Reversion of Texas male-sterile cytoplasm maize in culture to give fertile, T-toxin resistant plants

Comparative mitochondrial genome analysis reveals the evolutionary rearrangement mechanism in Brassica

The genus Brassica has many species that are important for oil, vegetable and other food products. Three mitochondrial genome types (mitotype) originated from its common ancestor. In this paper, a B. nigra mitochondrial main circle genome with 232,407 bp was generated through de novo assembly. Synteny analysis showed that the mitochondrial genomes of B. rapa and B. oleracea had a better syntenic relationship than B. nigra. Principal components analysis and development of a phylogenetic tree indicated maternal ancestors of three allotetraploid species in Us triangle of Brassica. Diversified mitotypes were found in allotetraploid B. napus, in which napus-type B. napus was derived from B. oleracea, while polima-type B. napus was inherited from B. rapa. In addition, the mitochondrial genome of napus-type B. napus was closer to botrytis-type than capitata-type B. oleracea. The sub-stoichiometric shifting of several mitochondrial genes suggested that mitochondrial genome rearrangement underwent evolutionary selection during domestication and/or plant breeding. Our findings clarify the role of diploid species in the maternal origin of allotetraploid species in Brassica and suggest the possibility of breeding selection of the mitochondrial genome.

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.

Molecular interactions ofBipolaris maydisT-toxin and maize

The toxins (T-toxins) produced by the fungal pathogens Bipolaris maydis race T (BmT) and Phyllosticta maydis (Pm) target the mitochondrial receptor, URF13, in maize (Zea mays L.) plants containing the Texas male-sterile cytoplasm (cms-T). URF13, a 13-kDa protein, is the product of the maize mitochondrial gene T-urf13, which is found only in the mitochondrial genome of cms-T maize and is thought to be responsible for cytoplasmically inherited male sterility and disease susceptibility. Pm-toxin binds specifically to URF13 in a cooperative manner, and Pm- and BmT-toxins compete for the same, or overlapping, binding sites. The binding of T-toxin to URF13 causes rapid permcabilization of the inner mitochondrial membrane, which results in leakage of NAD+and other ions from the matrix. A pore consisting of at least six transmembrane α-helices is required for NAD+leakage. Cross-linking experiments showed that URF13 oligomers are present in the mitochondrial membrane. A model of the secondary structure of URF13 proposes that each monomer contains three transmembrane α-helices. Studies combining site-directed mutagenesis and chemical cross-linking of URF13 expressed by Escherichia coli cells indicate that the oligomers are composed of a central core of helices II that line the center of the URF13 pores. Key words: maize cytoplasmic male sterility, URF13, mitochondrial pores, T-toxin receptor, Bipolaris maydis race T, Phyllosticta maydis, Helminthosporium maydis.

URF13, a ligand-gated, pore-forming receptor for T-toxin in the inner membrane of cms-T mitochondria

URF13 is the product of a mitochondrial-encoded gene (T-urf13) found only in maize plants containing the Texas male-sterile cytoplasm (cms-T), and it is thought to be responsible for both cytoplasmic male sterility and the susceptibility of cms-T maize to the fungal pathogens Bipolaris maydis race T and Phyllosticata maydis. Mitochondria isolated from cms-T maize are uniquely sensitive to pathotoxins (T-toxin) produced by these fungi and to methomyl (a commercial insecticide). URF13 acts as a receptor that specifically binds T-toxin to produce hydrophilic pores in the inner mitochondrial membrane. When expressed in Escherichia coli cells, URF13 also forms hydrophilic pores in the plasma membrane if exposed to T-toxin or methomyl. Topological studies established that URF13 contains three membrane-spanning alpha-helices, two of which are amphipathic and can contribute to pore formation. Chemical cross-linking of URF13 was used to demonstrate the existence of URF13 oligomers in cms-T mitochondria and E. coli cells. The ability of the carboxylate-specific reagent, N,N'-dicyclohexycarbodiimide, to cross-link URF13 was used in conjunction with site-directed mutagenesis to establish that the URF13 tetramer has a central core consisting of a four-alpha-helical bundle which undergoes a conformational change after interaction with T-toxin or methomyl. Overall, the experimental evidence indicates that URF13 functions as a ligand-gated, pore-forming T-toxin receptor in cms-T mitochondria.

Biochemical and molecular analysis of plants derived from embryogenic tissue cultures of napier grass (Pennisetum purpureum K. Schum)

We have investigated the extent of biochemical and molecular variation in 63 plants of napier grass (Pennisetum purpureum K. Schum.) regenerated from 3- to 24-week-old embryogenic callus cultures. The calli were derived from cultured basal segments of young leaves and immature inflorescences obtained from a single fieldgrown donor plant. The entire population was analyzed for the activity of 14 isozyme systems, but no qualitative variation was found at any of the loci examined. Similarly, no restriction fragment length polymorphisms (RFLPs) were detected in the mitochondrial, plastid and nuclear genomes in a representative sample of regenerated plants. Our results confirm earlier reports of the genetic uniformity of plants derived from somatic embryos and highlight their value both for clonal propagation and for genetic transformation.

