Starch biosynthesis during pollen maturation is not well understood in terms of genes/proteins and intracellular controls that regulate it in developing pollen. We have studied two specific developmental stages: "early," characterized by the lack of starch, before or during pollen mitosis I; and "late," an actively starch-filling post-pollen mitosis I phase in S-type cytoplasmic male-sterile (S-CMS) and two related male-fertile genotypes. The male-fertile starch-positive, but not the CMS starch-deficient, genotypes showed changes in the expression patterns of a large number of genes during this metabolic transition. In addition to a battery of housekeeping genes of carbohydrate metabolism, we observed changes in hexose transporter, plasma membrane H ϩ -ATPase, ZmMADS1, and 14-3-3 proteins. Reduction or deficiency in 14-3-3 protein levels in all three major cellular sites (amyloplasts [starch], mitochondria, and cytosol) in male-sterile relative to male-fertile genotypes are of potential interest because of interorganellar communication in this CMS system. Further, the levels of hexose sugars were significantly reduced in male-sterile as compared with male-fertile tissues, not only at "early" and "late" stages but also at an earlier point during meiosis. Collectively, these data suggest that combined effects of both reduced sugars and their reduced flux in starch biosynthesis along with a strong possibility for altered redox passage may lead to the observed temporal changes in gene expressions, and ultimately pollen sterility.Several excellent reviews on male gametophyte development in plants (Mascarenhas, 1989; Bewley et al., 2000) and maize (Zea mays) in particular (Bedinger, 1992;McCormick, 1993) have been written recently and provide overviews of events from meiosis to mature pollen development. In brief, haploid gametes as tetrads are encased in a callose wall and are well nourished through the sporophytic cell layer, tapetum. The role of the tapetum in pollen development is recently elaborated (Liu et al., 2001). Release of single, free microspores from each tetrad is achieved by callase secreted from the tapetal cells, which degenerate and lead to the symplastic isolation of microspores from the mother plant. All nourishments for developing microspores are transported presumably from the nutrient-rich locular fluid inside the anthers. Most importantly, symplastic discontinuity requires that the individual microspores be programmed with appropriate signals or at least be activated for major functions, including the two mitotic divisions, intracellular vacuolar biogenesis, and several metabolic changes such as starch biosynthesis.Starch biosynthesis during the final phases of pollen maturation is critical not only because starch is a reserve source of energy for pollen germination but it also serves as a checkpoint of pollen maturity. Very often, pollen maturation appears to be prematurely terminated if starch levels remain lower than a certain threshold point as evident from several genetically controlled...

We present cellular-and ultracellular-level studies here to show developmental programmed cell death (PCD) of placentochalazal (P-C) cell layers in maternal pedicel tissue in developing caryopses of normal seed (Mn1) and in the invertasedeficient miniature (mn1) seed mutant in maize (Zea mays). PCD was evidenced by loss of nuclei and all subcellular membranous organizations in many P-C layers. The terminal deoxynucleotidyl transferase-mediated X-dUTP nick-end labeling (TUNEL) stain that is diagnostic of apoptotic-like PCD identified spatially and temporally two distinctive subdomains, which coincided with nucellar and integumental P-C layers based on their developmental origins. The early phase of PCD in the nucellar P-C was TUNEL negative and was specific to only the fertilized caryopses, indicating that the signaling for PCD in these maternal cells originated in the zygotic tissues. In fact, the initiation of PCD coincided with endosperm cellularization and was rapidly and coordinately completed prior to the beginning of the major storage phase in endosperm. Cell shape in these cell layers was also influenced by the genotype of filial endosperm. The later phase of PCD was restricted to the integumental P-C layers underneath the nucellar cells and was TUNEL positive in both genotypes. The two subdomains of the P-C layers were also distinguishable by unique cell wall-associated phenolic compounds. Based on collective evidence, we infer that the nucellar PCD may have osmolytic etiology and may lead to activation of the post-phloem transport function of the P-C layer, whereas the integumental PCD was senescent related, in particular, protecting the maturing seed against microbes that may be transported from the maternal tissue.Pedicel, a maternal tissue at the base of developing seeds of all higher plants, provides the major structural bridge in the transfer of photoassimilates and nutrients from the mother plant to the filial generation, endosperm, and embryo. In maize (Zea mays), directly underneath the basal endosperm cells and just above the phloem termini in pedicel, is a mat of cells that constitute the placento-chalazal (P-C) layer (Fig. 1B), which is believed to play a critical role in post-phloem transport of water, sugars, and nutrients for developing seeds (Kiesselbach, 1949; Felker and Shannon, 1980;Schel et al., 1984). Although described in various plant species, a P-C region is known to exhibit a high level of anatomical variability in its structural adaptations (for review, see Thorne, 1985). P-C layers are perhaps best developed in tropical crops such as maize and sorghum (Sorghum bicolor), which show assimilate transport from only the base of the caryopsis. In fact, the functional importance of a normal P-C layer is best exemplified in the miniature 1 (mn1) seed mutation in maize that shows greatly reduced size of the endosperm due to a premature withdrawal of the P-C layer from developing seed, causing a physical discontinuity, a gap, between the source and sink tissues (Lowe and Nelson, 1946). T...

