Thiamine in plants: Aspects of its metabolism and functions

Thiamine diphosphate (vitamin B₁) plays a fundamental role as an enzymatic cofactor in universal metabolic pathways including glycolysis, the pentose phosphate pathway, and the tricarboxylic acid cycle. In addition, thiamine diphosphate has recently been shown to have functions other than as a cofactor in response to abiotic and biotic stress in plants. Recently, several steps of the plant thiamine biosynthetic pathway have been characterized, and a mechanism of feedback regulation of thiamine biosynthesis via riboswitch has been unraveled. This review focuses on these most recent advances made in our understanding of thiamine metabolism and functions in plants. Phenotypes of plant mutants affected in thiamine biosynthesis are described, and genomics, proteomics, and metabolomics data that have increased further our knowledge of plant thiamine metabolic pathways and functions are summarized. Aspects of thiamine metabolism such as catabolism, salvage, and transport in plants are discussed.     

Biogenesis of mitochondria 47

Summary This paper consolidates and refines the physical map of genetic loci previously established in our laboratory, by molecular analysis of seven genetically characterized new petites (deletion mutants of mtDNA). A modified DNA-DNA hybridization procedure employing filters simultaneously bound with mtDNA from two different petites has been used to measure the overlaps in mtDNA sequences between the different petite mutants.Thus, by analysis of three new petites carrying the antibiotic-resistance loci, ery1, cap1 and par1 on their mitochondrial genomes, it has now been possible to improve our estimation of the maximum distance between the cap1 and ery1 loci. The cap1, ery1 loci, and the 21S ribosomal RNA gene have now been mapped within 5 units in the same region (map position 0 to 5 units). Similarly, by analysis of four new petites carrying the O _II and/or par1 loci on their mtDNAs, the map position of the O _II locus is also more accurately determined within 2 units in a region (map position 34 to 36 units) between the par1 and ana1 loci. The positions of other loci including par1, the 15S ribosomal RNA gene, and some mit ^- loci are also discussed.We have thus extended our library of genetically and molecularly defined petite mutants, resulting in a set of petites having overlapping regions distributed throughout the entire wild-type mitochondrial genome, consistent with the idea that yeast mtDNA is physically circular.     

Molecular mapping around the centromere of tomato chromosome 6 using irradiation-induced deletions

 Irradiation-induced deletion mapping was exploited to construct a detailed locus-order map around the centromere of tomato chromosome 6 (CEN  6). An F₁ hybrid heterozygous for the marker loci thiamineless (tl), yellow virescent (yv) and potato leaf (c), and homozygous recessive for the nematode resistance gene mi, was pollinated with γ-irradiated pollen from cultivar VFNT Cherry carrying the wild-type alleles at the corresponding loci. A dose of 100 Gy was found optimal for inducing mutants. By screening for pseudo-dominant plants showing the marker phenotypes and/or nematode susceptibility, 30 deletions encompassing one or more of the four loci were detected in the M₁ generation. Molecular-marker analysis revealed that 29 of these mutants included the tl and mi loci on the short arm and originated from terminal deletions of different sizes. Remarkably, the breakpoints of these deletions were not randomly distributed along the short arm but located within the centromeric heterochromatin. Only one yv interstitial deletion and no c mutations on the long arm of the chromosome were detected. Mapping of the various chromosomal breakpoints in the isolated mutants permitted the resolution of a cluster of molecular markers from the centromeric heterochromatin that was hitherto unresolvable by genetic linkage analysis. The usefulness of such a deletion-mapping approach for whole-genome mapping is discussed. Received: 4 March 1997 / Accepted: 2 June 1997     

Gene locus specificity of the glucose effect in the thiamine pathway of the angiosperm, Arabidopsis

All mutants at 3 loci in Arabidopsis thaliana (L.) Heynh., a higher plant, that are associated with the synthesis or coupling of the thiazole moiety of thiamine are susceptible to reversible glucose inhibition. In contrast, several different alleles involved in the synthesis of the pyrimidine moiety of the vitamin are insensitive to glucose. Glucose and maltose are equally effective inhibitors while fructose, lactose, ribose, and xylose are toxic. This toxicity is not released by added thiamine.     

