Division and differentiation during normal and liguleless-1 maize leaf development

The maize leaf is composed of a blade and a sheath, which are separated at the ligular region by a ligule and an auricle. Mutants homozygous for the recessive liguleless-1 (lg1) allele exhibit loss of normal ligule and auricle. The cellular events associated with development of these structures in both normal and liguleless plants are investigated with respect to the timing of cell division and differentiation. A new method is used to assess orientation of anticlinal division planes during development and to determine a division index based on recent epidermal cross-wall deposition. A normal leaf follows three stages of development: first is a preligule stage, in which the primordium is undifferentiated and dividing throughout its length. This stage ends when a row of cells in the preligule region divides more rapidly in both transverse and longitudinal anticlinal planes. During the second stage, ligule and auricle form, blade grows more rapidly than sheath, divisions in the blade become exclusively transverse in orientation, and differentiation begins. The third stage is marked by rapid increase in sheath length. The leaf does not have a distinct basal meristem. Instead, cell divisions are gradually restricted to the base of the leaf with localized sites of increased division at the preligule region. Divisions are not localized to the base of the sheath until near the end of development. The liguleless-1 homozygote shows no alteration in this overall pattern of growth, but does show distinct alteration in the anticlinal division pattern in the preligule region. Two abnormal patterns are observed: either the increase in division rate at the preligule site is absent or it exhibits loss of all longitudinal divisions so that only transverse (or cell-file producing) divisions are present. This pattern is particularly apparent in developing adult leaves on older lg1 plants, in which sporadic ligule vestiges form. From these and results previously published (Becraft et al. (1990) Devl Biol. 14), we conclude that the information carried by the Lg1+ gene product acts earlier in development than formation of the ligule proper. We hypothesize that Lg1+ may be effective at the stage when the blade-sheath boundary is first determined.     

The indeterminate gametophyte1 gene of maize encodes a LOB domain protein required for embryo Sac and leaf development

Angiosperm embryo sac development begins with a phase of free nuclear division followed by cellularization and differentiation of cell types. The indeterminate gametophyte1 (ig1) gene of maize (Zea mays) restricts the proliferative phase of female gametophyte development. ig1 mutant female gametophytes have a prolonged phase of free nuclear divisions leading to a variety of embryo sac abnormalities, including extra egg cells, extra polar nuclei, and extra synergids. Positional cloning of ig1 was performed based on the genome sequence of the orthologous region in rice. ig1 encodes a LATERAL ORGAN BOUNDARIES domain protein with high similarity to ASYMMETRIC LEAVES2 of Arabidopsis thaliana. A second mutant allele of ig1 was identified in a noncomplementation screen using active Mutator transposable element lines. Homozygous ig1 mutants have abnormal leaf morphology as well as abnormal embryo sac development. Affected leaves have disrupted abaxial-adaxial polarity and fail to repress the expression of meristem-specific knotted-like homeobox (knox) genes in leaf primordia, causing a proliferative, stem cell identity to persist in these cells. Despite the superficial similarity of ig1-O leaves and embryo sacs, ectopic knox gene expression cannot be detected in ig1-O embryo sacs.     

Par-1b is required for morphogenesis and differentiation of myoepithelial cells during salivary gland development

The salivary epithelium initiates as a solid mass of epithelial cells that are organized into a primary bud that undergoes morphogenesis and differentiation to yield bilayered acini consisting of interior secretory acinar cells that are surrounded by contractile myoepithelial cells in mature salivary glands. How the primary bud transitions into acini has not been previously documented. We document here that the outer epithelial cells subsequently undergo a vertical compression as they express smooth muscle α-actin and differentiate into myoepithelial cells. The outermost layer of polarized epithelial cells assemble and organize the basal deposition of basement membrane, which requires basal positioning of the polarity protein, Par-1b. Whether Par-1b is required for the vertical compression and differentiation of the myoepithelial cells is unknown. Following manipulation of Par-1b in salivary gland organ explants, Par-1b-inhibited explants showed both a reduced vertical compression of differentiating myoepithelial cells and reduced levels of smooth muscle α-actin. Rac1 knockdown and inhibition of Rac GTPase function also inhibited branching morphogenesis. Since Rac regulates cellular morphology, we investigated a contribution for Rac in myoepithelial cell differentiation. Inhibition of Rac GTPase activity showed a similar reduction in vertical compression and smooth muscle α-actin levels while decreasing the levels of Par-1b protein and altering its basal localization in the outer cells. Inhibition of ROCK, which is required for basal positioning of Par-1b, resulted in mislocalization of Par-1b and loss of vertical cellular compression, but did not significantly alter levels of smooth muscle α-actin in these cells. Overexpression of Par-1b in the presence of Rac inhibition restored basement membrane protein levels and localization. Our results indicate that the basal localization of Par-1b in the outer epithelial cells is required for myoepithelial cell compression, and Par-1b is required for myoepithelial differentiation, regardless of its localization.     