The turf-type bermudagrasses include diploid Cynodon transvaalensis Burtt Davy, tetraploid C. dactylon (L.) Pers., and sterile triploid hybrids produced by crosses of these species. The objective of thiis study was to develop a set of mierosatellite markers that could distinguish among commercially important turf-type cultivars. A genomic library enriched for tfie [CA/GT]^ repeat motif was constructed from DNA of the Tifway' fiybrid and sequenced to identify mierosatellite regions. Twenty-five microsatellite-flanking primer sets were developed and used to genotype two plant introductions and 12 turf-type cultivars. These primer sets produced an average of 10 amplicons across tfie 14 DNA templates. Sequences of selected amplicons revealed polymorphism resulting from expansion and/or contraction of the microsatellite and from indel mutations in the microsatellite flanking regions. As few as two primer sets were sufficient to differentiate all unrelated introduction lines and cultivars. Tfie primer sets failed to distinguish among closely related cultivars developed by selection of natural variants, but one primer set uniquely distinguished the cultivar TifEagle from its irradiated parent cultivar. Tfiese genomic microsatellites were not derived from gene coding sequences and will supplement tfie existing expressed sequence tag (EST)-based bermudagrass microsatellites. Tfiey will be most useful for evaluating tfie genetic diversity of Cynodon accessions and distinguishing among cultivars that exploit thils diversity.

Biofuel crops such as napiergrass possess traits characteristic of invasive plant species, raising concern that biofuels might escape cultivation and invade surrounding agricultural and natural areas. Napiergrass biofuel types are being developed to have reduced invasion risk, but these might be cultivated in areas where naturalized populations of this species are already present. The successful management of napiergrass biofuel plantations will therefore require techniques to monitor for escaped biofuels as distinguished from existing naturalized populations. Here we used 20 microsatellite DNA markers developed for pearl millet to genotype 16 entries of napiergrass, including naturalized populations and accessions selected for biofuel traits. Use of the markers showed a clear genetic separation between the biofuel types and naturalized entries and revealed naturalized populations undergoing genetic isolation by distance. These findings demonstrated the utility of microsatellite marker transfer in the development of an important tool for managing the invasion risk of a potential biofuel crop.

Mitochondria execute key pathways of central metabolism and serve as cellular sensing and signaling entities, functions that depend upon interactions between mitochondrial and nuclear genetic systems. This is exemplified in cytoplasmic male sterility type S (CMS-S) of Zea mays, where novel mitochondrial open reading frames are associated with a pollen collapse phenotype, but nuclear restorer-of-fertility (restorer) mutations rescue pollen function. To better understand these genetic interactions, we screened Activator-Dissociation (Ac-Ds), Enhancer/Suppressor-mutator (En/Spm), and Mutator (Mu) transposon-active CMS-S stocks to recover new restorer mutants. The frequency of restorer mutations increased in transposon-active stocks compared to transposon-inactive stocks, but most mutants recovered from Ac-Ds and En/Spm stocks were unstable, reverting upon backcrossing to CMS-S inbred lines. However, 10 independent restorer mutations recovered from CMS-S Mu transposon stocks were stable upon backcrossing. Many restorer mutations condition seed-lethal phenotypes that provide a convenient test for allelism. Eight such mutants recovered in this study included one pair of allelic mutations that were also allelic to the previously described rfl2-1 mutant. Targeted analysis of mitochondrial proteins by immunoblot identified two features that consistently distinguished restored CMS-S pollen from comparably staged, normal-cytoplasm, nonmutant pollen: increased abundance of nuclear-encoded alternative oxidase relative to mitochondria-encoded cytochrome oxidase and decreased abundance of mitochondria-encoded ATP synthase subunit 1 compared to nuclear-encoded ATP synthase subunit 2. CMS-S restorer mutants thus revealed a metabolic plasticity in maize pollen, and further study of these mutants will provide new insights into mitochondrial functions that are critical to pollen and seed development.