Isolation of nutritional mutants inArabidopsis thaliana

An accumulation method was devised for the isolation of nutritional mutants of the CruciferArabidopsis thaliana. Six mutants were obtained, in all of which the synthesis of thiamine was blocked. The mutants showed chlorophyll deficiencies and were more or less depressed in growth. Thiamine, when added to the substrate, fully restored normal development. Three of the mutants also grew on a substrate containing the pyrimidine moiety of the vitamin molecule, one on a medium with the thiazole part, one on a substrate containing a mixture of both compounds, whereas the sixth mutant required the complete thiamine molecule for normal growth. Two of the three pyrimidine-less mutants were allelic, the third, when crossed with each of the other two, yielded F₁'s showing wild type growth in their youth but deficiency symptoms in later stages. In each of the other mutants different loci were involved. The relation between the method employed and the type of mutant isolated is discussed.     

A transiently expressed transposase system to generate <Emphasis Type="Italic">Ds</Emphasis>-tagged mutants for functional genomics in sorghum

Six T-DNA/Ds launch pad lines (T₀) previously generated by Agrobacterium-mediated transformation of M 35-1 genotype of sorghum were confirmed by PCR. T₁ plants of all six lines showed 3:1 segregation when sprayed with 12 ppm Basta herbicide, indicating single copy insertion, which was also confirmed by left border flanking sequence tag. Calli derived from pNU435-T₀(1) primary transformant was co-infected with Agrobacterium-carrying iAc construct for transient expression of transposase to generate stable Ds-tagged mutants in the T₀ generation. All nine regenerants were PCR-positive for Ds. However, four contained intact T-DNA/Ds launch pad, while five plants carried empty launch pad, indicating transposition of the Ds. One of these plants, IDs-T₀(8), was negative for iAc PCR, indicating that it was a stable Ds-tagged mutant. Of the four plants with intact T-DNA/Ds, IDs-T₀(5) carrying iAc was a double transformant and mutagenic, which can generate mutants in the subsequent generation. Hence, the transient expression of transposase system in sorghum reported here can be employed for high throughput mutagenesis.     

The red Mutations Impair the Regulation of Flavinogenesis and Metal Homeostasis in Yeast Pichia guilliermondii

A method of positive selection of mutants with impaired regulation of flavinogenesis and metal homeostasis in yeast Pichia guilliermondii was developed. This positive selection system was based on the isolation of pseudo-wild-type revertants (the Rib⁺ phenotype) in riboflavin-dependent rib1-86 mutant (the Rib^– phenotype) of yeast P. guilliermondii. Mutation rib1-86 blocks activity of the GTP cyclohydrolase II catalyzing the first step in riboflavin (RF) biosynthesis. Study of a collection of spontaneous Rib⁺ revertants allowed the identification of a considerably large number of genetic loci responsible for the suppression of rib1-86, which include both previously identified three loci (rib80, rib81, andhit1) and six new loci designated red1–red6 (reduction). A comparative analysis of the wild-type strain and red mutants revealed that these mutants had higher activity levels of GTP cyclohydrolase and RF-synthase, elevated levels of RF biosynthesis, enhanced Fe/Cu reductase activity and higher total iron content in cells and that they are characterized by enhanced sensitivity to transition metals (Fe(III), Cu(II), Cd(II), Co(II), Zn(II), Ag(I)) and to H₂O₂. The metal hypersensitivity of mutant cells can be prevented by an increased amount of extracellular iron ions. Mutations red1 and red6 synergistically interact with the locus rib81 in the course of RF biosynthesis. Obviously, each RED gene plays an important role in the regulation of both flavinogenesis and metal homeostasis in P. guilliermondii cells.     

