Resistance responses to Phoma lingam of plants regenerated from selected cell and embryogenic cultures of haploid Brassica napus

The current scenario and future perspectives of transgenic oilseed mustard by CRISPR-Cas9

PURPOSE

Production of a designer crop having added attributes is the primary goal of all plant biotechnologists. Specifically, development of a crop with a simple biotechnological approach and at a rapid pace is most desirable. Genetic engineering enables us to displace genes among species. The newly incorporated foreign gene(s) in the host genome can create a new trait(s) by regulating the genotypes and/or phenotypes. The advent of the CRISPR-Cas9 tools has enabled the modification of a plant genome easily by introducing mutation or replacing genomic fragment. Oilseed mustard varieties (e.g., Brassica juncea, Brassica nigra, Brassica napus, and Brassica carinata) are one such plants, which have been transformed with different genes isolated from the wide range of species. Current reports proved that the yield and value of oilseed mustard has been tremendously improved by the introduction of stably inherited new traits such as insect and herbicide resistance. However, the genetic transformation of oilseed mustard remains incompetent due to lack of potential plant transformation systems. To solve numerous complications involved in genetically modified oilseed mustard crop varieties regeneration procedures, scientific research is being conducted to rectify the unwanted complications. Thus, this study provides a broader overview of the present status of new traits introduced in each mentioned varieties of oilseed mustard plant by different genetical engineering tools, especially CRISPR-Cas9, which will be useful to improve the transformation system of oilseed mustard crop plants.

METHODS

This review presents recent improvements made in oilseed mustard genetic engineering methodologies by using CRISPR-Cas9 tools, present status of new traits introduced in oilseed mustard plant varieties.

RESULTS

The review highlighted that the transgenic oilseed mustard production is a challenging process and the transgenic varieties of oilseed mustard provide a powerful tool for enhanced mustard yield. Over expression studies and silencing of desired genes provide functional importance of genes involved in mustard growth and development under different biotic and abiotic stress conditions. Thus, it can be expected that in near future CRISPR can contribute enormously in improving the mustard plant's architecture and develop stress resilient oilseed mustard plant species.

In vitro selection for Fusarium wilt resistance in Gladiolus

Cormels pieces of four Fusarium susceptible Gladiolus cultivars (Friendship, Peter Pears, Victor Borge and Novalux) formed friable calli when cultured in vitro on Murashige and Skoog basal medium containing various concentrations of auxin and cytokinin. The friable calli established cell suspensions. Plantlet regeneration was obtained from the control callus, control cell suspension derived callus and in vitro selected Fusarium oxysporum Schlecht. resistant cell-lines of Friendship. The in vitro cormlets showed 85-95% germination after breaking dormancy of 8 weeks at 4 degrees C. Cell suspensions of all four Gladiolus cultivars were found to be highly sensitive to fusaric acid. Gradual increase in fusaric acid concentrations to the cell-suspension cultures decreased cell growth considerably. One albino plant was found from the second generation of the in vitro selected cell line of Friendship. The albino plant was found to be highly susceptible to F. oxysporum. The cormlets of all in vitro selected cell lines of Friendship were inoculated with a conidial suspension of the F. oxysporum before planting and were also sprayed with the same spore suspension for further characterization when the height of plants was about 6 cm. The four selected cell lines showed the same response whether or not they were inoculated with conidia of the F. oxysporum. Plantlets of all of the selected cell lines exhibited significant growth as compared with the control after application of conidia of the F. oxysporum.

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.

In vitro mutation and selection of doubled-haploid Brassica napus lines with improved resistance to Sclerotinia sclerotiorum

This paper describes a new protocol to develop doubled-haploid (DH) Brassica napus lines with improved resistance to Sclerotinia sclerotiorum. In this protocol, haploid seedlings derived from microspore cultures of B. napus were used to produce haploid calli for in vitro mutation-selection. For routine screening, mutation was induced by EMS (ethylmethane sulfonate) or occurred spontaneously, and screening for resistant mutants occurred on media with added oxalic acid (OA) as a selection agent. In tests with selected lines, the optimal concentration of EMS for mutation was determined to be 0.15%, and the optimal concentration of OA for in vitro screening was 3 mmol/l (half lethal dose was 3.1 mmol/l) for the first cycle of screening. There was an accumulated effect of OA toxicity on calli over two cycles of screening, but the growth and capacity of the surviving calli for regenerating seedlings were not affected by OA. Of the 54 DH lines produced from the in vitro mutation-selection, two DH lines of resistant mutants, named M083 and M004, were selected following seedling and glasshouse tests. The resistance of M083 and M004 to S. sclerotiorum following tests with both mycelial inoculum and OA was greater than that of their donor lines and the resistant control Zhongyou 821. In both glasshouse and field disease nurseries, disease indices on M083 and M004 were less than 50% of those of the control. The time required for M083 and M004 to mature was 14 days and 10 days shorter, respectively, than that of their donor lines. Furthermore, M083 had more pods per inflorescence, a greater 1,000 seed weight and higher yield than its donor line. Random amplified polymorphic DNA characterisation showed that M083 had DNA band patterns that differed from its donor line.

