Ear secondary traits related to aflatoxin accumulation in commercial maize hybrids under artificial field inoculation

Aims of the research were: (1) to evaluate and compare 24 maize hybrids for Aspergillus flavus resistance and for aflatoxin accumulation under artificial inoculation in field experiments grown during 2005 and 2006; (2) to estimate the relationship of aflatoxin concentration with ear secondary traits. Primary ears were inoculated with a fresh spore suspension (mixture of five A. flavus isolates from Northern Italy), by spraying silks, as a modification of the non-wounding silk channel inoculation technique (SCIA); controls were both non-inoculated and sterile water-inoculated ears. Ear secondary traits, such as silk channel length measured at pollination and husk coverage at maturity, were recorded for each hybrid. The severity of ear A. flavus attack was estimated using rating scales based on the percentage of kernels with visible symptoms of infection. The aflatoxin concentration in the inoculated ears resulted, during both years, higher than in the controls; this indication confirmed that the A. flavus isolates used for the inoculum procedure were successful in accumulating mycotoxin in grains. Variability was found among the hybrids under study: aflatoxin accumulation, after artificial inoculation, ranged from 0.13 to 705.25 ng/g. The data herein presented supported the implication of two ear secondary traits in determining aflatoxin accumulation. Silk channel length recorded at pollination was negatively correlated (r = -0.54; P<0.05) with aflatoxin accumulation; on the contrary, a positive correlation (r = 0.48; P<0.05) between husk coverage rating at maturity and aflatoxin concentration suggested that a looser husk coverage is associated with higher aflatoxin accumulation. The correlation between the two mentioned ear-related traits was negative (r = -0.73; P<0.05); this indicated that hybrids showing a good coverage at pollination stage, are favoured in keeping the ear tip covered until maturity, reducing the risk of aflatoxin accumulation.

Assessment of genetic diversity and relationships among maize (Zea mays L.) Italian landraces by morphological traits and AFLP profiling

In the present study we have analyzed the genetic diversity pattern in a sample of 54 Italian maize landraces, using morphological traits and molecular markers. Although the 54 landraces surveyed in this study were restricted to Lombardy, the core region of maize production in Italy, our data revealed a large genetic heterogeneity for both morphological and molecular traits in the accessions analyzed. Additionally, our data confirm that the AFLP markers produced a high frequency of polymorphic bands and were able to unequivocally fingerprint each of the landraces considered. Cluster analysis based on AFLP markers displayed a clearer separation of the accessions in comparison to morphological data. Different populations were divided into four major clusters reflecting the geographical origin and seasonal employment of the landraces analyzed. Molecular analysis of variance showed significant (P < 0.01) differences among groups, among populations within groups, and among individuals within populations. Approximately 74% of the total variance could be attributed to differences within populations. Conversely, a lower level of differentiation was detected among groups (approximately 4%). Regarding population structures, the genetic distance between populations (FST = 0.25 +/- 0.3) and the degree of inbreeding within groups (FSC = 0.22 +/- 0.2), did not diverge significantly, while both significantly differed from the degree of relatedness between markers within groups (FCT = 0.04 +/- 0.03). Results are discussed in relation to a suitable conservation method.

Microsatellite and AFLP markers in the Prunus persica [L. (Batsch)]xP. ferganensis BC(1)linkage map: saturation and coverage improvement

A set of 146 single sequence repeats (SSRs) and 14 amplified fragment length polymorphism (AFLP) primer combinations were used to enrich a previously developed linkage map obtained from a (Prunus persicaxP. ferganensis)xP. persica BC(1) progeny. Forty-one SSR primer pairs gave polymorphic patterns detecting 42 loci. The restriction/selective primer AFLP combinations produced a total of 79 segregating fragments. The resulting map is composed of 216 loci covering 665 cM with an average distance of 3.1 cM. Novel regions were covered by the newly mapped loci for a total of 159 cM. Eight linkage groups were assembled instead of the earlier 10 as two small groups (G1a and G8b), previously independent, were joined to their respective major groups (G1b and G8a). Several gaps were also reduced resulting in an improved saturation of the map. Twelve gaps >or=10 cm are still present. A comparative analysis against the Prunus reference map (71 anchor loci) pointed out an almost complete synteny and colinearity. Six loci were not syntenic and only two were not colinear. Genetic distances were significantly longer in our map than in the reference one.