Reduction in chlorophyll content without a corresponding reduction in photosynthesis and carbon assimilation enzymes in yellow-green oil yellow mutants of maize

The photosynthetic properties of a yellow lethal mutant, Oy/oy, and two yellow-green mutants of maize which are allelic (a homozygous recessive oy/oy and a heterozygous dominant Oy/+) were examined. Although Oy/oy had little or no chlorophyll or capacity for CO₂ fixation compared to normal siblings, it had 28% as much ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) activity, and from 40% to near normal activities of C₄ cycle enzymes.Both yellow-green mutants had only half as much chlorophyll per leaf area as normal green seedlings in greenhouse-grown plants in winter and spring. However, the absorbance of light by the mutants was relatively high, as their transmittance was only 5 to 8% greater than normal leaves. In winter-grown greenhouse plants, the activities of Rubisco and several C₄ cycle enzymes in the mutants were unaffected and similar to those of normal seedlings on a leaf area basis. After allowing for small differences in leaf absorbance, the light response curves for photosynthesis in the mutants were similar on a leaf area basis but much higher on a chlorophyll basis than those of the normal seedlings. In spring-grown greenhouse plants the enzyme activities and photosynthesis rates were about 30% lower per leaf area in the yellow-green mutant leaves compared to the wild type. The maximum carboxylation efficiency (measured under low CO₂ and 1000 mol quanta m^-2 s^-1) in the mutants and normal leaves was similar on a Rubisco protein basis. The results indicate that maize can undergo a 50% reduction in chlorophyll content without a corresponding reduction in enzymes of carbon assimilation, and still maintain a high capacity for photosynthesis.Abbreviations Chl chlorophyll - PEP phosphoenolypruvate - Rubisco ribulose-1,5-bisphosphate carboxylase oxygenase This research was supported by CSIRO and by USDA Competitive Grant 86-CRCR-1-2036.     

Defective kernel mutants of maize. I. Genetic and lethality studies

A planting of 3,919 M₁ kernels from normal ears crossed by EMS-treated pollen produced 3,461 M₁ plants and 3,172 selfed ears. These plants yielded 2,477 (72%) total heritable changes; the selfed ears yielded 2,457 (78%) recessive mutants, including 855 (27%) recessive kernel mutants and 8 (0.23%) viable dominant mutants. The ratio of recessive to dominant mutants was 201:1. The average mutation frequency for four known loci was three per 3,172 genomes analyzed. The estimated total number of loci mutated was 535 and the estimated number of kernel mutant loci mutated was 285. Among the 855 kernel mutants, 432 had a nonviable embryo, and 59 germinated but had a lethal seedling. A sample of 194 of the latter two types was tested for heritability, lethality, chromosome arm location and endosperm-embryo interaction between mutant and nonmutant tissues in special hyper-hypoploid combinations produced by manipulation of B-A translocations. The selected 194 mutants were characterized and catalogued according to endosperm phenotype and investigated to determine their effects on the morphology and development of the associated embryo. The possibility of rescuing some of the lethal mutants by covering the mutant embryo with a normal endosperm was investigated. Ninety of these 194 mutants were located on 17 of the 18 chromosome arms tested. Nineteen of the located mutants were examined to determine the effect of having a normal embryo in the same kernel with a mutant endosperm, and vice versa, as compared to the expression observed in kernels with both embryo and endosperm in a mutant condition. In the first situation, for three of the 19 mutants, the mutant endosperm was less extreme (the embryo helped); for seven cases, the mutant endosperm was more extreme (the embryo hindered); and for nine cases, there was no change. In the reverse situation, for four cases the normal endosperm helped the mutant embryo; for 14 cases there was no change and one case was inconclusive.     