Post-meiotic deficient anther1 (PDA1) encodes an ABC transporter required for the development of anther cuticle and pollen exine in rice

The tapetum of the anther locule encloses the male reproductive cells and plays a supportive role for normal pollen development. However, the underlying mechanism remains less understood. Previously, we identified a complete recessive male sterile mutant, post-meiotic deficient anther1 (pda1), with abnormal postmeiotic tapetal development. In this study we comprehensively characterized pda1. Chemical analysis uncovered that pda1 anther had significant lower levels of cutin monomers and cuticular waxes. PDA1 gene encodes an ATP-binding cassette (ABC) half-transporter, namely OsABCG15, which is conserved from algae to higher plants. In situ RNA hybridization assay showed that PDA1 is strongly expressed in tapetal cells, and weakly in microspores during the anther development. Additionally, the expression of two pollen exine biosynthetic genes CYP704B2 and CYP703A3 was dramatically reduced in pda1 mutant anthers. Altogether, these observations suggest that the tapetum-expressed ABC transporter PDA1 plays a crucial role in secreting lipidic precursors from the tapetum to developing microspores and the anther epidermis.     

Defective pollen wall is required for anther and microspore development in rice and encodes a fatty acyl carrier protein reductase

Aliphatic alcohols naturally exist in many organisms as important cellular components; however, their roles in extracellular polymer biosynthesis are poorly defined. We report here the isolation and characterization of a rice (Oryza sativa) male-sterile mutant, defective pollen wall (dpw), which displays defective anther development and degenerated pollen grains with an irregular exine. Chemical analysis revealed that dpw anthers had a dramatic reduction in cutin monomers and an altered composition of cuticular wax, as well as soluble fatty acids and alcohols. Using map-based cloning, we identified the DPW gene, which is expressed in both tapetal cells and microspores during anther development. Biochemical analysis of the recombinant DPW enzyme shows that it is a novel fatty acid reductase that produces 1-hexadecanol and exhibits >270-fold higher specificity for palmiltoyl-acyl carrier protein than for C16:0 CoA substrates. DPW was predominantly targeted to plastids mediated by its N-terminal transit peptide. Moreover, we demonstrate that the monocot DPW from rice complements the dicot Arabidopsis thaliana male sterile2 (ms2) mutant and is the probable ortholog of MS2. These data suggest that DPWs participate in a conserved step in primary fatty alcohol synthesis for anther cuticle and pollen sporopollenin biosynthesis in monocots and dicots.     

sucker encodes a zebrafish Endothelin-1 required for ventral pharyngeal arch development

Mutation of sucker (suc) disrupts development of the lower jaw and other ventral cartilages in pharyngeal segments of the zebrafish head. Our sequencing, cosegregation and rescue results indicate that suc encodes an Endothelin-1 (Et-1). Like mouse and chick Et-1, suc/et-1 is expressed in a central core of arch paraxial mesoderm and in arch epithelia, both surface ectoderm and pharyngeal endoderm, but not in skeletogenic neural crest. Long before chondrogenesis, suc/et-1 mutant embryos have severe defects in ventral arch neural crest expression of dHAND, dlx2, msxE, gsc, dlx3 and EphA3 in the anterior arches. Dorsal expression patterns are unaffected. Later in development, suc/et-1 mutant embryos display defects in mesodermal and endodermal tissues of the pharynx. Ventral premyogenic condensations fail to express myoD, which correlates with a ventral muscle defect. Further, expression of shh in endoderm of the first pharyngeal pouch fails to extend as far laterally as in wild types. We use mosaic analyses to show that suc/et-1 functions nonautonomously in neural crest cells, and is thus required in the environment of postmigratory neural crest cells to specify ventral arch fates. Our mosaic analyses further show that suc/et-1 nonautonomously functions in mesendoderm for ventral arch muscle formation. Collectively our results support a model for dorsoventral patterning of the gnathostome pharyngeal arches in which Et-1 in the environment of the postmigratory cranial neural crest specifies the lower jaw and other ventral arch fates.     