The maize WD-repeat gene ZmRbAp1 encodes a member of the MSI/RbAp sub-family and is differentially expressed during endosperm development

Members of the MSI/RbAp sub-family of WD-repeat proteins are widespread in eukaryotic organisms and form part of multiprotein complexes that are involved in various biological pathways, including chromatin assembly, regulation of gene transcription, and cell division. In this study we report the isolation and characterization of a cDNA sequence from Zea mays, which encodes an RbAp-like protein (ZmRbAp1) that binds acetylated histones H3 and H4 and suppresses mutations that have a negative effect on the Ras/cAMP pathway in yeast. The ZmRbAp genes form a gene family and are expressed in different tissues of Z. mays L. plants. Determination of its expression pattern during maize seed development revealed that ZmRbAp transcripts are abundant during the initial stages of endosperm formation. In addition, the transcripts are specifically localized in shoot apical meristem and leaf primordia of the embryo. A possible role for the ZmRbAp genes in early endosperm differentiation and plant development is discussed.

AFLP markers in a molecular linkage map of maize: codominant scoring and linkage group ditsribution

We exploited the AFLP technique to saturate a RFLP linkage map derived from a maize mapping population. By using two restriction enzyme, EcoRI and PstI, differing in methylation sensitivity, both in combination with MseI, we detected 1568 bands of which 340 where polymorphic. These were added to the exitsing RFLP marker data to study the effects of incorporation of AFLPs produced by different restriction-enzyme combinations upon genetic maps. Addition of the AFLP data resulted in greater genome coverage, both through linking previously separate groups and the extension of other groups. The increase of the total map length was mainly caused by the addition of markers to telomeric regions, where RFLP markers were poorly represented. The percentage of informative loci was significantly different between the EcoRI and PstI assays. There was also evidence that PstI AFLP markers were more randomly distributed across chromosomes and chromosome regions, while EcoRI AFLP markers clustered mainly at centomeric regions. The more-random ditsribution of PstI AFLP markers on the genetic map reported here may reflect a preferential localisation of the markers in the hypomethylated telomeric regions of the chromosomes.

Identification and characterisation of an RPD3 homologue from maize (Zea mays L.) that is able to complement an rpd3 null mutant of Saccharomyces cerevisiae

In mammals, yeast and Drosophila, the histone deacetylase RPD3 proteins can alter the expression of genes involved in fundamental biological processes by affecting the degree of acetylation of histones and changing chromatin structure. Here we report the isolation of a cDNA sequence encoding an RPD3 homologue from maize, which is able to complement the phenotype of an rpd3 null mutant of the yeast Saccharomyces cerevisiae. The expression of the corresponding gene(s) was assessed in different maize tissues. The number of homologous loci was estimated by Southern hybridisation to be in the range of two to three, and the chromosomal location of one of these loci was determined. Phylogenetic analysis and tests for relative divergence rates, using related RPD3 sequences from different species, were performed, and suggest that different polymorphic forms of RPD3-like proteins that evolve at distinct rates are present in the species considered.

Analysis of the methylation pattern of the maize opaque-2 (O2) promoter and in vitro binding studies indicate that the O2 B-Zip protein and other endosperm factors can bind to methylated target sequences

The maize opaque-2 locus (o2) has an endosperm-specific expression and is positively autoregulated by its gene product, a b-Zip protein, to a TGACGTTG motif. The genomic sequencing method was used here to describe, in leaf and endosperm, the methylation pattern of a 390-base pair region of the o2 promoter. In leaf, 96% of the C residues are methylated, whereas in endosperm the 5-methylcytosine content is 84%. Comparison of these methylation patterns indicates that the o2 tissue-specific expression does not result from the demethylation of any specific C residue and that, in vivo, O2 interacts with a methylated target sequence. Consistently, gel-shift experiments using a CpG-methylated, partially methylated, and hemimethylated o2 promoter fragments showed that, in vitro, the O2 protein binds to the major groove of a methylated target sequence, although its binding activity decreases at increasing levels of C-methylation and is more effectively reduced by methylation of the lower strand than of the upper strand of the DNA. Using partially purified endosperm cell extracts, we also show that, besides the O2 protein, other proteins specifically bind to a partially methylated o2 promoter fragment, therefore indicating that in plants a subset of different proteins may mediate the expression of a naturally occurring methylated o2 promoter.