Strain typing of polish Leptosphaeria maculans isolates supports at the genomic level the multi-species concept of aggressive and non-aggressive strains

47 Polish isolates of the blackleg fungus Leptosphaeria maculans (Phoma lingam) were compared with eight well-defined reference strains from Germany, France, Denmark, Australia and one Polish isolate of Phoma nigrificans. The isolates were tested (i) for growth characteristics, (ii) for their ability to form sirodesmins, (iii) for cellulolytic enzymes, and (iv) for pathotype-differentiating molecular markers generated by RAPD-PCR, PCR analysis with pathotype-specific primer pairs and PFGE. With two exceptions all Polish isolates do not form sirodesmins. grow rapidly without penetrating into the substrate and form in most cases yellow or brown pigments in Czapek-Dox liquid cultures. With respect to cellulase secretion and molecular fingerprinting Polish A strains (aggressive) fit into the general picture of the aggressive pathotype group, whereas the NA isolates (non-aggressive) display a higher degree of heterogeneity. This matches with inoculation tests on rape seedlings, which revealed a considerable number of isolates ranging in aggressivity between the conventional A and NA pathotype group. Molecular fingerprinting techniques unequivocally sorted intermediately aggressive isolates into the NA pathotype group. Isolate Ph Bial, which produces sirodesmin but groups within NA isolates according to molecular and physiological markers, may represent a novel third group besides A and NA strains with intermediate aggressivity (IA). We hybridized Southern blots of electrophoretically separated chromosomes with radioactively labelled PCR fragments used for differentiation between A and NA isolates. The specificity of diagnostic PCR amplicons is reflected at the genomic level. The A probe reveals a single hybridizing chromosome exclusively in A strains. The NA probe reveals several chromosomes and is specific for the NA pathotype group. Chromosomes from intermediately aggressive strains are equally well recognized by the NA probe as are Polish isolates with low aggressivity and give no signal with the A probe. Both diagnostic DNA sequences are highly specific for the pathotype group they were derived from. The lack of correspondence of both genetic elements between A and NA strains strongly supports the idea of ascribing the pathotype groups to different species. Whereas the A pathotype group is genetically homogeneous and congruent with the species Leptosphaeria maculans, the NA group needs to be revised taxonomically. NA isolates will presumably have to be split into several independent species.

High frequency production of embryos from liquid flask cultures of oilseed rape

As part of the program to scale-up the production of artificial seeds of winter oilseed rape, Brassica napus ssp. oleifera, we established a liquid flask culture system that enables the high frequency production of freely suspended embryos. As many as 4000 embryos could be obtained from 1 mL packed-cell-volume of cells. For initiation of liquid flask cultures, four different types of callus tissues were used. Among them, the most embryogenic cell suspension cultures were obtained from spontaneous callus developed on the surface of secondary embryos precultured in medium supplemented with 2,4-dichlorophenoxyacetic acid (4.52 microM) and kinetin (0.46 microM) (type B callus). Growth curves of the cell suspension were determined and the cell suspension was able to grow in medium without plant growth regulators. Embryos were observed to developed directly from the cells without going through an obvious callus phase. When subcultured to agar medium containing 44.38 microM benzylaminopurine, about 43% of the embryos developed into plants.

Differential responses of citrus calli and protoplasts to culture filtrate and toxin of Phoma tracheiphila

Nucellar calli from two citrus cultivars with known tolerance to mal secco disease were chosen as experimental material, to test the pathogen's response to culture filtrate (CF) and partially purified toxin (PPT). The response of the two calli to the CF was in reverse order to the known response of the two cultivars to natural and artificial inoculations with Phoma tracheiphila. HPLC analysis of P. tracheiphila CF indicated the presence of a relatively high level of indole acetic acid (IAA). The response of the two calli and protoplasts derived from these calli to increasing amounts of IAA in the culture media was in the same order as that of calli and protoplasts to CF. In contrast, the responses to PPT of calli and protoplasts from these two types confirmed the relative tolerance of 'Femminello' lemon and 'Tarocco' orange trees to mal secco disease.

