Comparative analysis of splicing of the complete set of chloroplast group II introns in three higher plant mutants.
(65/5543)
The barley mutant albostrians and the maize mutants crs1 and crs2 are defective in the splicing of various plastid group II introns. By analysing tRNA precursors and several mRNAs not previously examined, the investigation of in vivo splicing defects in these mutants has been completed. The albostrians mutation causes the loss of plastid ribosomes resulting secondarily in a disruption of splicing of all subgroup IIA introns in the chloroplast. Thus MatK, the only putative chloroplast intron-specific maturase of higher plants, might have evolved to function in splicing of multiple introns. We show that in the case of tRNA-Ala(UGC)the first step of splicing is affected, as suggested by the absence of lariat molecules. Thus the plastid-encoded splicing factor lacking in albostrians must participate in the formation of the catalytically active structure. In contrast, a mutation in the nuclear gene crs1 prevents splicing of only one intron but causes specific additional effects as precursor transcripts for tRNA-Ile(GAU), tRNA-Ala(UGC), tRNA-Lys(UUU)and tRNA-Val(UAC), but not tRNA-Gly(UCC), have significantly enhanced steady-state levels in this mutant. Our data provide evidence for a variety of splicing factors and pathways in the chloroplast, some encoded by nuclear and some by chloroplast genes, and possibly for a dual function of some of these factors. (+info)
Characterization of a plant mitochondrial active chromosome.
(66/5543)
A method is presented for the partial purification of a plant mitochondrial active chromosome (MAC). This method is based on the presence of the mitochondrial chromosome in the insoluble mitochondrial fraction which allows for its rapid purification from the bulk of detergent-solubilized proteins by ultra-centrifugation. The resuspended MAC carrying DNA and RNA-binding proteins retains DNA synthesis and transcription activities comparable to the ones found in isolated mitochondria. In comparison, tRNA-nucleotidyl terminal transferase taken as an example of RNA modifying activities remains in the soluble fraction. MAC purification is proposed as a rapid and efficient first step in the purification of DNA-binding proteins involved in DNA replication and transcription. (+info)
Starch and the control of kernel number in maize at low water potentials.
(67/5543)
After reproduction is initiated in plants, subsequent reproductive development is sometimes interrupted, which decreases the final number of seeds and fruits. We subjected maize (Zea mays L.) to low water potentials (psi(w)) that frequently cause this kind of failure. We observed metabolite pools and enzyme activities in the developing ovaries while we manipulated the sugar stream by feeding sucrose (Suc) to the stems. Low psi(w) imposed for 5 d around pollination allowed embryos to form, but abortion occurred and kernel number decreased markedly. The ovary contained starch that nearly disappeared during this abortion. Analyses showed that all of the intermediates in starch synthesis were depleted. However, when labeled Suc was fed to the stems, label arrived at the ovaries. Solute accumulated and caused osmotic adjustment. Suc accumulated, but other intermediates did not, showing that a partial block in starch synthesis occurred at the first step in Suc utilization. This step was mediated by invertase, which had low activity. Because of the block, Suc feeding only partially prevented starch disappearance and abortion. These results indicate that young embryos abort when the sugar stream is interrupted sufficiently to deplete starch during early ovary development, and this abortion results in a loss of mature seeds and fruits. At low psi(w), maintaining the sugar stream partially prevented the abortion, but invertase regulated the synthesis of ovary starch and partially prevented full recovery. (+info)
Mannose induces an endonuclease responsible for DNA laddering in plant cells.
(68/5543)
The effect of D-mannose (Man) on plant cells was studied in two different systems: Arabidopsis roots and maize (Zea mays) suspension-cultured cells. In both systems, exposure to D-Man was associated with a subset of features characteristic of apoptosis, as assessed by oligonucleosomal fragmentation and microscopy analysis. Furthermore, D-Man induced the release of cytochrome c from mitochondria. The specificity of D-Man was evaluated by comparing the effects of diastereomers such as L-Man, D-glucose, and D-galactose. Of these treatments, only D-Man caused a reduction in final fresh weight with concomitant oligonucleosomal fragmentation. Man-induced DNA laddering coincided with the activation of a DNase in maize cytosolic extracts and with the appearance of single 35-kD band detected using an in-gel DNase assay. The DNase activity was further confirmed by using covalently closed circular plasmid DNA as a substrate. It appears that D-Man, a safe and readily accessible compound, offers remarkable features for the study of apoptosis in plant cells. (+info)
Identification of cis-acting elements important for expression of the starch-branching enzyme I gene in maize endosperm.