Young Leaf Chlorosis 1, a chloroplast-localized gene required for chlorophyll and lutein accumulation during early leaf development in rice

Chlorophyll (Chl) and lutein are the two most abundant and essential components in photosynthetic apparatus, and play critical roles in plant development. In this study, we characterized a rice mutant named young leaf chlorosis 1 (ylc1) from a ⁶⁰Co-irradiated population. Young leaves of the ylc1 mutant showed decreased levels of Chl and lutein compared to those of wild type, and transmission electron microscopy analysis revealed that the thylakoid lamellar structures were obviously loosely arranged. Whereas, the mutant turns green gradually and approaches normal green at the maximum tillering stage. The Young Leaf Chlorosis 1 (YLC1) gene was isolated via map-based cloning and identified to encode a protein of unknown function belonging to the DUF3353 superfamily. Complementation and RNA-interference tests confirmed the role of the YLC1 gene, which expressed in all tested rice tissues, especially in the leaves. Real-time PCR analyses showed that the expression levels of the genes associated with Chl biosynthesis and photosynthesis were affected in ylc1 mutant at different temperatures. In rice protoplasts, the YLC1 protein displayed a typical chloroplast location pattern. The N-terminal 50 amino acid residues were confirmed to be necessary and sufficient for chloroplast targeting. These data suggested that the YLC1 protein may be involved in Chl and lutein accumulation and chloroplast development at early leaf development in rice.     

Wax-deficient anther1 is involved in cuticle and wax production in rice anther walls and is required for pollen development

In vegetative leaf tissues, cuticles including cuticular waxes are important for protection against nonstomatal water loss and pathogen infection as well as for adaptations to environmental stress. However, their roles in the anther wall are rarely studied. The innermost layer of the anther wall (the tapetum) is essential for generating male gametes. Here, we report the characterization of a T-DNA insertional mutant in the Wax-deficient anther1 (Wda1) gene of rice (Oryza sativa), which shows significant defects in the biosynthesis of very-long-chain fatty acids in both layers. This gene is strongly expressed in the epidermal cells of anthers. Scanning electron microscopy analyses showed that epicuticular wax crystals were absent in the outer layer of the anther and that microspore development was severely retarded and finally disrupted as a result of defective pollen exine formation in the mutant anthers. These biochemical and developmental defects in tapetum found in wda1 mutants are earlier events than those in other male-sterile mutants, which showed defects of lipidic molecules in exine. Our findings provide new insights into the biochemical and developmental aspects of the role of waxes in microspore exine development in the tapetum as well as the role of epicuticular waxes in anther expansion.     

fear of intimacy encodes a novel transmembrane protein required for gonad morphogenesis in Drosophila

Gonad formation requires specific interactions between germ cells and specialized somatic cells, along with the elaborate morphogenetic movements of these cells to create an ovary or testis. We have identified mutations in the fear of intimacy (foi) gene that cause defects in the formation of the embryonic gonad in DROSOPHILA: foi is of particular interest because it affects gonad formation without affecting gonad cell identity, and is therefore specifically required for the morphogenesis of this organ. foi is also required for tracheal branch fusion during tracheal development. E-cadherin/shotgun is similarly required for both gonad coalescence and tracheal branch fusion, suggesting that E-cadherin and FOI cooperate to mediate these processes. foi encodes a member of a novel family of transmembrane proteins that includes the closely related human protein LIV1. Our findings that FOI is a cell-surface protein required in the mesoderm for gonad morphogenesis shed light on the function of this new family of proteins and on the molecular mechanisms of organogenesis.     

The Tangled1 gene is required for spatial control of cytoskeletal arrays associated with cell division during maize leaf development.

The cytoskeleton plays a major role in the spatial regulation of plant cell division and morphogenesis. Arrays of microtubules and actin filaments present in the cell cortex during prophase mark sites to which phragmoplasts and associated cell plates are guided during cytokinesis. During interphase, cortical microtubules are believed to influence the orientation of cell expansion by guiding the pattern in which cell wall material is laid down. Little is known about the mechanisms that regulate these cytoskeleton-dependent processes critical for plant development. Previous work showed that the Tangled1 (Tan1) gene of maize is required for spatial regulation of cytokinesis during maize leaf development but not for leaf morphogenesis. Here, we examine the cytoskeletal arrays associated with cell division and morphogenesis during the development of tan1 and wild-type leaves. Our analysis leads to the conclusion that Tan1 is required both for the positioning of cytoskeletal arrays that establish planes of cell division during prophase and for spatial guidance of expanding phragmoplasts toward preestablished cortical division sites during cytokinesis. Observations on the organization of interphase cortical microtubules suggest that regional influences may play a role in coordinating cell expansion patterns among groups of cells during leaf morphogenesis.     

