Plastid Signals and the Bundle Sheath： Mesophyll Development in Reticulate Mutants

The development of a plant leaf is a meticulously orchestrated sequence of events producing a complex organ comprising diverse cell types. The reticulate class of leaf variegation mutants displays contrasting pigmentation between veins and interveinal regions due to specific aberrations in the development of mesophyll cells. Thus, the reticulate mutants offer a potent tool to investigate cell-type-specific developmental processes. The discovery that most mutants are affected in plastid-localized, metabolic pathways that are strongly expressed in vasculature-associated tis- sues implicates a crucial role for the bundle sheath and their chloroplasts in proper development of the mesophyll cells. Here, we review the reticulate mutants and their phenotypic characteristics, with a focus on those in Arabidopsis thali- ana. Two alternative models have been put forward to explain the relationship between plastid metabolism and meso- phyll cell development, which we call here the supply and the signaling hypotheses. We critically assess these proposed models and discuss their implications for leaf development and bundle sheath function in C3 species. The characteriza- tion of the reticulate mutants supports the significance of plastid retrograde signaling in cell development and highlights the significance of the bundle sheath in C3 photosynthesis.     

Wrinkled petals and stamens 1, is required for the morphogenesis of petals and stamens in Lotus japonicus

Although much progress has been made in understanding how floral organ identity is determined during the floral development, less is known about how floral organ is elaborated in the late floral developmental stages. Here we describe a novel floral mutant, wrinkled petals and stamens1 （wps1）, which shows defects in the development of petals and stamens. Genetic analysis indicates that wpsl mutant is corresponding to a single recessive locus at the long arm of chromosome 3. The early development of floral organs in wpsl mutant is similar to that in wild type, and the malfunction of the mutant commences in late developmental stages, displaying a defect on the appearance of petals and stamens. In the mature flower, petals and stamen filaments in the mutant are wrinkled or folded, and the cellular morphology under L1 layer of petals and stamen filaments is abnormal. It is found that the expression patterns of floral organ identity genes are not affected in wpsl mutants compared with that of wild type, consistent with the unaltered development of all floral organs. Furthermore, the identities of epidermal cells in different type of petals are maintained. The histological analysis shows that in wpsl flowers all petals are irregularly folded, and there are knotted structures in the petals, while the shape and arrangement of inner cells are malformed and unorganized. Based on these results, we propose that Wpsl acts downstream to the class B floral organ identity genes, and functions to modulate the cellular differentiation during the late flower developmental stages.     

Recent Progress on Rice Genetics in China

Through thousands of years of evolution and cultivation, tremendously rich genetic diversity has been accumulated in rice （Oryza sativa L.）, developing a large germplasm pool from which people can select varieties with morphologies of Interest and other important agronomic traits. With the development of modern genetics, scientists have paid more attention to the genetic value of these elite varieties and germplasms, and such rich rice resources provide a good foundation for genetic research in China. Approximately 100 000 accessions of radiation-, chemical- or insertion-induced mutagenesis have been generated since the 1980s, and great progress has been made on rice molecular genetics. So far at least 16 variant/mutant genes Including MOC1, BC1, SKC1, and Rfgenes have been isolated and characterized in China. These achievements greatly promote the research on functional genomics, understanding the mechanism of plant development and molecular design breeding of rice in China. Here we review the progress of three aspects of rice genetics in China： moving forward at the molecular level, genetic research on elite varieties and germplasms, and new gene screening and genetic analysis using mutants. The prospects of rice genetics are also discussed.     

Differential effects of p63 mutants on transactivation of p53 and/or p63 responsive genes

p63, known to play a role in development, has more recently also been implicated in cancer progression. Mutations in p63 have been shown to be responsible for several human developmental diseases. Differential splicing of the p63 gene gives rise to p63 isoforms, which can act either as tumor suppressors or as oncogene. In this report, we studied the effects of naturally occurring TAp637 mutants on the regulation of p53/p63 and p63 specific target genes. We observed significant differences among p63 mutants to regulate the p53/p63 and p63 specific target genes. Additionally, we observed a differential effect of p63 mutants on wildtype-p63-mediated induction ofp53/p63 and p63 specific target genes. We also demonstrated that these mutants differentially regulate the binding of wildtype p63 to the promoter of target genes. Furthermore, the effects of these mutants on cell death and survival were consistent with their ability to regulate the downstream targets when compared to wildtype TAp63T. In summary, our data demonstrate that p63 mutants exhibit differential effects on p63 and p53/p63 specific target genes and on the induction of apoptosis, and provide further insight into the function of p63.     

