In vitro screening of rice germplasm for resistance to brown spot disease using phytotoxin

In vitro selection of blackberry (Rubus fruticosus 'Tupy') plants resistant to Botrytis cinerea using gamma ray-irradiated shoot tips

Blackberry is an economically important crop in Mexico, and its yield is substantially reduced by gray mold, a disease caused by Botrytis cinerea. One of the means to obtain B. cinerea-resistant plants is gamma irradiation. Shoot tips of in vitro-micropropagated blackberry plants (Rubus fruticosus 'Tupy') were irradiated with five doses of Cobalt-60 gamma radiation (0, 15, 30, 45, and 60 Gy) and cultured on Murashige and Skoog basal medium containing 1.0 mg l^-1 benzylaminopurine and 0.06 mg l^-1 indole-3-butyric acid (MSB medium). After 28 days of culture, survival was evaluated to determine mean lethal dose (LD₅₀), and 200 shoots were further irradiated at the determined LD₅₀ (30.8 Gy). After 28 days, the surviving shoots were micropropagated on MSB medium for 60 days. Non-irradiated shoots were screened for the in vitro selection of resistant B. cinerea, exposing them to different concentrations of sterile culture filtrate of B. cinerea (0, 2, 4, 6, 8, and 10 g l^-1) for 28 days to determine mean lethal concentration (LC₅₀), and the irradiated surviving shoots were further exposed to the determined LC₅₀ (4.6 g l^-1). Three surviving lines (rfgum5, rfgum6, and rfgum17) that did not present changes compared with the control shoots were micropropagated to obtain plantlets, which were further subjected to in vitro resistance assays using detached leaves inoculated with B. cinerea (1×10³ spores ml^-1). Plants of rfgum5 and rfgum6 mutant lines were highly resistant and presented similar growth to control plants. Therefore, this methodology is useful to obtain B. cinerea-resistant blackberry plants.

In vitro generation of somaclonal variant plants of sugarcane for tolerance to Fusarium sacchari

KEY MESSAGE : A combination of in vitro culture and mutagenesis using ethyl methanesulfonate (EMS) followed by culture filtrate-mediated selection produced variant sugarcane plants tolerant and resistant to Fusarium sacchari. Eldana saccharina is a destructive pest of the sugarcane crop in South Africa. Fusarium sacchari PNG40 (a fungal strain harmful to E. saccharina) has the potential to be an endophytic biological control agent of the stalk borer. However, the fungus causes Fusarium stalk rot in sugarcane. In the current study, sugarcane plants tolerant and resistant to F. sacchari PNG40 were produced by exposing embryogenic calli to the chemical mutagen ethyl methanesulfonate (EMS), followed by in vitro selection during somatic embryogenesis and plantlet regeneration on media containing F. sacchari culture filtrates (CF). The incorporation of 100 ppm CF in the culture media at the embryo maturation stage, at germination, or at both, resulted in callus necrosis and consequent reduced plantlet yield. Subsequent trimming of the roots of regenerated plants and their exposure to 1,500 ppm CF served as a further selection treatment. Plants produced from EMS-treated calli displayed improved root re-growth in the presence of CF pressure compared with those from non-treated calli. The tolerance of CF-selected plants was confirmed in greenhouse tests by inoculation with F. sacchari PNG40, re-isolation of Fusarium spp. from undamaged tissue of asymptomatic plants and establishment of the identity of fungal isolates as PNG40 using molecular analysis. The restriction of PNG40 presence to the inoculation lesion in some plants suggested their resistance to the fungus. Genotypes exhibiting symptomless endophytic colonization by PNG40 were identified and will be utilised for testing biological control strategies against E. saccharina.

Genetic and molecular analysis of a purple sheath somaclonal mutant in japonica rice

Natural and artificially induced mutants have provided valuable resources for plant genetic studies and crop improvement. In this study, we investigated the genetic and molecular basis of the purple sheath trait in a somaclonal mutant Z418, which was regenerated from a green sheath rice variety C418 through tissue culture. The purple sheath trait in Z418 was heritable and stable based on our 10 years of evaluation. Genetic analysis revealed that the purple sheath trait of the mutant was controlled by a single dominant gene. To map the gene, we scored 89 polymorphic SSRs markers in a F(2) population of 232 plants derived from a cross between Z418 and HX-3, an indica variety with green sheath trait. The gene was initially mapped to the short arm of chromosome 6 between two SSR markers, RPM5 and RM402, with a genetic distance of 1.1 and 10.3 cM, respectively. Thirty-one SSR and indel markers located within the target region were further used to fine-map the gene to a 153-kb interval between two SSR markers (RPM8 and RPM11). The OsC1 gene, which locates within the region and encodes a MYB family transcription factor, was chosen as the candidate gene controlling the purple sheath trait in Z418. Sequencing analysis revealed that OsC1 gene and its transcript in Z418 was 34 bp longer than that in C418. The possible mechanisms for the gene mutation, the developmental and tissue-specific expression of purple anthocyanin pigmentation in Z418, were finally discussed.

