Gibberellin mutants

Research on gibberellin (GA) mutants is reviewed, focusing on reports, published since 1993. The mutants have usually been identified via a shoot elongation screen. This screen exposes mutations influencing GA synthesis, deactivation and reception, and also those acting further down the elongation pathway. Mutations blocking synthesis lead to a dwarf. GA-responsive phenotype. Numerous such mutations are now known. For some steps homologous mutations are known across 4 to 6 model species. Examples include the early step, geranylgeranyl diphosphate to copalyl diphosphate, and the activation step, GA₂₆to GA₁. Several GA-synthesis mutations have now been characterised at the molecular level and all are in structural genes. It is now clear some steps are controlled by gene families with distinct tissue specificity. Further, some enzymes control more than one step in the biosynthetic pathway. The only mutation known to block deactivation. sin in pea, leads to an elongated phenotype. The GA response mutants are less well understood and are a more diverse group. They include elongated mutants with a constitutive GA response (spy in arabidopsis. la cry-s in pea and sln in barley) or an enhanced GA response (phyB in arabidopsis. lv in pea and Ih in cucumber). Short response mutants include at least three types. One group accumulates GAs and are mostly unresponsive to applied GA (gai in arabidopsis. D8 in maize. Rht3 in wheat). A recently identified group, exemplified by Igr in pea and gas in barley, have a short stature and reduced response but attain full responses with very high doses of exogenous GA. How close these mutations act to GA reception remains to be determined. Lastly, a number of mutants with short stature and reduced GA response differ in overall phenotype from GA-deficient plants and cannot be made to mimic wild type even at high GA application rates. These mutations act beyond GA reception and some have already proved useful in elucidating other pathways that affect shoot elongation. For example, the lk and lkb mutations in pea appear to block brassinolide synthesis and this in turn prevents normal GA-mediated elongation responses.     

Plastome mutants

Summary Since the first reports seventy-five years ago on the non-Mendelian inheritance of variegation in plants, chloroplast gene mutations have been useful for genetical and physiological investigations. The mutations have been shown to affect the chloroplast translational apparatus, photosystem I, photosystem II, the cytochrome f/b₆ complex, carbon fixation, or the ATP synthase. They arose spontaneously or were induced by mutagens or by the action of nuclear mutator genes. Alterations of chloroplast DNA include point mutations, deletions, duplications, and inversions. In 1909, Correns discovered uniparental transmission of chloroplasts when he observed the maternal inheritance of a chlorophyll deficiency inMirabilis jalapa. At the same time, Baur (1909) reported crosses ofPelargonium zonale in which the offspring inherited chloroplasts from both parents (biparental transmission) with variegated leaves resulting as the green and white plastids sorted out. since the experiments of Baur and Correns, many non-Mendelian mutants have been isolated in both higher plants and algae (for reviews see Hagemann, 1964; Kirk and Tilney-Bassett, 1978; Gillham, 1978). Some of these are mitochondrial traits, including cytoplasmic male sterility in maize and several other plants (Hanson and Conde, 1985; Pring and Lonsdale, 1985). Several other traits have been tentatively identified as mitochondrial since their inheritance pattern differs from that of both nuclear and chloroplast genes, including the deformed leaf (“falsifolia”) syndrome ofOenothera (Stubbe, 1970), non-chromosomal stripe of maize (Coe, 1983), and inChlamydomonas, photoautotropism (Wiseman et al., 1977) and a minute colony phenotype (Alexander et al., 1974). A far larger number of extranuclear mutations affect the plastome (plastid genome). Among the algae,Euglena gracilis (Russell and Lyman, 1982),Scenedesmus obliquus (Bishop, 1982) andChlorella (Galling, 1982) have yielded interesting mutants, but unlikeChlamydomonas, they are not known to undergo sexual reproduction, and thus the Mendelian or non-Mendelian nature of the mutations has not been determined. Most of the plastome mutations which have been characterized have been isolated in higher plant lines or fromChlamydomonas.     