Distribution of sequences related to the Bg transposable element of maize in Zea and related genera

Thirty-four accessions from Zea and 10 accessions from related genera were assayed for the presence of Bg, a transposable element originally found in maize (Zea mays ssp. mays). Bg-like sequences, identified as hybridizing bands on Southern blots, were visualized in all Zea accessions and were present in approximately equal numbers in teosinte and maize. With the exception of Tripsacum dactyloides, all accessions from related genera failed to hybridize with the Bg probes, even at reduced stringency. A comparison of the restriction patterns of related inbred lines revealed numerous common hybridizing fragments. An index of molecular similarity (MS) was used to determine the degree of similarity between pairs of inbred lines. Computed MS values endorse an inbred relationship and are in good agreement with published results of cluster analysis on these inbred lines.

The transcriptional activator Opaque-2 controls the expression of a cytosolic form of pyruvate orthophosphate dikinase-1 in maize endosperms

The maize Opaque-2 (O2) protein is a transcription factor of the basic/leucine-zipper class, involved in the regulation of endosperm proteins including the 22kDa alpha-zein storage proteins and b32 protein. In this study we have focussed our attention on the relationship between O2 and the cyPPDK1 gene, which encodes a cytoplasmic pyruvate orthophosphate dikinase (PPDK) isoform. The results of this study showed that PPDK activity is detectable in wild-type maize endosperms, while in o2 mutant endosperms, the levels of PPDK protein, mRNA and enzymatic activity are reduced, indicating that O2 is involved in the regulation of cyPPDK1 in this tissue. By employing transient expression experiments in tobacco mesophyll protoplasts, we have demonstrated that the O2 protein can activate expression of a chloramphenicol acetyl transferase reporter gene placed under the control of the cyPPDK1 promoter. An in vitro binding assay and DNaseI footprint analysis demonstrated that a specific sequence in the cyPPDK1 promoter can be recognized and protected by maize O2 protein. The regulation by the O2 locus of cyPPDK1 reported here, and control of alpha-zein synthesis by O2 suggest that the O2 protein may play a more general role in maize endosperm development than previously thought.

Transposon tagging of the maize Glossy2 locus with the transposable element En/Spm

The Glossy2 (Gl2) locus of maize is required for the formation of the epicuticular wax layer of young plants. gl2 mutant seedlings can be visually identified because of their glossy leaf surface which is different from the dull surface of wild-type seedlings. The Gl2 locus was isolated by transposon tagging. Seven unstable mutations, gl2-m2 to gl2-m8, were induced in a parental strain carrying an active transposable Activator (Ac) element in the unstable wx-m7 allele. Genetic tests on the gl2-m2 allele indicated that it was not caused by the Ac element but by the insertion of the transposable element Enhancer/Suppressor-Mutator (En/Spm). A Sa/l restriction fragment segregating with the mutant phenotype was identified, by Southern analysis, using sequences from the En/Spm element as a probe. Part of the fragment was cloned and was shown to carry part of the unstable gl2-m2 allele. These gl2 sequences were used to identify a genomic fragment carrying the wild-type allele and to isolate its corresponding cDNA sequence. The predicted Glossy2 protein consists of 426 amino acids. No similar amino acid sequence was found in protein data banks and the biochemical function of the Gl2 gene product is still unknown. The wild-type Gl2 transcript is found predominantly in juvenile leaves. The transcript level in the leaves of seedlings homozygous for a stable recessive gl2-ref allele is hardly detectable.

