaces of lily was observed surrounding infection sites. By 3 h after inoculation, almost 70% of the conidia had germinated, but no penetration was observed. At 6 h after inoculation, almost one-third of germinated conidia had penetrated epidermal cells, and water-soaked lesions were associated with 20% of the penetrations. By 9 h after inoculation, approximately 60% of the germinated conidia had penetrated plant tissues, and water-soaked lesions were associated with 60% of the infections. Fluorescent microscopy with a specific fungal stain allowed assessment of successful infection and visualization of sub-epidermal hyphae. We conclude that penetration of abaxial foliar surfaces of oriental lilies by B. elliptica occurs via short swollen germ tube appressoria mostly near stomata.     

Interaction of root-lesion nematodes, Pratylenchus thornei and P. crenatus, with the fungus Thielaviopsis basicola and a grey sterile fungus on roots of pea (Pisum sativum)

Pratylenchus thornei invaded excised pea roots in agar in greater numbers and penetrated the cortex more deeply than P. crenatus . Both species fed on the roots ectoparasitically and displaced root cells into the surrounding medium. The cytoplasm of cortical cells near cither nematode became granulated, with enlarged vacuoles and nuclei. P. thornei also caused these responses in the endodermis. Infection of the root surface with a grey sterile fungus inhibited invasion by P. crenatus and P. thornei . Infection by Thielaviopsis basicola inhibited P. thornei invasion but encouraged penetration by P. crenatus and the hyphae were found deeper in the cortex when P. crenatus was present.