Leafy head2, which encodes a putative RNA-binding protein, regulates shoot development of rice

During vegetative development, higher plants continuously form new leaves in regular spatial and temporal patterns. Mutants with abnormal leaf developmental patterns not only provide a great insight into understanding the regulatory mechanism of plant architecture, but also enrich the ways to its modification by which crop yield could be improved. Here, we reported the characterization of the rice leafy-head2 （lhd2） mutant that exhibits shortened plastochron, dwarfism, reduced tiller number, and failure of phase transition from vegetative to reproductive growth. Anatomical and histological study revealed that the rapid emergence of leaves in lhd2 was resulted from the rapid initiation of leaf primordia whereas the reduced tiller number was a consequence of the suppression of the tiller bud outgrowth. The molecular and genetic analysis showed that LHD2 encodes a putative RNA binding protein with 67% similarity to maize TEl. Comparison of genome-scale expression profiles between wild-type and lhd2 plants suggested that LHD2 may regulate rice shoot development through KNOXand hormone-related genes. The similar phenotypes caused by LHD2 mutation and the conserved expression pattern of LHD2 indicated a conserved mechanism in controlling the temporal leaf initiation in grass.     

Genetic analysis of leaf form mutants from the Arabidopsis Information Service collection

Although a vast inventory of morphological mutants of Arabidopsis thaliana is available, only some have been used for genetic studies of leaf development. Such is the case with the Arabidopsis Information Service (AIS) Form Mutants collection, assembled by A. R. Kranz and currently stored at the Nottingham Arabidopsis Stock Centre, which includes a large number of mutant lines, most of which have been little studied. With the aim of contributing to the genetic dissection of leaf ontogeny, we have subjected 57 mutant lines isolated by others to genetic analysis; 47 of which were from the AIS collection. These are characterized by vegetative leaves of abnormal shape or size, and were chosen as candidates for mutations in genes required for leaf morphogenesis. The mutant phenotypes studied were shown to be inherited as single recessive Mendelian traits and were classified into 10 phenotypic classes. These mutant strains were found to fall into 37 complementation groups, 7 of which corresponded to known genes. Results of the phenotypic analysis and data on the genetic interactions of these mutants are presented, and their possible developmental defects discussed.     

Genetic analysis of leaf form mutants from the Arabidopsis Information Service collection.

Although a vast inventory of morphological mutants of Arabidopsis thaliana is available, only some have been used for genetic studies of leaf development. Such is the case with the Arabidopsis Information Service (AIS) Form Mutants collection, assembled by A. R. Kranz and currently stored at the Nottingham Arabidopsis Stock Centre, which includes a large number of mutant lines, most of which have been little studied. With the aim of contributing to the genetic dissection of leaf ontogeny, we have subjected 57 mutant lines isolated by others to genetic analysis; 47 of which were from the AIS collection. These are characterized by vegetative leaves of abnormal shape or size, and were chosen as candidates for mutations in genes required for leaf morphogenesis. The mutant phenotypes studied were shown to be inherited as single recessive Mendelian traits and were classified into 10 phenotypic classes. These mutant strains were found to fall into 37 complementation groups, 7 of which corresponded to known genes. Results of the phenotypic analysis and data on the genetic interactions of these mutants are presented, and their possible developmental defects discussed. Received: 28 October 1998 / Accepted: 21 February 1999     

Leaf phenomics: a systematic reverse genetic screen for Arabidopsis leaf mutants

The study and eventual manipulation of leaf development in plants requires a thorough understanding of the genetic basis of leaf organogenesis. Forward genetic screens have identified hundreds of Arabidopsis mutants with altered leaf development, but the genome has not yet been saturated. To identify genes required for leaf development we are screening the Arabidopsis Salk Unimutant collection. We have identified 608 lines that exhibit a leaf phenotype with full penetrance and almost constant expressivity and 98 additional lines with segregating mutant phenotypes. To allow indexing and integration with other mutants, the mutant phenotypes were described using a custom leaf phenotype ontology. We found that the indexed mutation is present in the annotated locus for 78% of the 553 mutants genotyped, and that in half of these the annotated T‐DNA is responsible for the phenotype. To quickly map non‐annotated T‐DNA insertions, we developed a reliable, cost‐effective and easy method based on whole‐genome sequencing. To enable comprehensive access to our data, we implemented a public web application named PhenoLeaf ( http://genetics.umh.es/phenoleaf ) that allows researchers to query the results of our screen, including text and visual phenotype information. We demonstrated how this new resource can facilitate gene function discovery by identifying and characterizing At1g77600, which we found to be required for proximal–distal cell cycle‐driven leaf growth, and At3g62870, which encodes a ribosomal protein needed for cell proliferation and chloroplast function. This collection provides a valuable tool for the study of leaf development, characterization of biomass feedstocks and examination of other traits in this fundamental photosynthetic organ.     