Physiological basis of QTLs for boron efficiency in Arabidopsis thaliana

Boron (B) is an essential micronutrient for higher plants, but the adaptability of plants to B deficiency varies widely both between and within species. On the basis of quantitative trait loci (QTL) analysis of the B efficiency coefficient (BEC) detected in an Arabidopsis thaliana Ler × Col recombinant inbred (RI) population, B efficiency was evaluated in the original parents (Ler and Col-4) and two F₈ lines (1938 and 1961), both of which were selected on the basis of phenotype and genotype of the RI population. The parent Ler and F₈ progeny 1938 had higher BEC and B utilization efficiency (BUE) values than those calculated for parent Col-4 and F₈ progeny 1961, respectively, when grown in nutrient solutions containing three different concentrations of B. The magnitude of the BEC and BUE-values was correlated closely with the combined phenotypic effect of the corresponding QTLs among the four genotypes. The F₈ line, 1938, inherited all four B-efficient QTLs, AtBE1-1, AtBE1-2, AtBE2 and AtBE5, from its two original parents. The four QTLs accounted for 65.2% of the total variation in BEC and 1938 showed the highest BEC (0.74) and BUE (10.5) values among the four genotypes when grown in nutrient solution that contained 0.324 μM B. Only one minor-effect QTL (AtBE1-1) was found in the parent, Col-4. This QTL accounted only for 8.8% of total BEC variation and resulted in the lowest BEC (0.39) and BUE (0.76) in Col-4 when it was grown in nutrient solution that contained 0.324 μM B. Phenotypic profile analysis showed that 1938 not only inherited the B utilization and distribution characteristics found in the silique of Ler, but also acquired the low-B requirement for root and shoot growth from Col-4. As a result, this genotype displayed the strongest tolerance to B deficiency. In addition, both B-efficient genotypes, 1938 and Ler, possessed the QTL (AtBE1-2) and both plants had high-seed yields and high-B distributions in their siliques. Therefore, we hypothesize that QTL AtBE1-2 plays a role in the utilization and/or the distribution of B to the silique when plants suffer from B deficiency. A close correlation between the B-efficient phenotype and the corresponding QTLs indicated that phenotypic differences depend on the genetic variation. Responsible Editor: Richard W. Bell.     

Interactive regulation of grass yield and chemical properties by defoliation,a salivary chemical,and inorganic nutrition

Summary Clones of 2 C₄ grass species, Sprobolus ioclados and S. pyramidalis, were obtained from more and less heavily grazed grasslands, respectively, in Tanzania's Serengeti National Park. Plants were grown in a factorial experiment to determine the effects of severe defoliation, nutrient limitation, and a salivary chemical (thiamine) on plant growth, nitrogen content, and non-structural carbohydrate content. The experimental design included: (1) species; (2) clipping, with plants either unclipped or clipped weekly to a height of 5 cm; (3) 0.2 ml of distilled water of 0.2 ml of 10 ppb aqueous thiamine sprayed on plants from an atomizer after clipping and identical treatments at the same time to unclipped plants; (4) phosphorus (P) at 0.2 or 1 mM; (5) nitrogen (N) at 3 or 15 mM. Clipping was the major variable affecting plant growth. Total and litter yields were reduced to half and residual plant yield was reduced to 30% of the values for unclipped plants. Clipping interacted strongly with other variables since they commonly had minor effects on clipped plants and major effects on unclipped plants. Exceptions to this generalization were generally due to better performance by S. ioclados under clipping. Compared to lower treatment levels, higher treatment levels promoted total yield of unclipped plants by 52% for N, 43% for thiamine, and 33% for P. In general, thiamine had greater effects than P but lesser effects than N. Thiamine promoted yield and modified the chemical balance of plants by promoting carbohydrate (CHO) concentrations and reducing N concentrations. N and P deficiencies promoted CHO accumulation. Clipping promoted the N of leaves and crowns and reduced the N levels in roots. Leaf blade water and N contents were positively correlated with very little scatter. The slope of the line was different for S. ioclados and S. pyramidalis. Leaf blade water and CHO contents were negatively related but there was more scatter and the species could not be distinguished. The species from more heavily grazed grasslands was conspicuously more sensitive to thiamine application. The results indicated that leaf treatment with thiamine, the only likely source of which in natural grasslands is saliva deposited by feeding herbivores, can have major effects on plant yield and metabolic balances at very low application levels. But under defoliation levels as severe as those imposed in this experiment, which reduced above ground plant biomass to a fourth of the level produced by unclipped plants, growth was so strongly limited by defoliation that neither thiamine nor inorganic nutrients affected plant yield residual from clipping. Therefore, whether chemicals such as thiamine that may be introduced onto grass foliage by grazing ungulates and other herbivores will influence the growth of grazed plants will depend upon the grazing intensity associated with the transfer.     