Insertion mutations at the maize Opaque2 locus induced by transposable element families Ac, En/Spm and Bg

Eight independently isolated unstable alleles of the Opaque2 (O2) locus were analysed genetically and at the DNA level. The whole series of mutations was isolated from a maize strain carrying a wild-type O2 allele and the transposable element Activator (Ac) at the wx-m7 allele. Previous work with another unstable allele of the same series has shown that it was indeed caused by the insertion of an Ac element. Unexpectedly, the remaining eight mutations were not caused by the designated Ac element, but by other insertions that are structurally similar or identical to one of two different autonomous transposable elements. Six mutations were caused by the insertion of a transposable element of the Enhancer/Suppressor-Mutator (En/Spm) family. Two mutations were the result of the insertion of a transposable element of the Bergamo (Bg) family. Genetic tests carried out with plants carrying the unstable mutations demonstrated that all were caused by the insertion of an autonomous transposable element.

In an elite cross of maize a major quantitative trait locus controls one-fourth of the genetic variation for grain yield

Quantitative trait loci (QTLs) for grain yield, dry matter content and test weight were identified in an F2 segregating population derived from a single cross between two elite maize lines (B73 and A7) and testcrossed to two genetically divergent in breds. Most of the QTLs inferred were consistent across locations, indicating that the expression of the genes influencing the traits under investigation was largely independent of the environment. By using two different tester lines we found that QTLs exhibited by one tester may not necessarily be detected with the second one. Only loci with larger effects were consistent across testers, suggesting that interaction with tester alleles may contribute to the identification of QTLs in a specific fashion. Analysis across both testers revealed four significant QTLs for grain yield that explained more than 35% of the phenotypic variation and showed an overall phenotypic effect of more than 2t/ha. The major QTL for grain yield, located in the proximity of the Nucleolus Organiser Region, accounted for 24.5% of the phenotypic variation for grain yield and showed an average effect of allele substitution of approximately 1 t/ha. Marker-assisted introgression of the superior A7 allele at this locus in the B73 genetic background will not differ from qualitative trait introgression and will eventually lead to new lines having superior testcross performance.

Molecular analysis of opaque-2 alleles from Zea mays L. reveals the nature of mutational events and the presence of a hypervariable region in the 5' part of the gene

Ten recessive Opaque-2 (O2) alleles of independent origin were characterized at the molecular level. The results revealed a high level of polymorphism at the O2 locus. In addition, our data suggest the possible cause for the recessive character of some of the alleles investigated, and allow us to infer some conclusions concerning the degree of relationship between the o2 mutations. Comparison of genomic sequences spanning the first exon and obtained from a series of wild-type and recessive alleles revealed the presence of a hypervariable region, involving different dipeptides, in the N-terminal part of the O2 protein.

Early event in maize leaf epidermis formation as revealed by cell lineage studies

The epidermal cells of the juvenile leaves of maize are covered by a wax layer. glossy mutants are known which reduce drastically wax deposition. We have used the somatically unstable glossy-1 mutable 8 allele to study the distribution on the epidermis of spontaneous revertant sectors of wild- type tissues. Sectors tend to start and end at positions that correlate with the location on the epidermis of the long costal cells of ribs. It is concluded that in the protoderm only a few cells have a role and position in the generation of each of the developmental modules located between leaf midrib and margin. The module consists of an epidermal strip of cells bordered by two lateral ribs. The module originates from at least 4 cells, with one cel l being the progenitor of the other three. Data are provided describing the mode of longitudinal anticlinal epidermal cell divisions within the module that are responsible for the increase in leaf width. The results suggest the existence of a clonal type of development during early leaf epidermis formation.

An unstable allele at the maize Opaque2 locus is caused by the insertion of a double Ac element

An unstable allele designated o2-m55, was isolated as a derivative of the o2-m5 allele. Whereas the o2-m5 allele is caused by the insertion of one copy of the transposable element Activator (Ac) into the first exon of the maize Opaque2(O2) gene, the o2-m55 allele contains two Ac elements. The position of one copy is identical to the position of the Ac element in the o2-m5 allele. The second copy is present within the first copy, thereby interrupting its structure shortly before the first ATG of the major reading frame of Ac. Both Ac sequences have the same orientation. Excision of the internal Ac element as well as excision of the complete double Ac element was detectable. Truncated double Ac elements comprising the complete internal Ac element and either the proximal or distal fragment of the interrupted Ac element can also be excised. The Ac elements exhibit a strong negative dosage effect: kernels which display revertant sectors in a mutant background are rarely seen in plants homozygous for the o2-m55 allele. If only one dose of the o2-m55 allele is present in endosperm tissue, revertant sectors can be detected. The amount of the transcript expressed from the internal Ac element of the o2-m55 allele is less than that derived from the single Ac element present in the o2-m5 allele.