<Emphasis Type="Italic">ASYMMETRIC LEAVES1</Emphasis>, an <Emphasis Type="Italic">Arabidopsis</Emphasis> gene that is involved in the control of cell differentiation in leaves

During leaf development, the formation of dorsal-ventral and proximal-distal axes is central to leaf morphogenesis. To investigate the genetic basis of dorsoventrality and proximodistality in the leaf, we screened for mutants of Arabidopsis thaliana (L.) Heynh. with defects in leaf morphogenesis. We describe here the phenotypic analysis of three mutant alleles that we have isolated. These mutants show varying degrees of abnormality including dwarfism, broad leaf lamina, and aberrant floral organs and fruits. Genetic analysis revealed that these mutations are alleles of the previously isolated mutant asymmetric leaves1 ( as1). In addition to the leaf phenotypes described previously, these alleles display other phenotypes that were not observed. These include: (i) some rosette leaves with petiole growth underneath the leaf lamina; (ii) leaf vein branching in the petiole; and (iii) a leaf lamina with an epidermis similar to that on the petiole. The mutant phenotypes suggest that the ASYMMETRIC LEAVES1 ( AS1) gene is involved in the control of cell differentiation in leaves. As the first step in determining a molecular function for AS1, we have identified the AS1 gene using map-based cloning. The AS1 gene encodes a MYB-domain protein that is homologous to the Antirrhinum PHANTASTICA ( PHAN) and maize ROUGH SHEATH2 ( RS2) genes. AS1 is expressed nearly ubiquitously, consistent with the pleiotropic mutant phenotypes. High levels of AS1 expression were found in tissues with highly proliferative cells, which further suggests a role in cell division and early cell differentiation.     

Maize mutants and variants altering developmental time and their heterochronic interactions.

It is useful to envision two fundamentally different ways by which the timing of plant development is regulated: developmental stage-transition mechanisms and time-to-flowering mechanisms. The existence of both mechanisms is indicated by the behavior of various mutants. Shoot stage transitions are defined by dominant mutants representing at least four different genes; each mutant retards transitions from juvenile shoot stages to more adult shoot stages. In addition, dominant leaf stage-transition mutants in at least seven different genes have similar phenotypes, but the leaf rather than the shoot is the focus (and at least two of these genes encode homeodomain proteins.) One mutant, Hairy sheath frayed 1-O (Hsf1-O) simultaneously affects shoot and leaf; this mutant's behavior initiated our interest in plant heterochronism. The second type of timekeeping involves time-to-flowering. As with most plant but not animal species, cultivars of the maize species vary greatly for the time-to-flowering quantitative trait: between 6 and 14 weeks is common. It is via the 'slipping time frames' interaction that takes place between stage-transition mutants and time-to-flowering genetic backgrounds that unexpected and radical phenotypes occur. We see a reservoir of previously unsuspected morphological possibilities among the few heterochronic genotypes we have constructed, possibilities that may mimic the sort of variation needed to fuel macroevolution without having to posit (as done by Goldschmidt) any special macromutational mechanisms.     

A conceptual framework for maize leaf development.

What is and is not known about the maize leaf is reviewed. Analysis of genetic mosaics and direct observation with the SEM have broken leaf development into three distinct phases: recruitment of cells within the meristem, cell division into the 0.6-mm tall primordium, and postprimordial division and differentiation into the mature leaf. New data are presented that imply that cell division rates in the leaf are coordinated by inductive signals from the internal cells. Leaf cells that tend to divide more are held in check by slower growing neighbors; this complicates the search for developmental compartments. Experiments with recessive mutants that remove the ligule and auricle have been important in identifying an inducer signal with the specific meaning "make ligule-auricle." We have studied many dominant mutant alleles at seven different genes. Each mutant alters the position of the ligule boundary. We conclude the following. First, the mutants act in particular domains of the primordium. Second, the dominant mutants all move the ligule boundary in the same direction. Third, the mutants all retard developmental stage transitions. Fourth, three and probably four of the seven genes for which dominant mutants have been studied specify homeodomain proteins in the wrong place. The concept of "maturation schedule" is used to explain these data. All of the dominant mutant phenotypes are seen as consequences of immature cells being in the wrong place when inductive signals pass through the leaf. Several specific questions of leaf development and especially questions as to source of inductive signals or homologies among juvenile and adult organ parts are recast in light of this "maturation schedule" hypothesis.     

A series of novel mutants of Arabidopsis thaliana that are defective in the formation of continuous vascular network: calling the auxin signal flow canalization hypothesis into question

For the genetic analysis of molecular mechanisms underlying temporal and spatial regulation of vascular pattern formation, we isolated mutants of Arabidopsis thaliana that are impaired in vascular patterning. Microscopic examination of the cotyledonary venation of 3,400 M(3) lines led to the identification of 12 mutant lines. Genetic analysis of 8 of these mutant lines indicated that vein pattern formation in these lines resulted from monogenic recessive mutations in 7 different genes, designated VAN1 through VAN7. Mutations in VAN1 through VAN6 genes caused fragmentation (disconnection or partial loss) of lateral veins of the cotyledon and tertiary veins of the rosette leaf whereas they were less injurious to the formation of major veins. Detailed characterization of the van3 mutant using pAthb8::GUS and pTED3::GUS, as molecular markers for the early stage of vascular tissue formation showed that the provascular tissue of the cotyledonary lateral veins was differentiated in fragments during late embryogenesis. These phenotypes of the van mutants are discussed in relation to the auxin signal flow canalization hypothesis and the diffusion-reaction prepattern hypothesis, with the fragility of the continuity in the minor vein formation favoring the latter hypothesis.