Mutations in three of the genes determining thiamine biosynthesis in Pisum sativum

Summary Twenty stable auxotrophs for the vitamin thiamine (Thi) were isolated in two cultivars of garden pea (Pisum sativum) and characterized. All thi mutations were recessive lethals. The mutant plants were indistinguishable from normal and heterozygous plants when provided exogenously with about 5 mg of Thi. Eighteen of the mutants were found to define three genes: ThiA, thiB and thiC. The thiA gene mapped very close to the marker k on chromosome 2. The thiB gene was found to be 11.3 crossover units away from pl on chromosome 6 and the thiC gene was located 20 crossover units from st on chromosome 3. The suppressive effects of supplementation with thiamine compounds on the phenotype of the mutants suggested that the thiA and thiC gene products participate in certain steps up to the biosynthesis of the thiazole and hydroxymethylpyrimidine moieties of thiamine, respectively, and that the thiB gene product participates in steps from thiazole and hydroxymethylpyrimidine to thiamine.     

Biochemical and genetic comparison of two nitrate reductase-deficient pea mutants disturbed in the cofactor

Two nitrate reductase (NaR)-deficient mutants of pea (Pisum sativum L.), E₁ and A300, both disturbed in the molybdenum cofactor function and isolated, respectively, from cv Rondo and cv Juneau, were tested for allelism and were compared in biochemical and growth characteristics. The F₁ plants of the cross E₁ × A300 possessed NaR and xanthine dehydrogenase (XDH) activities comparable to those of the wild types, indicating that these mutants belong to different complementation groups, representing two different loci. Therefore, mutant E₁ represents, besides mutant A300 and the allelic mutants A317 and A334, a third locus governing NaR and is assigned the gene destignation nar 3. In comparison with the wild types, cytochrome c reductase activity was increased in both mutants. The mutants had different cytochrome c reductase distribution patterns, indicating that mutant A300 could be disturbed in the ability to dimerize NaR apoprotein monomers, and mutant E₁ in the catalytic function of the molybdenum cofactor. In growth characteristics studied, A300 did not differ from the wild types, whereas fully grown leaves of mutant E₁ became necrotic in soil and in liquid media containing nitrate.     

A genetic and physiological analysis of late flowering mutants in Arabidopsis thaliana

Summary Monogenic mutants of the early ecotype Landsberg erecta were selected on the basis of late flowering under long day (LD) conditions after treatment with ethyl methanesulphonate or irradiation. In addition to later flowering the number of rosette and cauline leaves is proportionally higher in all mutants, although the correlation coefficient between the two parameters is not the same for all genotypes. Forty-two independently induced mutants were found to represent mutations at 11 loci. The mutations were either recessive, intermediate (co locus) or almost completely dominant (fwa locus). The loci are located at distinct positions on four of the five Arabidopsis chromosomes. Recombinants carrying mutations at different loci flower later than or as late as the later parental mutant. This distinction led to the assignment of eight of the loci to three epistatic groups. In wild type, vernalization promotes flowering to a small extent. For mutants at the loci fca, fve, fy and fpa, vernalization has a large effect both under LD and short day (SD) conditions, whereas co, gi, fd and fwa mutants are almost completely insensitive to this treatment. SD induces later flowering except for mutants at the co and gi loci, which flower with the same number of leaves under LD and SD conditions. This differential response of the mutants to environmental factors and their subdivision into epistatic groups is discussed in relation to a causal model for floral initiation in Arabidopsis thaliana.     

