Isolation of a recessive barley mutant resistant to S-(2-aminoethyl)L-cysteine

The biosynthesis and metabolism of the aspartate derived amino acids in higher plants

The essential amino acids lysine, threonine, methionine and isoleucine are synthesised in higher plants via a common pathway starting with aspartate. The regulation of the pathway is discussed in detail, and the properties of the key enzymes described. Recent data obtained from studies of regulation at the gene level and information derived from mutant and transgenic plants are also discussed. The herbicide target enzyme acetohydroxyacid synthase involved in the synthesis of the branched chain amino acids is reviewed.

Free proline contents in two different groups of rice mutants resistant to hydroxy-L-proline

In four rice (Oryza sativa L.) mutants resistant to hydroxy-L-proline (Hyp), HYP101, HYP203, HYP205 and HYP210, and in their original variety, Nipponbare, free proline and Hyp contents in the seeds and in the 14-day-old seedlings have been determined. The four mutants can be divided into two groups: HYP101 and HYP203 are classified as to recessive gene and the levels of free proline are similar to that of the original variety; the second group includes mutants HYP205 and HYP210 where the Hyp resistance is transmitted heterozygously and, both in the seeds and in the seedlings, a remarkable increase in free proline content is observed. In particular, free proline contents in the seeds of HYP205 and HYP210 are, respectively, 24 and 12 times that of the original variety. Hyp is detected only in the seedlings cultured with Hyp solution. In the Hyp resistant seedlings of HYP205 and HYP210, Hyp contents are twice that of the original variety and less than half in the seedlings of HYP101 and HYP203. Hyp resistance and differential proline levels are also evident in the callus initiated from the mutants. This suggests that the Hyp resistant mutants are good genetic markers both in planta and in vitro. The Hyp mutants are also discussed with regard to stress resistance.

Non-proline-accumulating rice mutants resistant to hydroxy-L-proline

Three rice (Oryza sativa L.) mutants resistant to hydroxy-L-proline (Hyp), HYP 101, HYP 202 and HYP 203, were selected from an ethylene imine mutagenized M2 population of the original variety, 'Nipponbare', and their biochemical and genetical characteristics were investigated. The sensitivity of the mutants to Hyp could be clearly differentiated from that of the original variety when seeds were germinated and cultured with 10(-4)∼10(-3) M Hyp for 10 days. A difference in Hyp sensitivity was also observed among the HYP mutant lines, HYP 101 being the most resistant line. When free amino acids in seeds and 15-day-old seedlings were analyzed, the composition of the amino acids in the mutants was somewhat different from that found in the original variety. However, free proline accumulation was not detected in either the HYP mutants or the original variety. In each mutant line, HYP resistance was transmitted with a single recessive nuclear gene (hpr). These results suggest that the mechanism of Hyp resistance controlled by the recessive gene do not involve free proline accumulation.

Advances in somatic cell genetics of higher plants - the protoplast approach in basic studies on mutagenesis and isolation of biochemical mutants

Selection strategies developed in microbial genetics were successfully extrapolated to in vitro cell culture systems of higher plants and are having a major impact in the elucidation of regulatory mechanisms of basic cellular processes in eukaryotes. Although an increasing number and wide spectrum of biochemical variants have been isolated in such cell culture systems, their routine selection, characterization, and manipulation have not yet been achieved. Methodological limitations are considered to be one of the major reasons. Suspension or callus cultures, so extensively employed during the last decade in mutation-selection experiments and so useful in demonstrating the potentialities of in vitro screening techniques in obtaining various biochemical markers, have inherent drawbacks which limit in our opinion their further contribution in this field. Protoplast cultures represent an ideal tool for mutation and selection experiments. It is the purpose of this review to show how, due to recent methodological advances in the manipulation of some model protoplast culture systems, essential aspects of mutagenesis and selection of biochemical mutants can be reconsidered. These systems are simple and efficient, and lend themselves to statistical interpretation. Genetic analysis of selected variants should help us to understand and define better the new set of problems and concepts revealed by the somatic cell genetics of higher plants; combined with biochemical analyses it should elucidate the basic relationship between control of biological processes at cellular and whole organism level.

The isolation and characterisation of a catalase-deficient mutant of barley (Hordeum vulgare L.)

A mutant line of barley, R(othamsted)-Pr 79/4, has been isolated which grows poorly in natural air, but normally in air enriched to 0.2% CO2. Analysis of the products of (14)CO2 fixation showed that there was no major block in photosynthetic or photorespiratory carbon metabolism in the mutant and that rates of CO2 fixation were only slightly lower than those measured in the wild type (c.v. Maris Mink). Leaves of the mutant line contained only 10% of the catalase (EC 1.11.1.6) activity found in the wild type; and the two major bands of catalase activity detected after starch-gel electrophoresis of extracts of normal leaves were missing from similar extracts of RPr 79/4. Peroxisomes isolated from mutant leaves contained negligible catalase activity, but normal levels of other enzymes involved in photorespiration. Genetic analysis has shown that the mutation is recessive and that both air-sensitivity and catalase-deficiency segregate together in F2 plants derived from a cross between the mutant and the cultivar Golden Promise. [1-(14)C]Glycollate was not converted to (14)CO2 faster in the mutant leaves than in the normal leaves. Thus there was no evidence that photorespiratory CO2 may be obtained by the chemical action of H2O2 on glyoxylate or hydroxypyruvate.