Structural and functional analysis of an Opaque-2-related gene from sorghum

The Opaque-2 (O2) gene from maize encodes a transcriptional activator of the b-ZIP class. We have isolated and characterized a gene from sorghum, related in sequence to the O2 gene from maize. A single copy of the gene is present in sorghum. Both genomic and cDNA sequences of the O2-related sorghum gene were determined. The sequence is highly homologous to maize O2 both in the promoter and in the coding region. The most closely related sequences contain the b-ZIP domain with only 11 amino acid substitutions in a total of 122 residues. In transient expression assays, the sorghum O2-related coding sequence, expressed from a CaMV 35S promoter, activates expression from the maize b-32 promoter as effectively as that obtained with the maize O2 sequence.

Functional expression of the transcriptional activator Opaque-2 of Zea mays in transformed yeast

The aim of this research was to determine whether the structural homology between the O2 gene, a maize transcriptional activator, and the GCN4 gene, a yeast transcriptional factor, is reflected at the level of function. The O2 cDNA was cloned in the yeast expression vector pEMBLyex4 under the control of a hybrid inducible promoter, and used to transform the yeast Saccharomyces cerevisiae. Transformed yeast cells produced O2 mRNA and a polypeptide immunoreactive with anti-O2 antibodies during growth in galactose. The heterologous protein was correctly translocated into the yeast nuclei, as demonstrated by immunofluorescence, indicating that the nuclear targeting sequences of maize are recognized by yeast cells. Further experiments demonstrated the ability of O2 to rescue a gcn4 mutant grown in the presence of aminotriazole, an inhibitor of the HIS3 gene product, suggesting that O2 activates the HIS3 gene, gene normally under control of GCN4. It was shown that the O2 protein is able to trans-activate the HIS4 promoter in yeast cells and binds to it in vitro. The sequence protected by O2, TGACTC, is also the binding site for GCN4. Finally, the expression of O2 protein in yeast did not produce alterations during batch growth at 30 degrees C, while transformants expressing O2 protein showed a conditionally lethal phenotype when grown in galactose at 36 degrees C; this phenotype mimics the behaviour of gcd mutants. The results support the idea that basic mechanisms of transcription control have been highly conserved in eukaryotes.

Translation of the mRNA of the maize transcriptional activator Opaque-2 is inhibited by upstream open reading frames present in the leader sequence

The protein encoded by the Opaque-2 (O2) gene is a transcription factor, translated from an mRNA that possesses an unusually long 5' leader sequence containing three upstream open reading frames (uORFs). The efficiency of translation of O2 mRNA has been tested in vivo by a transient assay in which the level of activation of the b32 promoter, a natural target of O2 protein, is measured. We show that uORF-less O2 alleles possess a higher transactivation value than the wild-type allele and that the reduction in transactivation due to the uORFs is a cis-dominant effect. The data presented indicate that both uORF1 and uORF2 are involved in the reducing effect and suggest that both are likely to be translated.