Activation of the maize transposable element Suppressor-mutator (Spm) in tissue culture

Previous experiments have revealed that the maize transposable element Activator (Ac) may become active during tissue culture. The objective of the present study was to determine whether a second transposable element, Suppressor-mutator (Spm), could also be activated in tissue culture and detected in regenerated maize plants. Approximately 500 R1 progeny of 143 regenerated plants (derived from 49 embryo cell lines) were crossed as males onto an Spm-responsive tester stock. Spm activity was observed in two R1 progeny of a single regenerated plant. This plant had been regenerated from Type II (friable embryogenic) callus of an A188 × B73 genetic background after 8 months in culture; the absence of Spm activity in four other plants regenerated from this same callus demonstrates that Spm activity was not present before culturing. Approximately 20 Spm-homologous DNA sequences were detected in each of the inbreds used to initiate the tissue cultures; it is presumed that one of these became active to give rise to Spm activity.

Study of two different recombination events in maize cmsT-regenerated plants during reversion to fertility

The change of phenotype from sterility to fertility for some cmsT callus tissue culture regenerated plants and their progenies has been correlated with changes in their mitochondrial genome. Those changes that have been analyzed here are the result of recombination events. Two different sets of repeated sequences have been found to be involved in those recombination events. The most common one is a recombination through a 127-bp repeat between various independently isolated revertants. The second one is a recombination through a 58-bp repeat. In every case the products of recombination containing the urf13 gene have been deleted.

In vitro selection for methomyl resistance in CMS-T maize

Many plants resistant to methomyl (Lannate), an insecticide which selectively damages maize with the Texas (T) type of cytoplasmic male sterility (CMS-T), were obtained by in vitro selection and also without selection. The selection procedure used 0.6-0.7mM methomyl and callus from CMS-T versions of several field and sweet corn genotypes (W182BN, Wf9, P39, MDM1, SW1 and hybrids of SW1, IL766A1, IL766A2, and 442 with W182BN-N). Addition of 1 mM methomyl to the regeneration medium greatly reduced recovery of methomyl-sensitive escapes. Resistance was linked with reversion to male fertility and maternally inherited. Most progeny of resistant plants exhibited stable maternally inherited resistance for two generations in field tests. First-generation progeny of seven culture-derived plants segregated for resistance and sensitivity; this suggests that ears of these seven regenerants were cytoplasmically chimeral. Resistance to methomyl was associated with resistance to T toxin from Helminthosporium maydis race T and with changes in mitochondrial physiology. Prolonged culture (14-16 months versus 6-8 months) increased the frequency of resistance among both selected and non-selected regenerants. Little or no resistance was found among regenerants from certain genotypes. Selection with methomyl may be useful for production of improved sweet corn lines and as a source of mitochondrial mutants. This system is also convenient for studies of the effects of nuclear background and of culture and selection systems on the generation of cytoplasmic mutants.

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'.

Loss of CMS-specific mitochondrial DNA arrangement in fertile segregants of Petunia hybrids

The progeny of somatic hybrid Petunia plants derived from the fusion of a male-fertile line and a cytoplasmic male-sterile (cms) line were examined. Male-fertile progeny derived from three different male-sterile somatic hybrid plants did not exhibit the mitochondrial DNA (mtDNA) arrangement which has previously been correlated with cms in Petunia. The cms-associated mtDNA arrangement was present in the male-sterile predecessors of these fertile revertants. Thus, it is concluded that the loss of this mtDNA arrangement is associated with reversion to fertility in the progeny of the unstable somatic hybrid petunia plants.

In vitro selection of barley and wheat for resistance against Helminthosporium sativum

Calli derived from immature embryos of barley and wheat genotypes were screened for their resistance to purified culture filtrate produced by the fungus Helminthosporium sativum P.K. and B. Two selection methods were used: a continuous method in which four cycles of selection were performed one after another on toxic medium and a discontinuous method in which a pause on non-toxic medium was given after the second or third cycle of selection. The latter was superior as it allowed the calli to regain their regeneration ability. About 3,000 calli from two barley genotypes and 2,000 from two wheat genotypes were used for selection. The selection with the pathotoxins resulted in 6% to 17% surviving calli. Toxin tolerant callus lines of barley were characterised by protein isozymes. Zymograms showed one more isozyme than with the unselected sensitive callus. Barley and wheat plants have been regenerated from callus lines surviving the toxin treatment and in vivo testing against pathogen revealed that the majority of these plants were less sensitive.