Sequential occurrence of mutations in a growing rice callus

Four mutations for early heading, albina, short culm and sterility were obtained in the progenies of twelve rice plants regenerated from a single callus of a rice seed. Studies on the segregation rates of these mutations revealed that for each mutation a single recessive gene was likely to be involved and that there was no linkage among the genes. The segregation pattern also showed that these mutations were induced in the following sequence: early heading, albina, and short culm and sterility during the stage of callus growth until the beginning of the regeneration of the rice plants.

Threonine accumulation in the seeds of a barley mutant with an altered aspartate kinase

Barley (Hordeum vulgare L.) mutants altered in the regulation of synthesis of aspartate-derived amino acids were sought by screening embryos for growth on a medium containing lysine plus threonine. One mutant, Rothamsted 2501, was selected with good growth. From the segregation of resistance in the following generations, it was concluded that the resistance was conferred by a dominant gene, Lt1. No homozygous Lt1/Lt1 fertile plants have been recovered. Partially purified aspartate kinase preparations from resistant and sensitive plants were separated on DEAE-cellulose chromatography into three peaks of activity (I, II, III) and the feedback regulatory properties of these peaks determined. These peaks are considered to be three isozymic forms of aspartate kinase, one predominantly sensitive to threonine and two sensitive to lysine or lysine plus S-adenosyl methionine. The feedback characteristics of one of the peaks of aspartate kinase activity from resistant plants were changed such that lysine was half-maximally inhibitory at 10 rather than 0.4 mM. Increases in te concentrations of the free pools of threonine (4x) and methionine (2x) were measured in young plants grown on a basal medium. Threonine in the soluble fraction of mature seeds from resistant plants was increased from 0.8 to 9.6% of the total threonine content. The total content of both threonine and methionine of the seeds was increased by 6% compared with grain of similar nitrogen content.

Proline accumulation in a barley mutant resistant to trans-4-hydroxy-L-proline

Five proline analogues were tested for inhibition of the growth of mature barley (Hordeum vulgare L.) embryos in sterile culture. Inhibition by all analogues was relieved by proline. Inhibition by trans-4-hydroxy-L-proline was relieved by low amounts of proline. Twenty thousand mature embryos were dissected from M2 seeds after sodium azide mutagenesis. Four plants (Rothamsted 5201, 6102, 6901, 6902) were selected with good growth on 4 mM trans-4-hydroxyproline. Properties of mutant R5201 were studied in detail. Selfed progeny of R5201 were all resistant to trans-4-hydroxyproline and also to L-thiazolidine-4-carboxylic acid and trans-3-hydroxy-L-proline but not L-azetidine-2-carboxylic acid. The content of soluble proline in progeny of R5201 was higher in leaves by a factor of up to six-fold. Proline content was measured in the soluble fraction of the terminal 20 mm of 4 d old plants subjected to severe water stress in 40% w/v polyethylene glycol. Leaves of the mutant contained more proline initially and accumulated proline morer rapidly than the parental leaves. As mutant leaves were larger and lost water more rapidly the greater increase in proline may have been caused by more severe water stress. Resistance to trans-4-hydroxyproline in R5201 was due to a single partially dominant nuclear gene.

Lysine metabolism in a barley mutant resistant to S(2-aminoethyl)cysteine

Lysine and S(2-aminoethyl)cysteine (AEC) metabolism were investigated in normal barley (Hordeum vulgare L. cv. Bomi) and a hemozygous recessive AEC-resistant mutant (R906). Feedback regulation of lysine and threonine synthesis from [(14)C] acetate was unimpaired in plants of the mutant 3 d after germination. Seeds of Bomi and R906 contained similar total amounts of lysine, threonine, methionine and isoleucine. Concentrations of these amino acids in the soluble fraction of plants grown 6 d without AEC were also similar. The concentration of AEC in R906 plants was less than in the parent variety when both were grown in the presence of 0.25 mM AEC for 6 d. The uptake of [(3)H]AEC and [(3)H]lysine by roots of R906 was, respectively, 33% and 32% of that by Bomi roots whereas the uptake of these compounds into the scutellum was the same in both the mutant and its parent. The uptake of [(3)H]leucine and its incorporation into proteins was also the same in Bomi and R906 plants. These results suggest that a transport system specific for lysine and AEC but not leucine is altered or lost in roots of the mutant R906. AEC is incorporated into protein and this could be the reason for inhibition of growth rather than action as a false-feedback inhibitor of lysine biosynthesis.