Genetic diversity of maize inbred lines within and among heterotic groups revealed by RFLPs

The objectives of this study were (1) to investigate genetic diversity for RFLPs in a set of important maize inbreds commonly used in Italian breeding programs, (2) to compare genetic similarities between unrelated lines from the same and different heterotic groups, and (3) to examine the potential of RFLPs for assigning maize inbreds to heterotic groups. Forty inbreds were analyzed for RFLPs with two restriction enzymes (EcoRI and HindIII) and 82 DNA clones uniformly distributed over the maize genome. Seventy clone-enzyme combinations gave single-banded RFLP patterns, and 79 gave multiple-banded RFLP patterns. The average number of RFLP patterns detected per clone-enzyme combination across all inbreds was 5.8. RFLP data revealed a wide range of genetic diversity within the two heterotic groups assayed, Iowa Stiff Stalk Synthetic (BSSS) and Lancaster Sure Crop (LSC). Genetic similarity (GS) between lines was estimated from binary RFLP data according to the method of Nei and Li (1979). The mean GS for line combinations of type BSSS × LSC (0.498) was substantially smaller than for unrelated line combinations or type BSSS × BSSS (0.584) but almost as great as for un-related line combinations of type LSC × LSC (0.506). Principal coordinate and cluster analyses based on GS values resulted in the separate groupings of lines, which is consistent with known pedigree information. A comparison between both methods for multivariate analyses of RFLP data is presented.

Three high-lysine mutations control the level of ATP-binding HSP70-like proteins in the maize endosperm

The synthesis and deposition of seed storage proteins in maize are affected by several dominant and recessive mutants. The effect of three independent mutations, floury-2 (fl2), Defective endosperm-B30 (De-B30), and Mucronate (Mc), that reduce zein level in the endosperm were investigated. These mutations also control the level of b-70, a polypeptide bound to protein bodies, which is separable into the two isoforms b-70I and b-70II by two-dimensional gel electrophoresis. Both isoforms are overexpressed 10-fold in fl2; however, only b-70I is present in De-B30 and Mc, which contain an amount of total b-70 isoforms fivefold higher than in the wild type. Both b-70I and b-70II resemble heat shock protein (HSP70) in that they bind ATP, cross-react with anti-HSP antibodies, and have N-terminal sequence homology to HSP70. All maize protein body-located b-70 characteristics are typical of those of chaperone-like HSPs. A third protein, b-70III, similar in size to but slightly more acidic than b-70I and b-70II, also binds ATP and reacts with the same antibody, providing evidence for the presence in endosperm extracts of a cytosolic chaperone-like protein. The level of b-70III was not altered by the mutations studied. The results suggested that the repression effect of the three mutations on zein accumulation may be mediated by the alteration of a zein transport or zein assembly process involving b-70I and b-70II.

Molecular analysis of the Bg-rbg transposable element system of Zea mays L

The two components of the Bg-rbg transposable element system of maize have been cloned. The Bg element, isolated from the mutable allele wx-m32:: Bg is inserted in the intron of the Waxy (Wx) gene between exons 12 and 13. The length of the element is of 4869 bp. Bg has 5 bp terminal inverted repeats, and generates upon insertion an 8 bp direct duplication of the target sequence. Both ends of the Bg element contain a 76 bp direct repeat adjacent to the terminal inverted repeats. The hexamer motif TATCGGC is here repeated several times in direct or inverse orientation. The rbg element was isolated from the mutable allele o2m(r) where it is located in the promoter region of the Opaque-2 (O2) gene. rbg is approximately 4.5 kb in length, has terminal inverted repeats identical to those of the Bg element, and is also flanked by an 8 bp direct duplication at the target site. Like Bg, rbg carries the 76 bp direct repeats. Restriction enzyme analysis reveals that, compared to Bg, the receptor element is distinguishable by small deletion and insertion events. Sequence data indicate that not more than 75% homology exists at the DNA level between the rbg element and the autonomous Bg element.

The maize regulatory locus Opaque-2 encodes a DNA-binding protein which activates the transcription of the b-32 gene

The maize locus, Opaque-2, controls the expression in developing endosperm of structural genes encoding a family of storage proteins, the 22 kd zeins, and an abundant albumin, termed b-32. It is shown that the promoter of the b-32 gene is activated in vivo in the presence of the O2 gene product and that the information necessary for this activation resides in a 440 bp DNA fragment containing five O2 binding sites (GATGAPyPuTGPu). Two of these sites are embedded in copies of the 'endosperm box', a motif thought to be involved in endosperm-specific expression, which is also represented in 22 kd zein promoters. The O2 protein is also shown to be capable of binding in vitro and activating in vivo, its own promoter.

