The ribosomal RNA genes from chloroplasts of mustard (Sinapis alba L.): mapping and sequencing of the leader region

Summary The genes coding for rRNAs from mustard chloroplasts were mapped within the inverted repeat regions of intact ctDNA and on ctDNA fragments cloned in pBR322. R-loop analysis and restriction endonuclease mapping show that the genes for 16S rRNA map at distances of 17 kb from the junctions of the repeat regions with the large unique region. The genes for 23S rRNA are located at distances of 2.8 kb from the junctions with the small unique region. Genes for 4.5S and 5S rRNA are located in close proximity to the 23S rRNA genes towards the small unique region. DNA sequencing of portions of the 5 terminal third from the mustard 16S rRNA gene shows 96–99% homology with the corresponding regions of the maize, tobacco and spinach chloroplast genes. Sequencing of the region proximal to the 16S rRNA gene reveals the presence of a tRNA^Val gene in nearly the same position and with identical sequence as in maize, tobacco and spinach. Somewhat less but still strong homology is also observed for the tDNA ^Val/16S rDNA intercistronic regions and for the regions upstream of the tRNA^Val gene. However, due to many small and also a few larger deletions and insertions in the leader region, common reading frames coding for homologous peptides larger than 44 amino acids can not be detected; it is therefore unlikely that this region contains a protein coding gene.     

The ultrastructural development of plastids in leaves of maize plants exposed to continuous illumination

Summary Chloroplast differentiation in relation to increasing leaf age has been investigated in maize plants exposed to continuous illumination. In the young leaves the proplastids differentiate into chloroplasts containing well organized grana as well as prolamellar bodies. In the older leaves, while plastids differentiate, the prolamellar bodies are no longer detectable. Chloroplast ability to build up prolamellar bodies does not seems so much a light dependent process as it is affected by cell differentiation rate.Supported by a grant of C.N.R.     

The meiotic drive system on maize abnormal chromosome 10 contains few essential genes

In maize, a distal portion of abnormal chromosome 10 (Ab10) causes the meiotic drive of itself as well as many unlinked heterochromatic regions known as knobs. The Ab10 drive system, which encodes trans- as well as cis-acting components, occupies a large region of chromosome 10L equivalent to 3% of the genome. Here we describe five new structural mutations of Ab10 (five deletions and a duplication) that arose from a screen for meiotic drive mutants. The high frequency of breakage events, detected both genetically and cytologically, suggest that the chromosome may be especially unstable. Very large deletions within the drive system are female-transmissible and plants homozygous for deficiencies lacking much of this interval can be grown to maturity. The data suggest that few genes required for normal growth and development lie within the portion of Ab10 responsible for meiotic drive. These and other published data suggest that meiotic drive systems tend to evolve in gene-sparse or otherwise information-poor regions of the genome where they are less likely to negatively affect individual fitness.     

Effect of 1.10-phenanthroline, a photodynamic herbicide on the development and structure of maize chloroplasts

Effect of low (5 mmol·dm⁻³) and high (10 or 20 mmol·dm⁻³) doses of 1.10-phenanthroline (Phe), a photodynamic herbicide, on the development of chloroplasts in etiolated and subsequently illuminated maize seedlings and on the structure of already developed chloroplasts of green maize seedlings was examined. Etiolated and then irradiated plants were resistant to 5 mmol·dm⁻³ of Phe with respect to morphology, however Phe caused inhibition of greening and of grana formation. Higher Phe concentrations followed by exposure to light caused not only total inhibition of greening but also dilation of thylakoids, swelling of chloroplasts, and finally total destruction of chloroplast structure. Application of Phe in the same concentrations to green plants revealed that they were resistant to low dose of Phe with respect to morphology and structure of chloroplasts, however 10 and 20 mmol·dm⁻³ Phe and illumination caused the loss of turgor of treated plants and other photooxidative damages seen at the ultrastructural level. We concluded that maize, as representant of monocotyledonous plants, is resistant to low (5 mmol·dm⁻³) Phe concentration. Higher (10 or 20 mmol·dm⁻³) concentrations, used to determine the site of damage and mode of action of Phe on the level of cell revealed that action of photodynamic herbicides is based on standard photoinhibition mechanism and also probably on their chelating properties.     