(69/5543)
The genes encoding the starch-branching enzymes (SBE) SBEI, SBEIIa, and SBEIIb in maize (Zea mays) are differentially regulated in tissue specificity and during kernel development. To gain insight into the regulatory mechanisms controlling their expression, we analyzed the 5'-flanking sequences of Sbe1 using a transient gene expression system. Although the 2.2-kb 5'-flanking sequence between -2,190 and +27 relative to the transcription initiation site was sufficient to promote transcription, the addition of the transcribed region between +28 and +228 containing the first exon and intron resulted in high-level expression in suspension-cultured maize endosperm cells. A series of 5' deletion and linker-substitution mutants identified two critical positive cis elements, -314 to -295 and -284 to -255. An electrophoretic mobility-shift assay showed that nuclear proteins prepared from maize kernels interact with the 60-bp fragment containing these two elements. Expression of the Sbe1 gene is regulated by sugar concentration in suspension-cultured maize endosperm cells, and the region -314 to -145 is essential for this effect. Interestingly, the expression of mEmBP-1, a bZIP transcription activator, in suspension-cultured maize endosperm cells resulted in a 5-fold decrease in Sbe1 promoter activity, suggesting a possible regulatory role of the G-box present in the Sbe1 promoter from -227 to -220. (+info)
TM20, a gene coding for a new class of transmembrane proteins expressed in the meristematic tissues of maize.
(70/5543)
In the course of the analysis of lachrima, a recessive, defective kernel, embryo-lethal mutation in Zea mays that alters embryo and endosperm development, a gene coding for a new class of transmembrane proteins was isolated. The mutant was produced by Ac transposon tagging, and a gene located in the insertion region of the transposon was isolated as well as the corresponding cDNA. The predicted protein contains twenty hydrophobic segments that can be grouped in five repeats formed by four segments that fulfill the criteria for membrane spanning domains, and for this reason the gene has been named TM20. The sequences of the domains in each position of each group can be aligned, indicating that TM20 is formed by a four-domain structure duplicated five times. During embryogenesis in wild-type embryos and in the growing plant, TM20 gene expression is associated with meristems. (+info)
A nuclear gene in maize required for the translation of the chloroplast atpB/E mRNA.
(71/5543)
To elucidate mechanisms that regulate chloroplast translation in land plants, we sought nuclear mutations in maize that disrupt the translation of subsets of chloroplast mRNAs. Evidence is presented for a nuclear gene whose function is required for the translation of the chloroplast atpB/E mRNA. A mutation in atp1 results in a failure to accumulate the chloroplast ATP synthase complex due to reduced synthesis of the AtpB subunit. This decrease in AtpB synthesis does not result from a change in atpB mRNA structure or abundance. Instead, the atpB mRNA is associated with abnormally few ribosomes in atp1-1 mutants, indicating that atp1 function is required during translation initiation or early in elongation. Previously, only one nuclear gene that is required for the translation of specific chloroplast mRNAs had been identified in a land plant. Thus, atp1 will be a useful tool for dissecting mechanisms of translational control in chloroplasts. (+info)
discordia mutations specifically misorient asymmetric cell divisions during development of the maize leaf epidermis.
(72/5543)
In plant cells, cytokinesis depends on a cytoskeletal structure called a phragmoplast, which directs the formation of a new cell wall between daughter nuclei after mitosis. The orientation of cell division depends on guidance of the phragmoplast during cytokinesis to a cortical site marked throughout prophase by another cytoskeletal structure called a preprophase band. Asymmetrically dividing cells become polarized and form asymmetric preprophase bands prior to mitosis; phragmoplasts are subsequently guided to these asymmetric cortical sites to form daughter cells of different shapes and/or sizes. Here we describe two new recessive mutations, discordia1 (dcd1) and discordia2 (dcd2), which disrupt the spatial regulation of cytokinesis during asymmetric cell divisions. Both mutations disrupt four classes of asymmetric cell divisions during the development of the maize leaf epidermis, without affecting the symmetric divisions through which most epidermal cells arise. The effects of dcd mutations on asymmetric cell division can be mimicked by cytochalasin D treatment, and divisions affected by dcd1 are hypersensitive to the effects of cytochalasin D. Analysis of actin and microtubule organization in these mutants showed no effect of either mutation on cell polarity, or on formation and localization of preprophase bands and spindles. In mutant cells, phragmoplasts in asymmetrically dividing cells are structurally normal and are initiated in the correct location, but often fail to move to the position formerly occupied by the preprophase band. We propose that dcd mutations disrupt an actin-dependent process necessary for the guidance of phragmoplasts during cytokinesis in asymmetrically dividing cells. (+info)