Characterization of temperature-sensitive mutants of bacteriophage PM2: Membrane mutants

Summary In an effort to understand the genetic regulation of membrane morphogenesis, twenty-nine temperature-sensitive mutants of the membrane-containing bacteriophage PM2 were isolated. Characterization at restrictive temperature revealed groups showing no lysis (Groups I–IV), partial lysis (Groups V–VIII), and full lysis (Groups IX–XII) of the host Pseudomonas BAL-31. When the cell lysis data are considered in conjunction with data on stimulation of viral DNA synthesis, at least six mutant groups are defined. Analysis by gel electrophoresis of the pattern of viral proteins synthesized under restrictive conditions further divides the mutants into twelve groups. Temperature shift experiments delineate early, intermediate and late mutants. Complementation data support some of these groupings. The observed low levels of complementation and recombination are discussed in terms of gene product/genome restriction, bound to the membrane at the site of infection.It is of particular interest to membrane morphogenesis that under restrictive conditions late mutants in Groups II, III and IV make empty-appearing vesicles inside the cell that are the size of virus membranes as seen in thin sections of cells in the electron microscope. Mutants ts 1 (Group II) and ts 12 (Group III) show defects in their ability to incorporate into membranes viral structural proteins sp 13 and sp 6.6. The possibility is discussed that either of these proteins control the size and shape of the viral membrane.     

Novel peroxisome clustering mutants and peroxisome biogenesis mutants of Saccharomyces cerevisiae

The goal of this research is to identify and characterize the protein machinery that functions in the intracellular translocation and assembly of peroxisomal proteins in Saccharomyces cerevisiae. Several genes encoding proteins that are essential for this process have been identified previously by Kunau and collaborators, but the mutant collection was incomplete. We have devised a positive selection procedure that identifies new mutants lacking peroxisomes or peroxisomal function. Immunofluorescence procedures for yeast were simplified so that these mutants could be rapidly and efficiently screened for those in which peroxisome biogenesis is impaired. With these tools, we have identified four complementation groups of peroxisome biogenesis mutants, and one group that appears to express reduced amounts of peroxisomal proteins. Two of our mutants lack recognizable peroxisomes, although they might contain peroxisomal membrane ghosts like those found in Zellweger syndrome. Two are selectively defective in packaging peroxisomal proteins and moreover show striking intracellular clustering of the peroxisomes. The distribution of mutants among complementation groups implies that the collection of peroxisome biogenesis mutants is still incomplete. With the procedures described, it should prove straightforward to isolate mutants from additional complementation groups.     

Interruption-deficient mutants of bacteriophage T5: analysis of single-site mutants.

The properties of viable mutants of bacteriophage T5 that lack, singly, each of the four major sites at which single-chain interruptions normally occur in T5 DNA are described. The mutations responsible for loss of each interruption were mapped by analysis with HhaI, a restriction endonuclease with a cleavage site (pGCGC) that occurs at the 5' termini of the major interruptions (B. P. Nichols and J. E. Donelson, J. Virol. 22:520-526, 1977). For each mutant tested, loss of a specific interruption resulted in loss of a specific HhaI cleavage site. Multiple single-site mutants were constructed to determine the effect of loss of more than one interruption on phage viability. These recombinants, including a phage that lacks the four major interruptible sites, were fully viable and did not exhibit a compensating increase in the frequency of minor interruptions. The effect of loss of a specific interruption on genetic recombination was tested in two-factor crosses with markers that occur close to, but on opposite sites of, the interruption. Loss of the interruptible site did not affect recombination frequency.     

Chilling-sensitive mutants of arabidopsis

Twenty-one mutants ofArabidopsis thaliana were isolated that developed chlorosis or necrosis upon incubation at low temperature (10°C to 15°C). Crosses among mutants in different phenotypic classes showed that mutants in three of four classes were found in a small number of loci. This article is reproduced fromWeeds World, vol. 1. For electronic access toWeeds World, see PMBR 12(4):302–303.     