Fertile revertants from S-type male-sterile maize grown in vitro

Plants were regenerated from callus cultures of maize inbred W182BN with the S(USDA) type of cytoplasmic male sterility (cms). Some regenerates from 16 of 18 separate cultures had fertile tassels. Many other regenerates, whose fertility could not be scored accurately because of abnormal plant morphology, produced fertile progeny after pollination with N cytoplasm W182BN. Revertant plants and/or progeny were obtained from all 18 cultures, which included the CA, D, LBN, and S sources of cmsS. More revertants were recovered from cultures maintained as callus for 12 months than from 3-4 month old cultures. Several types of evidence (absence of segregation for fertility after selfing or pollination of revertants with standard W182BN, pollen viability counts, failure of revertants to restore sterile cmsS lines to fertility, mitochondrial DNA analyses) indicated that the reversion to fertility involved cytoplasmic rather than nuclear alterations. All revertants examined lacked the S1 and S2 plasmid-like DNAs characteristic of the mitochondrial genome of sterile cmsS lines. Most callus cultures lost S1 and S2 after 13-20 months in vitro. No revertants were seen among thousands of W182BN cmsS plants grown from seed in the field or among plants from tissue cultures of W182BN with the C or T types of cms. The cytoplasmic revertants recovered from culture may be useful for the molecular analysis of cmsS.

Heritable somaclonal variation in wild barley (Hordeum spontaneum)

Progenies of H. spontaneum plants regenerated from immature embryo derived calli were analysed for somaclonal variation on the following traits: (1) organization of the intergenic spacer of the rRNA genes; (2) B and C hordein pattern on SDS-PAGE; (3) genomic organization of the B and C hordein coding sequences; (4) mitochondrial DNA organization assayed by hybridization of Southern blots of total DNA with mitochondrial coding genes; (5) cytology. One out of twelve progeny plants was characterized as variant for two traits: (a) a loss of 1.8 and 2.5 kb Taq I intergenic rDNA spacer fragments and (b) a variant pattern of hordeins on 1-D SDS-PAGE. No numerical or structural chromosome variation was detected among the control plants therefore it is assumed that the variation was caused by the in vitro culture and transmitted, through sexual reproduction, to the analysed progeny.

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.

Structural alterations in a transcribed region of the T type cytoplasmic male sterile maize mitochondrial genome

H. maydis toxin resistance, and cytoplasmic reversion from sterility to fertility in Zea mays T type cytoplasm are correlated with sequence changes in a 6.6 kb XhoI fragment of T mitochondrial DNA. Comparative Northern blot analysis of N (normal male fertile), and cmsT (cytoplasmic male sterile) mitochondrial RNAs using subclones of the 6.6 kb Xho I region of cmsT as probes, reveals different sized transcripts. In cmsT these RNA coding sequences have been mapped onto a 1.5 kb AvaI fragment completely internal to the 6.6 kb XhoI region. Comparative Southern analysis of AvaI digested mitochondrial DNA from cmsT, N and a T-type cytoplasm which has reverted to fertility (designated V3) (Brettel et al. 1979), positions the RNA coding sequences on a 1.5 kb, 2.1 kb and 2.1 kb fragment respectively. These sequence rearrangements in the fertile T revertants create a novel mtRNA transcript. Southern hybridization experiments of other higher plant mitochondrial DNAs using the 1.5 kb Ava I fragment from cmsT mtDNA as a probe, indicate the presence of homologous sequences.

Large scale selection of aluminum-resistant mutants from plant cell culture: expression and inheritance in seedlings

A large number of aluminum-resistant variants, selected from non-mutagenized homozygous diploid cell cultures of Nicotiana plumbaginifolia Viv., are characterized. Of 115 variants cloned and reselected from single cells, 67 retained Al resistance in callus cultures after 6-9 months of growth in the absence of Al. There was no association between Al resistance and callus growth in the absence of Al, suggesting that the Al-resistant phenotype is not detrimental in the absence of Al challenge and that Al resistance is not the result of increased vigor. Plants regenerated from initially resistant callus lines that subsequently lost their resistance failed, with one exception, to transmit resistance to their seedling progeny. Fertile plants were regenerated from 40 of the 67 variants that retained stable Al resistance in callus culture. All 40 transmitted Al resistance to their seedling progeny (selfed and backcrossed) in segregation ratios expected for a single dominant mutation. The selfed progeny of many variants also segregated for recessive lethal mutations which were attributed to independent mutations that occurred during cell culture.

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.