The maize regulatory locus Opaque-2 encodes a DNA-binding protein which activates the transcription of the b-32 gene

The maize locus, Opaque-2, controls the expression in developing endosperm of structural genes encoding a family of storage proteins, the 22 kd zeins, and an abundant albumin, termed b-32. It is shown that the promoter of the b-32 gene is activated in vivo in the presence of the O2 gene product and that the information necessary for this activation resides in a 440 bp DNA fragment containing five O2 binding sites (GATGAPyPuTGPu). Two of these sites are embedded in copies of the 'endosperm box', a motif thought to be involved in endosperm-specific expression, which is also represented in 22 kd zein promoters. The O2 protein is also shown to be capable of binding in vitro and activating in vivo, its own promoter.

Relationship between gene expression and hybrid vigor in primary root tips of young maize (Zea mays L.) plantlets

To provide an insight into the molecular basis of heterosis, we investigated gene expression in primary root tips of a heterotic maize hybrid (B73 × Mo17) and its parental lines (B73 and Mo17). This analysis was carried out (i) by differential plaque hybridization of a recombinant cDNA library made to poly(A) RNA isolated from B73 × Mo17 primary root tips, and (ii) by comparing with two-dimensional gel electrophoresis proteins synthesized in vitro in the rabbit reticulocyte system by poly(A) RNA isolated, at different stages of development, from the three genotypes. The results showed that there are sets of proteins and mRNAs that are differentially synthesized and expressed in the F1 primary root tips in comparison to the parental lines. Moreover, results from the survey of 21 major in-vitrosynthesized polypeptide variants, from mRNAs of primary root tips of the parental lines and their F1 hybrid, indicated that in seven instances hybrid proteins translated in vitro were more abundant or possibly new. In most of the remaining cases, hybrid spots were similar in intensity to the same protein produced by one of the two parental lines.

The Mu1 transposable element of maize contains two promoter signals recognized by the Escherichia coli RNA polymerase

The galactokinase (GalK) expression plasmid vector system pKO-1 has been used to screen for promoter elements in the maize transposable element Mu1 that function in Escherichia coli. Two transcriptional start points, named S1 and S2, were identified, which are located in the two direct repeats of the transposable element. This paper demonstrates that sequence elements exist in a plant transposable element which function as prokaryotic promotors.

The O2 gene which regulates zein deposition in maize endosperm encodes a protein with structural homologies to transcriptional activators

The structure of the zein regulatory gene Opaque 2 of Zea mays has been determined by sequence analysis of genomic and cDNA clones. The size of O2 mRNA is 1751 bp [poly(A) tail not included] containing a major open reading frame (ORF) of 1380 bp preceded by three short ORFs of 3, 21 and 20 amino acid residues. The main ORF comprises 1362 bp and is composed of six exons ranging in size from 465 to 61 bp and five introns of 678 bp to 83 bp. A putative protein 454 amino acids long was derived by the theoretical translation of the genomic sequences corresponding to exons. The opaque 2 protein contains a domain similar to the leucine zipper motif identified in DNA binding proteins of animal protooncogenes such as fos, jun and myc, and in the transcriptional activators GCN4 and C/EBP. The region of 30 amino acid residues next to the leucine repeats towards the N terminus is rich in basic amino acids and is also homologous to a domain present in fos, jun and GCN4. Moreover, in the carboxy terminal region an amino acid motif closely resembling a metal binding domain is present.

The b-32 protein from maize endosperm, an albumin regulated by the O2 locus: nucleic acid (cDNA) and amino acid sequences

The cDNA coding for the b-32 protein, an albumin expressed in maize endosperm cells under the control of the O2 and O6 loci, has been cloned and the complete amino acid sequence of the protein derived. A lambda gt11 cDNA library from mRNA of immature maize endosperm was screened for the expression of the b-32 protein using antibodies against the purified protein. One of the positive clones obtained was used to isolate a full-length cDNA clone. By Northern analysis, the size of the b-32 mRNA was estimated to be 1.2 kb. Hybrid-selected translation assays show that the message codes for a protein with an apparent molecular weight of 30-35 kDa. The nucleotide sequence shows that several internal repeats are present. The protein has a length of 303 amino acid residues (mol. wt. 32430 dalton) and its sequence shows the following features: no signal peptide is observable; it contains seven tryptophan residues, an amino acid absent in maize storage proteins; polar and hydrophobic residues are spread along the sequence; several pairs of basic residues are present in the N-terminal region; the secondary structure allows the prediction of two structural domains for the b-32 protein that would fold up giving rise to a globular shape. The cloning of this gene may help in understanding the role of the O2 and O6 loci in regulating the deposition of zein, the major storage protein of maize endosperm.