The response of maize (Zea mays) to Azospirillum inoculation in various types of soils in the field

Application of a peat-based powder inoculant of Azospirillum brasilense, as well as a granular inoculant (each containing 0.5–1.0×10⁷ Azospirillum/g moist peat), in the seed furrows of Zea mays resulted in significantly increased yields (11 to 14%) in light soils at low rates of N fertilization. In general, there was no effect of inoculation on plant yields in heavier soils nor when N fertilization was high. Pre-emergence application of granular inoculant and inoculation associated with irrigation were more efficient in increasing yield than inoculation post-emergence or seed coating.E. Fallik is with ARO-The Volcani Centre, Department of Postharvest Science of Fresh Produce, Bet-Dagan 50250, Israel. Y. Okon is with The Hebrew University of Jerusalem, Faculty of Agriculture, Department of Plant Pathology and Microbiology, Rehovot 76100, Israel     

Chromosomal and cell size analysis of cold tolerant maize

Summary C-band number, guard cell length, and chloroplast number per guard cell were determined for eight maize populations. These populations consisted of maize selected for cold tolerance at the University of Nebraska as well as the original unselected populations. The genome size of these populations had previously been determined. C-band number fluctuated concertedly with the changes in genome size indicating that deletions and additions of constitutive heterochromatin occurred during selection, resulting in altered genome sizes. Guard cell size of all the cold tolerant populations was greater than the cell size of the respective nonselected populations. Chloroplast number per guard cell was also higher in all the cold tolerant populations than in their parental populations, but the increases were not statistically significant. The results indicate that changes in genome size that occurred during selection for cold tolerance are the result of changes in amounts of C-band heterochromatin and that the selection process results in an increase in cell size in the cold tolerant populations.     

Zinc deficiency and pollen fertility in maize (Zea mays)

Zinc deficiency decreased pollen viability in maize (Zea mays L. cv. G2) grown in sand culture. On restoring normal zinc supply to zinc-deficient plants before the pollen mother cell stage of anther development, the vegetative yield of plants and pollen fertility could be recovered to a large extent, but the recovery treatment was not effective when given after the release of microspores from the tetrads. If zinc deficiency was induced prior to microsporogenesis it did not significantly affect vegetative yield and ovule fertility, but decreased the fertility of pollen grains, even of those which visibly appeared normal. If the deficiency was induced after the release of microspores from the tetrads, not only vegetative yield and ovule fertility but pollen fertility also remained unaffected.     

Chlorophyll-deficient gene and morphological variations in Korean populations of maize (Zea mays)

We studied cultivated and naturalized Korean maize populations to determine the extent to which the chlorophylldeficient mutation and the phenotypic variations of two morphological characters (i.e., red coleoptiles and epicotyls, and the number of the first root hairs) are maintained. The frequency of the chlorophyll-deficient mutant gene (2.73% on average) was highly variable. Frequencies of red coleoptiles and epicotyls also were higher than expected from a mutation-selection balance. The average number of hairy phenotypes within populations was 1.8, ranging from 0.0 to 4.0. Naturalized populations were closely related to with cultivated communities. Most striking, however, was the more significant difference among populations than within populations with regard to both the frequency of chlorophyll-deficient mutant genes and the phenotypic variations of our two morphological characters. On a per-gene basis, the majority of the phenotypic variation (mean of 73.3%) resided among populations.     

In vitro plant regeneration from quality protein maize (QPM)

Breeding efforts to obtain more nutritious maize materials aimed at alleviating dietary deficiencies in developing countries have resulted in an improved maize germplasm known as quality protein maize (QPM). Quality protein maize has higher contents of tryptophan, lysine, and leucine than common maize, but suffers from some major agronomic drawbacks found in common inbred maize lines, such as susceptibility to insect pests and fungal and bacterial diseases and herbicide sensitivity. The development of a reproducible and efficient protocol for tissue culture of QPM is expected to solve some of these deficiencies. In this work, we have evaluated different formulations for in vitro induction of morphogenic responses in three QPM lines developed by the International Maize and Wheat Improvement Center (CIMMYT): CML (CIMMYT maize line)-145, CML-176, and CML-186. Only CML-176 and CML-186 have proven to be responsive to the in vitro conditions considered in this work, with CML-176 showing the highest efficiency in regenerable callus formation and growth. N6C1 medium was found to be efficient for in vitro culture of QPM, whereas no plants could be regenerated by using MPC medium. From CML-176 embyogenic calli cultured on N6C1 medium, we were able to regenerate up to 0.3 plants per 500 mg fresh weight (FW) callus. Further modifications in this experimental protocol, including the replacement of 3,6-dichloro-o-anisic acid with 2,4-dichlorophenoxyacetic acid and modification of the N6C1 vitamin balance, significantly increased the regeneration response of the induced calli, with up to 16.8 and 9.3 plants recovered per 500 mg FW callus for CML-176 and CML-186, respectively.     