Stably expressed <Emphasis Type="SmallCaps">d</Emphasis>-genome-derived HMW glutenin subunit genes transformed into different durum wheat genotypes change dough mixing properties

Durum wheat (Triticum turgidum L. var. durum) is traditionally used for the production of numerous types of pasta, and significant amounts are also used for bread-making, particularly in southern Italy. The research reported here centres on the glutenin subunits 1Dx5 and 1Dy10 encoded by chromosome 1D, and whose presence in hexaploid wheats is positively correlated with higher dough strength. In order to study the effects of stable expression of the 1Dx5 and 1Dy10 glutenin subunits in different durum wheat genotypes, four cultivars commonly grown in the Mediterranean area (‘Svevo’, ‘Creso’, ‘Varano’ and ‘Latino’) were co-transformed, via particle bombardment of cultured immature embryos, with the two wheat genes Glu-D1-1d and Glu-D1-2b encoding the glutenin subunits, and a third plasmid containing the bar gene as a selectable marker. Protein gel analyses of T₁ generation seed extracts showed expression of one or both glutenin genes in four different transformed durum wheat plants. One of these transgenic lines, DC2-65, showed co-suppression of all HMW-GS, including the endogenous ones. Transgene stability in the transgenic lines has been studied over four generations (T₁–T₄). Fluorescence in situ hybridization (FISH) analysis of metaphase chromosomes from T₄ plants showed that the integration of transgenes occurred in both telomeric and centromeric regions. The three plasmids were found inserted at a single locus in two lines and in two loci on the same chromosome arm in one line. The fourth line had two transgenic loci on different chromosomes: one with both glutenin plasmids and a different one containing only the construct with the gene encoding the 1Dy10 glutenin subunit. Segregation of these two loci in subsequent generations allowed establishment of two sublines, one containing both 1Dx5 and 1Dy10 and the other containing only 1Dy10. Small-scale quality tests showed that accumulation of Dx5, Dy10 or both in transgenic durum wheat seeds resulted in doughs with stronger mixing characteristics. A. Gadaleta and A. E. Blechl have contributed equally to this work.     

Ac/Ds transposon mutagenesis in Arabidopsis thaliana: mutant spectrum and frequency of Ds insertion mutants

Using a two-component Ac/Ds system consisting of a stabilized Ac element (Ac_c1) and a non-autonomous element (Ds_A), 650 families of plants carrying independent germinal Ds_A excisions/transpositions were isolated. Progenies of 559 of these Ac_c1/Ds_A families, together with 43 families of plants selected for excision/transposition of wild-type (wt)Ac, were subjected to a broad screening program for mutants exhibiting visible alterations. This resulted in the identification of 48 mutants showing a wide variety of mutant phenotypes, including embryo lethality (24 mutants), chlorophyll defects (5 mutants), defective seedlings (2 mutants), reduced fertility (5 mutants), reduced size (3 mutants), altered leaf morphology (2 mutants), dark green, unexpanded rosette leaves (3 mutants), and aberrant flower or shoot morphology (4 mutants). To test whether these mutants were due to transposon insertions, a series of Southern blot experiments was performed on 28 families, comparing in each case several mutant plants with others showing the wild-type phenotype. A preliminary analysis revealed in 4 of the 28 families analyzed a common, novel Ds_A fragment in all mutant plants, which was present only in heterozygous plants with wt phenotype, as expected for Ds_A insertion mutations. These four mutants included two showing embryo lethality, one with dark green, unexpanded rosette leaves and stunted inflorescences, and one with curly growth of stems, leaves and siliques. Further evidence for Ds_A insertion mutations was obtained for one embryo lethal mutant and for the stunted mutant, while in case of the second embryo lethal mutant, the Ds_A insertion could be separated from the mutant locus by genetic recombination.     