Toxin-based in-vitro selection and its potential application to date palm for resistance to the bayoud Fusarium wilt

Date palm (Phoenix dactylifera L.) is qualified as a 'tree' of great ecological and socio-economical importance in desert oases. Unfortunately, it is being decimated, especially in Morocco and Algeria, by a fusariosis wilt called bayoud and caused by Fusarium oxysporum f. sp. albedinis (Fao). Controlling this disease requires the implementation of an integrated management program. Breeding for resistance is one of the most promising component strategies of this program. Few naturally resistant cultivars with a mediocre fruit quality (dates) are known. Conventional and non-conventional methods are under development and have to use the simplest and easiest methods to screen for resistant individuals. The use of pathogen toxins as selective agents at the tissue culture step might be a source of variability that can lead to the selection of individuals with suitable levels of resistance to the toxin and/or to the pathogen among the genetic material available. Foa produces toxins such as fusaric, succinic, 3-phenyl lactic acids and their derivatives, marasmins and peptidic toxins. These toxins can be used bulked or separately as selective agents. The aim of this contribution was to give a brief overview on toxins and their use as a mean to select resistant lines and to initiate a discussion about the potential use of this approach for the date palm-Foa pathosystem. This review does not pretend to be comprehensive or exhaustive and was prepared mainly to highlight the potential use of Foa toxins for selecting date palm individuals with a suitable resistance level to bayoud using toxin-based selective media.

Effects ofVerticillium albo-atrum culture filtrate on somatic embryogenesis in alfalfa

Cell suspensions derived from young petioles of alfalfa (Medicago sativa L.) were cultured in the presence and absence of aVerticillium albo-atrum culture filtrate (20% v/v) for 6 cycles. The frequency of somatic embryogenesis and the growth rate of the suspension cultures were investigated at each cycle. Somatic embryogenesis in the filtrate-treated cultures declined but was still at a relatively high level after 6 subcultures, compared to controls cultures which virtually lost the capacity for embryo formation in the same period. The decline in the embryogenic capacity of filtrate treated-cultures was accompanied by a six-fold increase in the rate of growth of the cultures.

Antibacterial Activity of Sirodesmin PL Phytotoxin: Application to the Selection of Phytotoxin-Deficient Mutants

Sirodesmin PL, a phytotoxin and mycotoxin produced by Leptosphaeria maculans , the causal agent of stem-canker disease of crucifers, exhibited antibacterial activity against gram-positive bacteria and particularly Bacillus subtilis. The importance of the disulfide bridge of the molecule in antibacterial activity was demonstrated. A simple and reliable bioassay based on the antibacterial activity of the toxin was performed for screening sirodesmin PL-deficient mutants when grown on solid culture medium. A mutant was selected and found to produce 3,700-fold less toxin than did the wild-type strain. A sensitive procedure for quantification of the toxin by high-pressure liquid chromatography was developed. Levels of product as low as 100 ng could be detected by this procedure.

Differences in response to the toxin sirodesmin PL produced by Phoma lingam (Tode ex fr.) Desm. on protoplasts, cell aggregates and intact plants of resistant and susceptible Brassica accessions

The selective property of sirodesmin PL, a toxin produced by Phoma lingam, was studied on protoplasts, cell aggregates, leaves and roots. Directly after isolation, protoplasts from all the different Brassica accessions were sensitive when treated with toxin in a concentration higher than 1 μM. When more differentiated plant tissue. i.e. cell aggregates, leaves or roots, were investigated, insensitivity to the toxin was found in the plant material resistant to P. lingam, while the plant material susceptible to P. lingam was sensitive. The results reveal that a clear correlation between resistance to P. lingam and insensitivity to sirodesmin PL is present, and that the toxin can be used to distinguish resistant plant material from susceptible both in vivo and in vitro.

In vitro selection of alfalfa plants resistant to Fusarium oxysporum f. sp. medicaginis

From two lines of Medicago sativa characterized by a high regeneration capability, calli resistant to culture filtrate of Fusarium oxysporum f. sp. medicaginis have been selected. In these calli regeneration capability was greatly reduced and only one plant per callus was recovered. Regenerated plants have been evaluated for resistance to culture filtrate and for in vivo resistance to the pathogen. Three plants out of eight were resistant to the fungus and a high correlation between resistance to culture filtrate and in vivo resistance was observed.

A single dominant gene for Fusarium wilt resistance in protoplast-derived tomato plants

Tomato plants resistant to the fungal pathogen, Fusarium oxysporum f. sp. lycopersici, race 2, were obtained using in vitro selection against fusaric acid, a non-specific toxin, as well as non-challenged cells. Protoplasts were isolated from cotyledonary tissue of tomato cv. 'UC-82', which is susceptible to Fusarium race 2. Protoplasts were challenged with the toxin, and the resistant calli were further subjected to the toxin. Plants regenerated from toxin-resistant calli were screened for resistance to the pathogen by using the Fusarium slurry inoculation technique. Seeds were collected from the surviving individuals, germinated and rescreened for resistance to the pathogen. Data obtained from this test showed a ratio of three resistant to one susceptible among R1 progenies. Further analysis of the R2 progenies confirmed that the fusarium-resistant plants were either homozygous or heterozygous dominant for the gene conferring the resistance. Similar results were recorded for tomato plants regenerated from cells that received no selection pressure. The nature of this single dominant gene-type of resistance is under investigation.

Is the Phytophthora citrophthora culture filtrate a reliable tool for the in vitro selection of resistant Citrus variants?

Nucellar calli from four Citrus cultivars with known resistance to the Phytophthora citrophthora pathogen were chosen as experimental material to test the pathogen's response to culture filtrate (CF). Sensitivity of the four calli to CF of the fungus was in reverse order to what is known on the susceptibility of the cultivars in vivo. Sensitivity of protoplasts derived from the same four calli to 2,4-dichlorophenoxyacetic acid (2,4-D) was in the same order as that of calli to CF. Protoplasts derived from calli selected for tolerance to CF showed a higher plating efficiency with increasing concentration of CF in the medium. TLC and GLC determinations showed the presence of indole acetic acid in the culture filtrate. Results indicate that CF of P. citrophthora cannot be used as a selection tool in vitro.