Multiple proteins bind to the P2 promoter region of the zein gene pMS1 of maize

Summary A 216 by promoter fragment of the 19 kDa protein zein gene pMS1, containing the CCAAT and TATA boxes, was analysed by a variety of techniques for in vitro interactions with nuclear proteins from endosperm tissue. HMG proteins were found to form stable complexes with these A/T-rich promoter sequences and several specific DNA-binding proteins appear to be involved in the formation of DNA-protein complexes with this fragment. A 29 bp region spanning the two CCAAT boxes was protected from DNase I digestion in footprinting experiments.     

Isolation and characterization of pollen-specific maize genes with sequence homology to ragweed allergens and pectate lyases

A cDNA clone (Zm58.1) was isolated by differential screening from a cDNA library made to mature Zea mays pollen, and shown to be pollen-specific by RNA blot analysis. When this partial-length clone was used to probe a genomic library, a similar but distinct pollen-specific genomic clone (68% sequence identity) was isolated (Zm58.2). The putative proteins coded for by these two clones show sequence homology to several flower-expressed gene products from various plant species, including known pollen allergens from short ragweed (Ambrosia artemisiifolia), and to pectate lyases from the plant pathogenic bacteria Erwinia spp. The two genes map to different chromosomes.     

Haplo-diploid gene expression and pollen selection for tolerance to acetochlor in maize

The objectives of this research were to determine if genes controlling the reaction to the herbicide acetochlor in maize (Zea mays L.) are active during both the haploid and the diploid phases of the life cycle and if pollen selection can be utilized for improving sporophytic resistance. Pollen of eight inbred lines, previously characterized through sporophytic analysis for the level of tolerance to acetochlor, showed a differential reaction to the herbicide forin vitro tube length; moreover, such pollen reactions proved to be significantly correlated (r =0.786^*,df=6) with those of the sporophytes producing the pollen. Pollen analysis of two inbred lines (i.e. Mo17, tolerant, and B79, susceptible) and their single cross showed that thein vitro pollen-tube length reaction of the hybrid was intermediate between those of two parents. An experiment on pollen selection was then performed by growing tassels of Mo17xB79 in the presence of the herbicide. Pollen obtained from treated tassels showed a greater tolerance to acetochlor, assessed asin vitro tube length reaction, than pollen obtained from control tassels. Moreover, the backcross [B79 (Mo17xB79)] sporophytic population obtained using pollen from the treated tassels was more tolerant (as indicated by the fresh weight of plants grown in the presence of the herbicide) than was the control backcross population. The two populations did not differ when grown without the herbicide. These findings indicate that genes controlling the reaction to acetochlor in maize have haplodiploid expression; consequently, pollen selection can be applied for improving plant tolerance.     

Loss or retention of chloroplast DNA in maize seedlings is affected by both light and genotype

We examined the chloroplast DNA (cpDNA) from plastids obtained from wild type maize (Zea mays L.) seedlings grown under different light conditions and from photosynthetic mutants grown under white light. The cpDNA was evaluated by real-time quantitative PCR, quantitative DNA fluorescence, and blot-hybridization following pulsed-field gel electrophoresis. The amount of DNA per plastid in light-grown seedlings declines greatly from stalk to leaf blade during proplastid-to-chloroplast development, and this decline is due to cpDNA degradation. In contrast, during proplastid-to-etioplast development in the dark, the cpDNA levels increase from the stalk to the blade. Our results suggest that DNA replication continues in the etioplasts of the upper regions of the stalk and in the leaves. The cpDNA level decreases rapidly, however, after dark-grown seedlings are transferred to light and the etioplasts develop into photosynthetically active chloroplasts. Light, therefore, triggers the degradation of DNA in maize chloroplasts. The cpDNA is retained in the leaf blade of seedlings grown under red, but not blue light. We suggest that light signaling pathways are involved in mediating cpDNA levels, and that red light promotes replication and inhibits degradation and blue light promotes degradation. For five of nine photosynthetic mutants, cpDNA levels in expanded leaves are higher than in wild type, indicating that nuclear genotype can affect the loss or retention of cpDNA.