Background Cytoplasmic male sterility (CMS) is a maternally inherited failure to produce functional pollen that most commonly results from expression of novel, chimeric mitochondrial genes. In Zea mays, cytoplasmic male sterility type S (CMS-S) is characterized by the collapse of immature, bi-cellular pollen. Molecular and cellular features of developing CMS-S and normal (N) cytoplasm pollen were compared to determine the role of mitochondria in these differing developmental fates. Results Terminal deoxynucleotidyl transferase dUTP nick end labeling revealed both chromatin and nuclear fragmentation in the collapsed CMS-S pollen, demonstrating a programmed cell death (PCD) event sharing morphological features with mitochondria-signaled apoptosis in animals. Maize plants expressing mitochondria-targeted green fluorescent protein (GFP) demonstrated dynamic changes in mitochondrial morphology and association with actin filaments through the course of N-cytoplasm pollen development, whereas mitochondrial targeting of GFP was lost and actin filaments were disorganized in developing CMS-S pollen. Immunoblotting revealed significant developmental regulation of mitochondrial biogenesis in both CMS-S and N mito-types. Nuclear and mitochondrial genome encoded components of the cytochrome respiratory pathway and ATP synthase were of low abundance at the microspore stage, but microspores accumulated abundant nuclear-encoded alternative oxidase (AOX). Cytochrome pathway and ATP synthase components accumulated whereas AOX levels declined during the maturation of N bi-cellular pollen. Increased abundance of cytochrome pathway components and declining AOX also characterized collapsed CMS-S pollen. The accumulation and robust RNA editing of mitochondrial transcripts implicated translational or post-translational control for the developmentally regulated accumulation of mitochondria-encoded proteins in both mito-types. Conclusions CMS-S pollen collapse is a PCD event coincident with developmentally programmed mitochondrial events including the accumulation of mitochondrial respiratory proteins and declining protection against mitochondrial generation of reactive oxygen species.

The Australian finger lime is a unique citrus species that has gained importance due to its unique fruit characteristics and perceived tolerance to Huanglongbing (HLB), an often-fatal disease of citrus trees. In this study, we developed allotetraploid finger lime hybrids and cybrids by utilizing somatic cell fusion techniques to fuse diploid ‘OLL8’ sweet orange or ‘Page’ tangelo callus-derived protoplasts with finger lime (FL) mesophyll-derived protoplasts. Six somatic fusions were regenerated from the ‘OLL8’ + FL fusion, while three putative cybrids were regenerated from the ‘Page’ + FL fusion. Ploidy levels and nuclear-expressed sequence tag derived simple sequence repeat (EST-SSR) markers confirmed the somatic hybrid production, and mitochondrial DNA primer sets confirmed the cybrid nature. Several trees produced by the somatic fusion remained HLB negative even after 6 years of growth in an HLB-endemic environment. Pathogenesis related (PR) and other genes that are often upregulated in HLB-tolerant trees were also upregulated in our somatic fusions. These newly developed somatic fusions and cybrids could potentially be used as breeding parents to develop the next generation of improved HLB-tolerant rootstocks and scions.

Plant genomes are comprised of nuclear, plastid and mitochondrial components characterized by different patterns of inheritance and evolution. Genetic markers from the three genomes provide complementary tools for investigations of inheritance, genetic relationships and phenotypic contributions. Plant mitochondrial genomes are challenging for universal marker development because they are highly variable in terms of size, gene order and intergenic sequences and highly conserved with respect to protein-coding sequences. PCR amplification of introns with primers that anneal to conserved, flanking exons is effective for the development of polymorphic nuclear genome markers. The potential for plant mitochondrial intron polymorphisms to distinguish between congeneric species or intraspecific varieties has not been systematically investigated and is possibly constrained by requirements for intron secondary structure and interactions with co-evolved organelle intron splicing factors. To explore the potential for broadly applicable plant mitochondrial intron markers, PCR primer sets based upon conserved sequences flanking 11 introns common to seven angiosperm species were tested across a range of plant orders. PCR-amplified introns were screened for indel polymorphisms among a group of cross-compatible Citrus species and relatives; two Raphanus sativus mitotypes; representatives of the two Phaseolus vulgaris gene pools; and congeneric pairs of Cynodon, Cenchrus, Solanum, and Vaccinium species. All introns were successfully amplified from each plant entry. Length polymorphisms distinguishable by gel electrophoresis were common among genera but infrequent within genera. Sequencing of three introns amplified from 16 entries identified additional short indel polymorphisms and nucleotide substitutions that separated Citrus, Cynodon, Cenchrus and Vaccinium congeners, but failed to distinguish Solanum congeners or representatives of the Phaseolus vulgaris major gene pools. The ability of primer sets to amplify a wider range of plant species’ introns and the presence of intron polymorphisms that distinguish congeners was confirmed by in silico analysis. While mitochondrial intron variation is limited in comparison to nuclear introns, these exon-based primer sets provide robust tools for the amplification of mitochondrial introns across a wide range of plant species wherein useful polymorphisms can be identified.