GENETIC BLOCKS IN THE THIAMINE SYNTHESIS OF THE ANGIOSPERM ARABIDOPSIS,,

Five thiamine-requiring mutants were obtained at two loci. Two are blocked in the synthesis of the pyrimidine part of the vitamin, the other three have lost the ability to make the thiazole moiety. None of the tested substances suggested as possible or likely precursors of the pyrimidine or the thiazole components of thiamine displayed any activity in the mutants. These conditional lethals responded to remarkably small supplements of thiamine. The pyrimidine-requiring mutants utilized to some extent the anti-vitamin neopyrithiamine. The thiazole-less mutants grew on basal media supplemented only with the analog, oxythiamine. Thiamine deficiency, irrespective of the position of the genetic block in the synthesis, results in a characteristic anomaly of pigmentation. The position of the py locus in the second linkage group has been determined. Allelic complementation has not been detected. The frequency of mutations affecting thiamine synthesis appears about the same in Arabidopsis as in fungi. The general frequency of reparable genetic lesions is, however, one to two orders of magnitude lower in Arabidopsis than that in fungi or bacteria.     

Production of doubled haploids through anther culture of M<Subscript>1</Subscript> rice plants derived from mutagenized fertilized egg cells

To produce stable mutants from Mankeumbyeo, a japonica rice (Oryza sativa L.) variety, we estimated the mutation efficiency of ethyl methane sulfonate (EMS) and N-methyl-N-nitrosourea (MNU) on fertilized egg cells using doubled haploids (DHs) derived from anther culture of M₁ plants. M₁ seed production and germination were higher in 1 mM MNU than in 94.2 mM EMS. A total of 68 DHs (35.4%) were regenerated by anther culture of M₁ plants. Twenty-one DHs (30.9%) were stable mutants, 14 DHs (20.6%) were unstable mutants, and the remainder (48.5%) were normal. The frequencies of stable mutants following EMS and MNU treatments were 20.7% (three semidwarfs, one early maturation and one glabrous line) and 38.5% (three semidwarfs, two early maturation, four glabrous and one long grain line), respectively. In a field trial of seven stable mutants for yield potential, five mutants did not show a significant difference in yield as compared with the original variety. Among these five, three glabrous mutants (MK-MAC 1, MK-MAC 4 and MK-MAC 26) with a smooth leaf and hull may be considered to be improved mutant lines because of the health benefits (reduced skin damage and generation of less dust compared to the original variety) to farmers handling the plant materials. MK-MAC 26, a glabrous mutant, had also less shattering resistance than that of the original variety. These stable mutants could be used as new breeding materials.Communicated by P.P. Kumar     

Allelspezifische suppressormutationen vonSchizosaccharomyces pombe

By comparing the intragenic distribution of suppressor sensitive mutants in fine structure maps, 13 allele specific suppressor mutations (isolated from revertants in adenine dependent mutants of constitutionad ₇) have been analyzed for their allele specific patterns of action in three different groups of mutants blocked in adenine biosynthesis. The 13 suppressor mutations, which have resulted from mutations at seven different suppressor loci, are characterized by four different suppression patterns. Three of these patterns, which partially overlap, are not locus specific since they include sensitive mutants at each of the three lociad ₇, ad₆ andad ₁ studied. The relative frequency of mutants sensitive to one or the other of the suppressors of this type, the absence of osmotic-remedial strains among the suppressor sensitive mutants, and the polarized complementation behaviour of one suppressiblead ₆ mutant and two suppressiblead ₁ mutants capable of interallelic complementation, suggest that the suppression mechanism involves misreading of a mutant triplet of the nonsense type.