Cytokinesis-defective mutants of Arabidopsis

We have identified mutations in six previously uncharacterized genes of Arabidopsis, named club, bublina, massue, rod, bloated, and bims, that are required for cytokinesis. The mutants are seedling lethal, have morphological abnormalities, and are characterized by cell wall stubs, gapped walls, and multinucleate cells. In these and other respects, the new mutants are phenotypically similar to knolle, keule, hinkel, and pleiade mutants. The mutants display a gradient of stomatal phenotypes, correlating roughly with the severity of their cytokinesis defect. Similarly, the extent to which the different mutant lines were capable of growing in tissue culture correlated well with the severity of the cytokinesis defect. Phenotypic analysis of the novel and previously characterized loci indicated that the secondary consequences of a primary defect in cytokinesis include anomalies in body organization, organ number, and cellular differentiation, as well as organ fusions and perturbations of the nuclear cycle. Two of the 10 loci are required for both cytokinesis and root hair morphogenesis. The results have implications for the identification of novel cytokinesis genes and highlight the mechanistic similarity between cytokinesis and root hair morphogenesis, two processes that result in a rapid deposition of new cell walls via polarized secretion.     

Dynamics of escape mutants

We use multi-type Galton-Watson branching processes to model the evolution of populations that, due to a small reproductive ratio of the individuals, are doomed to extinction. Yet, mutations occurring during the reproduction process, may lead to the appearance of new types of individuals that are able to escape extinction. We provide examples of such populations in medical, biological and environmental contexts and give results on (i) the probability of escape/extinction, (ii) the distribution of the waiting time to produce the first individual whose lineage does not get extinct and (iii) the distribution of the time it takes for the number of mutants to reach a high level. Special attention is dedicated to the case where the probability of mutation is very small and approximations for (i)-(iii) are derived.     

Photomorphogenic mutants of tomato

Photomorphogenesis of tomato (Lycopersicon esculentum Mill.) is being studied with the aid of mutants which are modified either in their photoreceptor composition or in their signal transduction chain(s). Phytochrome chromophore mutants, presumably deficient in all phytochromes, and mutants specifically deficient in phytochrome A (phy A) or B1 (phyB1) have been used to study the roles played by phytochromes in photomorphogenesis. In addition, other mutants, including transgenic lines overproducing phyA, exhibit exaggerated photomorphogenesis. Studies using these mutants are reviewed, with emphasis being placed on anthocyanin biosynthesis and plastid development as model systems for the dissection of the complex interactions between photoreceptors and to elucidate the nature of photoreceptor transduction chains. Recently, new mutants have been isolated by screening in a phyA, phyB1-deficient background. The novel phenotypes selected are candidates for mutants in additional photoreceptors or their transduction chains.     

Plant hormone mutants

Many of the basic facts about plant hormones are still obscure, including biosynthetic pathways and their regulation. Furthermore, our knowledge of the molecular steps between hormones and their action is extremely limited. The increasing collection of isogenic genotypes differing in hormone synthesis or responses offers great promise for future research.     

Cell wall mutants

An ever growing collection of cell wall mutants is yielding new insights into the mechanisms underlying the synthesis and assembly of cell walls in plants. In this review, we will provide an update on the use of genetic tools in plant cell wall research and we will discuss the lessons that can be drawn from the study of the first generation of mutants.     

Phenocopies ofBithorax mutants

Summary Exposure to ether of wild-type embryos of different strains ofDrosophila melanogaster causes phenocopies of different alleles of thebithorax system. Clonal analysis of the phenocopy spots has shown that the transformation caused by the treatment is maintained by cell heredity. Embryos heterozygous for several recessive mutant alleles ofbithorax show the same frequency of phenocopies as wild-type homozygous sib controls. The same holds for embryos heterozygous for the dominant mutant allelesCbx andUbx ¹ which are point mutants in thecis-regulatory region of the system. However, for dominant mutants which have breakpoints in this region (Ubx ⁸⁰,Ubx ¹³⁰ andHm) the frequency of phenocopies is about twice that of their sib controls. Embryos with increasing numbers of copies (from 1 to 4) of thebithorax system show a decreasing frequency of phenocopies. A model is proposed that explains bithorax phenocopies as resulting from disturbances in the distribution of positional information signals for segments (inductor molecules) which compete with the product of a regulator gene (repressor) and thecis-regulatory region of thebithorax system. On this model, the initiation of a metathoracic developmental pathway would result from the derepression of thebithorax system.     