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.

Selection of peach cells for insensitivity to culture filtrates of Xanthomonas campestris pv. pruni and regeneration of resistant plants

Individual callus cultures were initiated from 400 immature embryos of bacterial leaf spot-susceptible 'Sunhigh' peach. Each was subjected to several selection cycles of a toxic culture filtrate produced by Xanthomonas campestris pv. pruni, the causal agent of leaf spot of peach. Progressively higher concentrations of the filtrate were used in each cycle. Two calli survived, and two plants were regenerated from each of the surviving calli. Each of the four clones was propagated in vitro and tested for whole plant resistance to X. c. pv. pruni. Results from bioassays on greenhouse-grown plants indicated that two out of the four selected clones were significantly more resistant to X. c. pv. pruni than the parental cv 'Sunhigh'. In addition, one clone was significantly more resistant than the moderately resistant cv 'Redhaven'.

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.

THE EFFECT OF A PHYTOTOXIN FROM ALTERNARIA BRASSICICOLA ON BRASSICA POLLEN

A methanol-soluble extract from the culture filtrate of Alternaria brassicicola (Schw.) Wilts., grey leaf spot, markedly inhibited pollen germination and pollen tube growth of Brassica napus L., Brassica campestris L. and Brassica oleracea L. in hanging drop culture at concentrations comparable to those causing death of B. napus secondary embryoids. The extract was partially purified by Sephadex column chromatography, and the fractions obtained were tested for their inhibitory capacity. Two fractions which killed secondary embryoids also inhibited pollen germination. The methanol-soluble extract and inhibitory fractions from Sephadex chromatography were further tested using a thin-layer chromatographic bioassay which showed that the original extract could be separated into at least four inhibitory compounds. The potential of the techniques used for screening and selection for resistance to compounds toxic to plants is briefly outlined.

TOWARDS THE SELECTION IN VITRO FOR RESISTANCE TO ALTERNARIA BRASSICICOLA (SCHW.) WILTS., IN BRASSICA NAPUS SSP. OLEIFERA (METZG.) SINSK., WINTER OILSEED RAPE

Culture filtrates of Alternaria brassicicola (Schw.) Wilts, were found to be toxic to secondary embryoids of Brassica napus ssp. oleifera (Metzg.) Sinsk. The culture filtrates were partially purified and added to tissue culture media to produce a selection medium. Different secondary embryoid lines of B. napus were found to differ in their sensitivity to the selection medium, and it was possible to select lines which showed increased resistance to the selection medium. Some plants that were regenerated from secondary embryoids were found to be more resistant to the pathogen than were seed-grown plants of the same cultivar, although there was no correlation between sensitivity to the selection medium and susceptibility to the pathogen.

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.

Selection and regeneration of toxin-insensitive plants from tissue cultures of oats (Avena sativa) susceptible to Helminthosporium victoriae

Insensitivity to the pathotoxin victorin, which is produced by the fungus Helminthosporium victoriae (Meehan and Murphy), was selected in tissue cultures of oat (Avena sativa L.) lines heterozygous for the dominant sensitive allele Vb. The Vb allele imparts both susceptibility to H. victoriae and resistance to several races of oat crown rust (Puccinia coronata var. 'avenae', Fraser and E. Led.). None of 84 homozygous Vb Vb oat calli survived when grown on victorin-containing medium. Among 175 calli of heterozygous Vb vb cultures grown on toxin-containing medium, 16 representing 13 separate embryo-derived culture lines produced surviving callus sectors or shoots. Based on leaf bioassays of plants regenerated after toxin selection, nine culture lines gave toxin-insensitive plants and two gave plants showing the toxin sensitivity of the parent. Two selected lines failed to regenerate. Plants regenerated from 30 culture lines which had never been exposed to toxin-containing selection medium were all toxin sensitive. The toxin insensitivity of the regenerants from the toxin-selected culture lines was heritable since progeny of these plants were all insensitive. The toxin-insensitive selected lines all were found to have coincidentally lost the Vb crown rust resistance of the original line. In cytological analysis of meiotic cells of regenerants from the selected cultures, no chromosomal deficiency was found which could be associated with, and thus account for, the loss of sensitivity to the toxin. Somatic recombination and mutation to vb vb are other possible origins of toxin insensitivity in the selections. The victorin selection demonstrates that specific resistance can be selected in tissue cultures of oats. It also provides a highly sensitive scheme to test effects of culture conditions and chemical agents on induction of genetic and chromosomal changes in tissue cultures.