Genotype-dependent leaf senescence in maize : inheritance and effects of pollination-prevention

Objective of research was (a) to evaluate the influence of pollination-prevention on various metabolic parameters of the two maize inbreds B73 and B14A and their F1, and (b) to gain information on the inheritance of leaf senescence, in response to pollination-prevention. The results show that the visual pattern of leaf senescence, in response to prevention of ear pollination, contrasts markedly between the two inbred lines. Relative to control plants, prevention of ear pollination, causes a premature senescence in B73 and B73 x B14A plants, while leaves of unpollinated B14A remain green and similar in appearance to pollinated controls. Furthermore, prevention of ear pollination induces a sizable reduction of dry matter accumulation of all above-ground material and changes in various metabolic parameters. An accumulation of sucrose in the leaves of unpollinated B73 and B73 x B14A plants is correlated with the development of premature senescence. Finally, the genetic analysis supports suggestions that a single dominant gene is responsible for the differences observed, in the visual pattern of leaf senescence, in response to prevention of ear pollination.

Ds-Induced Alleles at the OPAQUE-2 Locus of Maize

Transposon mutagenesis has been used to isolate mutable alleles at the Opaque-2 (O2) locus of maize. Plants with the Activator-Dissociation (Ac-Ds) system of transposable elements and O2 were crossed as males to a stable o2 tester line. Among a population of 200,000 kernels, 198 exceptional kernels with somatic instability were recovered. In four cases, designated O2-m1, o2-m2, O2-m3 and O2-m4, variegated phenotypes appeared in F(2) and subsequent generations. Genetic analyses indicated that the presence of Ds near or within the O2 gene was responsible for the observed somatic instability at the O2 locus. The phenotypes of the newly induced alleles were of two types. Alleles O2-m1, O2-m3 and O2-m4, in the absence of Ac, were characterized by kernel phenotypes indistinguishable from the wild type; in the presence of Ac they generated kernels with opaque sectors interspersed within a vitreous background. In contrast, the mutable allele o2-m2, in the absence of Ac, was characterized by kernels with a recessive phenotype similar to o2 recessive mutants. In the presence of Ac, it reverted somatically to wild-type-producing kernels with vitreous spots in an o2 background. The association of the Ds element with the O2 locus may prove a valuable tool directed to the isolation of DNA fragments bearing the O2 gene.

Combining ability estimates of sulfate uptake efficiency in maize

Plant root nutrient uptake efficiency may be expressed by the kinetic parameters, Vmax and Km, as well as by normal enzymatic reactions. These parameters are apparently useful indices of the level of adaptation of genotypes to the nutrient conditions in the soil. Moreover, sulfate uptake capacity has been considered a valuable index for selecting superior hybrid characterized by both high grain yield and efficiency in nutrient uptake. Therefore, the purpose of this research was to determine combining ability for sulfate uptake, in a diallel series of maize hybrids among five inbreds. Wide differences among the 20 single crosses were obtained for Vmax and Km. The general and specific combining ability mean squares were significant and important for each trait, indicating the presence of considerable amount of both additive and nonadditive gene effects in the control of sulfate uptake. In addition, maternal and nonmaternal components of F1 reciprocal variation showed sizeable effects on all the traits considered. A relatively high correlation was also detected between Vmax and Km. However, both traits displayed enough variation to suggest that simultaneous improvement of both Vmax and Km should be feasible. A further noteworthy finding in this study was the identification of one inbred line, which was the best overall parent for improving both affinity and velocity strategies of sulfate uptake.