Plant hormone mutants

The techniques used for the production and identification of plant hormone mutants are described. The properties used to classify these mutants into the broad synthesis and response categories are discussed, and the genetic considerations needed to allow their effective use in plant hormone research examined. A brief outline of significant work on gibberellin (GA), abscisic acid (ABA), auxin, ethylene, cytokinin and phytochrome mutants is provided. The molecular action of these genes is discussed where available and recent rapid advances made in Arabidopsis highlighted. Suggestions for future emphasis are made, particularly relating to an examination of the tissue and ontogenetic specificity of the plant hormone genes.     

Phytochrome chromophore-deficient mutants

Phytochrome chromophore-deficient mutants have been used as phytochrome-deficient plants to study many aspects of plant development. However, there are still a number of important questions to be resolved concerning both the targets and the phenotypic consequences of these mutations. Recently, progress has been made in our understanding of the molecular basis of the chromophore deficiency in these mutants. Biochemical assays for the committed steps of chromophore synthesis have been developed and used to demonstrate that the pcd1 and yellow-green-2 mutants of pea and tomato, respectively, are unable to synthesize biliverdin IXα from heme while pcd2 and aurea are deficient in phytochromobilin synthase activity. This review focuses on how this information can be used to help understand the basis of other chromophore-deficient mutants, such as the hy1 and hy2 mutants of Arabidopsis, and discusses how the phenotype of chromophore-deficient mutants is related to lesions in the chromophore biosynthesis pathway.     

Saving zebrafish mutants

At present, the zebrafish Danio rerio is the only vertebrate species for which a large-scale mutagenesis effort to identify developmental genes has been reported. Several laboratories are now intensely pursuing the molecular characterization of the genes affected by these mutations. One important criterion for the identity of the mutated gene is the rescue of the mutant phenotype by a wild-type (wt) copy of the gene. Until recently, most rescue attempts were carried out by injecting wt messenger RNA (mRNA) into fertilized eggs. A report by Yan and collaborators shows the partial rescue of floatinghead mutants by injection of genomic fragments cloned in either bacterial artificial chromosomes or bacteriophage lambda vectors. Combined with other ongoing efforts to characterize the zebrafish genome, this approach of mutant rescue opens interesting avenues for a systematic functional analysis of vertebrate genes.     

The axolotl mutants

The Mexican axolotl (Ambystoma mexicanum) has enjoyed wide use in experimental embryology for over 100 yr. Its usefulness has been extended into the area of developmental genetics largely due to the contributions of R. Briggs and R. R. Humphrey at Indiana University. To date over 30 mutants have been described, almost all of which affect development. Some of these have been discovered in inbred strains while others have been uncovered in recent Mexican imports. These mutants can be subdivided into several major classes. Maternal effect mutations lead to deficiencies in informational, structural, or metabolic components of the egg essential to early development prior to the time at which the embryo's own genome becomes active. In contrast, the developmental lethals affect later stages in embryogenesis when both morphogenetic and biochemical events are determined exclusively by the genotype of the embryo. Most lead to death at about feeding stage. Some, the cell lethals, are believed to suffer from fundamental metabolic defects affecting all parts of the embryo. Others affect the development of specific organs or tissues. The developmental nonlethals also affect specific systems, but ones that are not essential to survival. Some affect the development and survival of pigment cells and these, along with isozyme variants, are useful as markers in developmental experiments. A number of the mutants have been studied in detail, but others scarcely at all. The purpose of this review is to bring them to the attention of all developmental biologists in the hope that their potential will be even more widely recognized.