Correlations between in vivo resistance to Fusarium and in vitro response to fungal elicitors and toxic substances in carnation

With the aim of ascertaining the existance of a correlation between in vivo resistance to Fusarium oxysporum f. sp. dianthi and in vitro response to fungal elicitors and toxic substances, phenylalanine ammonialyase and phytoalexin accumulation, on one hand, and resistance to culture filtrate, on the other, were assayed in "in vitro" cultures of three susceptible and four resistant Dianthus caryophyllus cultivars. Cultivars showing varying degrees of resistance in vivo either tolerated higher culture filtrate concentrations ('Niki') or showed high PAL activity and phytoalexin production when treated with Fusarium elicitor ('Duca'), or responded positively to both treatments ('Mei-Ling', 'Pulcino'). No such responses were shown in tissue cultures of susceptible cultivars. The differential response to the fungal elicitor seemed to be highly specific as genetic differences between cultivars were not observed in tissue cultures treated with other biotic (Phytophthora infestans) and abiotic (HgCl2) elicitors.

Introduction of genetic material into plant cells

The tumor-inducing (Ti) plasmid of the soil microorganism Agrobacterium tumefaciens is the agent of crown gall disease in dicotyledonous plants. The Ti plasmid contains two regions that are essential for the production of transformed cells. One of these regions, termed transfer DNA, induces tumor formation and is found in all established plant tumor lines; the other, termed the virulence region, is essential for the formation but not the maintenance of tumors. Transfer DNA, which transfers to the plant genomes in a somewhat predictable manner, can be increased in size by the insertion of foreign DNA without its transferring ability being affected. The tumor-causing genes can be removed so that they no longer interfere with normal plant growth and differentiation. This modified Ti plasmid can thus be used as a vector for the transfer of foreign genes into plants.

Somaclonal genetics of rice, Oryza sativa L

The inheritance and variations of some traits of more than 2,000 somatic cell derived plants of rice (Oryza sativa L.) were investigated in the second and third generations (T2 and T3) of regenerated plants (somaclones). The percentages of multiploids occurring in somaclones ranged from 0-13.3 in nine varieties (or hybrids) of 'Hsien' (indica) group, but no multiploid was found in nine varieties (or hybrid) of 'Keng' (japonica) group. A dwarf mutant with the height of only 20 cm was isolated in the T2 of 'Tai-Zhong-Yu 39'. Genetic analysis indicated that it was controlled by a single recessive gene. The frequencies of chlorophyll mutations probably controlled by cytoplasmic genes decreased gradually with the advance of generations. The variations of five quantitative traits - plant height, grain weight, etc. - in 950 T2 pedigree lines of four varieties were also studied. Only 24.4% of the lines were normal in all the traits studied. Variation frequencies of different traits were from 11.5% to 39.5%. And there was an obvious tendency for the plant height to become shorter, number of productive tillers to increase and 1,000 grain weight to be lighter, whatever the variety studied. Traits were uniform within each of more than 90% of all T2 lines studied. What is more interesting, variations phenotyped in T2 proved to breed true. Causes of somaclonal variations are discussed, as well as their potentials in breeding.

Resistant tobacco plants from protoplast-derived calluses selected for their resistance to Pseudomonas and Alternaria toxins

Protoplast-derived calluses of tobacco (Nicotiana tabacum cv. 'Samsun') were selected for their resistance to toxins from Pseudomonas syringae pv. tabaci, which causes wildfire disease, and from Alternaria alternata pathotype tobacco, which causes brown spot. A number of plants were regenerated from each of the toxin-selected protoplast-derived calluses. A large percentage of the plants obtained from the second selection cycle calluses were resistant to infection by these pathogens. Resistance to wildfire disease, however, seems to be unrelated to resistance to brown spot disease. Variations in the morphological characteristics of the regenerated plants were found. Results of an assay of the R1 generation indicate that the resistance shown by R0 plants against both disease is heritable.