Quantitative variation in components of the maize mitochondrial genome between tissues and between plants with different male-sterile cytoplasms

The amounts of a 1.9 kb mitochondrial plasmid relative to sequences in another mitochondrial DNA replicon and also to nuclear ribosomal DNA sequences have been compared in maize leaves and anthers. Similar comparisons have been made between plants with the same nuclear genotype but containing normal, S, or T cytoplasms. The ratio of 1.9 kb plasmid to nuclear rDNA is lower in plants with normal cytoplasm than in plants with S or T cytoplasm. It also differs between leaves and anthers. Furthermore, the relative concentration of the mitochondrial DNA sequences belonging to different replicons differs between leaves and anthers. It is concluded that components of different mitochondrial replicons are not maintained in fixed ratios during development and that the concentration of the 1.9 kb plasmid is regulated, in part, by cytoplasmically-inherited determinants. The 1.9 kb plasmid is absent from lines with the Vg cytoplasm, but related sequences are found in the maize nuclear genome.

Variability among plants and their progeny regenerated from protoplasts of Su/su heterozygotes of Nicotiana tabacum

Mesophyll protoplasts of Nicotiana tabacum, heterozygous for the sulfur locus (Su/su), were isolated and more than 2,200 calli were cultured. More than 8,000 regenerated shoots were analyzed for leaf colour. Cell culture regimes included media for normal and stressed growth conditions with both short and long culture periods. An additional treatment included N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis. An analysis of the regenerated shoots showed that an extended culture period led to an enhanced frequency of variant colony types, i.e. colonies producing both parental (Su/su) and non-parental (Su/Su or su/su) plants. NNG at 10 mg/l also enhanced the frequency of variant colony types. In some treatments there was also an increase in non-morphogenic colonies but this was independent of genetic changes at the sulfur locus. The frequency of dark green spots and twin spots, presumed to result from somatic crossing-over, was higher in the leaf cells of regenerated plants after both prolonged cell culture and chemical mutagenesis. Genetic analysis of the progeny of selfed regenerants revealed additional tissue culture induced variability with respect to segregation ratios of the different sulfur phenotypes. About two thirds of the lines tested segregated in accordance with a 1∶2∶1 Mendelian ratio. The remainder deviated from the expected segregation pattern and some lines also showed heterogeneity between progeny families derived from different seed capsules of the same plant. These results demonstrate that genetic changes affecting a specific locus and segregation patterns in progeny of regenerated plants are induced during cell culture.

Mitochondrial DNA analyses of fertile and sterile maize plants derived from tissue culture with the texas male sterile cytoplasm

Maize plants carrying Texas (T) cytoplasm are male-sterile and sensitive to Drechslera maydis race T toxin, whereas plants carrying Normal (N) cytoplasm are male-fertile and resistant to the toxin. Some plants regenerated from T cytoplasm tissue cultures exhibit a N cytoplasm-like phenotype with respect to malefertility and toxin-resistance. Analysis of the high molecular weight mitochondrial DNA (mtDNA) of such regenerants has shown that the plants do not contain N cytoplasm mtDNA. However, their mtDNAs do show sequence differences from each other and from the mtDNA of plants possessing T cytoplasm. No single alteration detected correlates with the change to malefertility or toxin resistance. Sequence alterations were also evident in high molecular weight mtDNA isolated from a plant regenerated from N cytoplasm callus. No changes in low molecular weight mtDNA molecules were observed in regenerants from N or T cytoplasm callus.

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

Resistant plants and plants with reduced susceptibility against the pathogen Phoma lingam could be regenerated from selected callus and embryogenic cultures of haploid rape (Brassica napus) previously treated with mutagens. In the two in vitro selection systems used - absence of fungus growth on the cultures after incubation with parasite spores and resistance to the toxic filtrate - the resistance to the toxin was effective. In addition, some regenerants with increased tolerance were obtained from unselected cultures. Resistance tests on regenerated plants were carried out by inoculation of whole plants in the greenhouse, reproducing as much as possible the infection mechanisms which take place under natural conditions. Preliminary results on resistance of the progeny of single susceptible and tolerant regenerants seem to indicate that the acquired resistances are of a genetic nature.

Mitochondrial sensitivity to Drechslera maydis T-toxin and the synthesis of a variant mitochondrial polypeptide in plants derived from maize tissue cultures with texas male-sterile cytoplasm

Tissue cultures of maize carrying cms-T cytoplasm have been found to regenerate fertile, T-toxin resistant plants, with and without a selective treatment with T-toxin. Progeny of these plants were tested for mitochondrial sensitivity to T-toxin and the translation products synthesised by isolated mitochondria were analysed. The results confirm previous indications of a close correlation between susceptibility to T-toxin and the synthesis of a variant 13,000 Mr mitochondrial polypeptide. Interestingly, there appeared to be a critical level at about 33% maximum synthesis of the 13,000 Mr polypeptide above which male sterility and sensitivity to T-toxin are jointly expressed. The possibility that there is a causal link between synthesis of this additional mitochondrial polypeptide, pollen abortion and sensitivity to T-toxin is discussed.