MTR1 encodes a secretory fasciclin glycoprotein required for male reproductive development in rice

1. Download : Download high-res image (356KB)
2. Download : Download full-size image

Highlights? The fasciclin glycoprotein MTR1 regulates male reproductive development in rice ? Rice male reproductive cells secrete MTR1 to control somatic tapetum development     

Nucleostemin-like 1 is required for embryogenesis and leaf development in Arabidopsis

Arabidopsis NSN1 encodes a nucleolar GTP-binding protein and is required for flower development. Defective flowers were formed in heterozygous nsn1/+?plants. Homozygous nsn1 plants were dwarf and exhibited severe defects in reproduction. Arrests in embryo development in nsn1 could occur at any stage of embryogenesis. Cotyledon initiation and development during embryogenesis were distorted in nsn1 plants. At the seedling stage, cotyledons and leaves of nsn1 formed upward curls. The curled leaves developed meristem-like outgrowths or hyperplasia tissues in the adaxial epidermis. Long and enlarged pavement cells, characteristic of the abaxial epidermis of wild type plants, were found in the adaxial epidermis in nsn1 leaves, suggesting a disoriented leaf polarity in the mutant. The important role of NSN1 in embryo development and leaf differentiation was consistent with the high level expression of the NSN1 gene in the developing embryos and the primordia of cotyledons and leaves. The CLAVATA 3 (CLV3) gene, a stem cell marker in the Arabidopsis shoot apical meristem (SAM), was expressed in expanded regions surrounding the SAM of nsn1 plants, and induced ectopically in the meristem-like outgrowths in cotyledons and leaves. The nsn1 mutation up-regulated the expression levels of several genes implicated in the meristem identity and the abaxial cell fate, and repressed the expression of other genes related to the specification of cotyledon boundary and abaxial identity. These results demonstrate that NSN1 represents a novel GTPase required for embryogenesis, leaf development and leaf polarity establishment in Arabidopsis.     

VARICOSE, a WD-domain protein, is required for leaf blade development

To gain insight into the processes controlling leaf development, we characterized an Arabidopsis mutant, varicose (vcs), with leaf and shoot apical meristem defects. The vcs phenotype is temperature dependent; low temperature growth largely suppressed defects, whereas high growth temperatures resulted in severe leaf and meristem defects. VCS encodes a putative WD-domain containing protein, suggesting a function involving protein-protein interactions. Temperature shift experiments indicated that VCS is required throughout leaf development, but normal secondary vein patterning required low temperature early in leaf development. The low-temperature vcs phenotype is enhanced in axr1-3 vcs double mutants and in vcs mutants grown in the presence of polar auxin transport inhibitors, however, vcs has apparently normal auxin responses. Taken together, these observations suggest a role for VCS in leaf blade formation.     

Pollen semi-sterility1 encodes a kinesin-1-like protein important for male meiosis, anther dehiscence, and fertility in rice

In flowering plants, male meiosis produces four microspores, which develop into pollen grains and are released by anther dehiscence to pollinate female gametophytes. The molecular and cellular mechanisms regulating male meiosis in rice (Oryza sativa) remain poorly understood. Here, we describe a rice pollen semi-sterility1 (pss1) mutant, which displays reduced spikelet fertility (~40%) primarily caused by reduced pollen viability (~50% viable), and defective anther dehiscence. Map-based molecular cloning revealed that PSS1 encodes a kinesin-1-like protein. PSS1 is broadly expressed in various organs, with highest expression in panicles. Furthermore, PSS1 expression is significantly upregulated during anther development and peaks during male meiosis. The PSS1-green fluorescent protein fusion is predominantly localized in the cytoplasm of rice protoplasts. Substitution of a conserved Arg (Arg-289) to His in the PSS1 motor domain nearly abolishes its microtubule-stimulated ATPase activity. Consistent with this, lagging chromosomes and chromosomal bridges were found at anaphase I and anaphase II of male meiosis in the pss1 mutant. Together, our results suggest that PSS1 defines a novel member of the kinesin-1 family essential for male meiotic chromosomal dynamics, male gametogenesis, and anther dehiscence in rice.