Spatial regulation in the expression of structural and regulatory storage-protein genes inZea mays endosperm

The regulation of zein biosynthesis in maize endosperm

We review the current knowledge regarding the regulation of zein storage proteins biosynthesis and protein body formation, which are crucial processes for the successful accumulation of nutrients in maize kernels. Storage proteins in the seeds of crops in the grass family (Poaceae) are a major source of dietary protein for humans. In maize (Zea mays), proteins are the second largest nutrient component in the kernels, accounting for ~ 10% of the kernel weight. Over half of the storage proteins in maize kernels are zeins, which lack two essential amino acids, lysine and tryptophan. This deficiency limits the use of maize proteins in the food and feed industries. Zeins are encoded by a large super-gene family. During endosperm development, zeins accumulate in protein bodies, which are derived from the rough endoplasmic reticulum. In recent years, our knowledge of the pathways of zein biosynthesis and their deposition within the endosperm has been greatly expanded. In this review, we summarize the current understanding of zeins, including the genes encoding these proteins, their expression patterns and transcriptional regulation, the process of protein body formation, and other biological processes affecting zein accumulation.

A PCR-based forward genetics screening, using expression domain-specific markers, identifies mutants in endosperm transfer cell development

Mutant collections are an invaluable source of material on which forward genetic approaches allow the identification of genes affecting a wide variety of biological processes. However, some particular developmental stages and morphological structures may resist analysis due to their physical inaccessibility or to deleterious effects associated to their modification. Furthermore, lethal mutations acting early in development may escape detection. We have approached the characterization of 101 maize seed mutants, selected from a collection of 27,500 visually screened Mu-insertion lines, using a molecular marker approach based on a set of genes previously ascribed to different tissue compartments within the early developing kernel. A streamlined combination of qRT-PCR assays has allowed us to preliminary pinpoint the affected compartment, establish developmental comparisons to WT siblings and select mutant lines with alterations in the different compartments. Furthermore, clusters of markers co-affected by the underlying mutation were identified. We have analyzed more extensively a set of lines presenting significant variation in transfer cell-associated expression markers, and have performed morphological observations, and immunolocalization experiments to confirm the results, validating this approach as an efficient mutant description tool.

Expressional profiling study revealed unique expressional patterns and dramatic expressional divergence of maize alpha-zein super gene family

The alpha-zein super gene family encodes the most predominant storage protein in maize (Zea mays) endosperm. In maize inbred line B73, it consists of four gene families with 41 member genes. In this study, we combined quantitative real-time PCR and random clone sequencing to successfully profile the expression of alpha-zein super gene family during endosperm development. We found that only 18 of the 41 member genes were expressed, and their expression levels diverge greatly. At the gene family level, all families had characteristic "up-and-down" oscillating expressional patterns that diverged into two major groups. At the individual gene level, member genes showed dramatic divergence of expression patterns, indicating fast differentiation of their expression regulation. A comparison study among different inbred lines revealed significantly different expressed gene sets, indicating the existence of highly diverged haplotypes. Large gene families containing long gene clusters, e.g. z1A or z1C, mainly contributed the highly divergent haplotypes. In addition, allelic genes also showed significant divergence in their expressional levels. These results indicated a highly dynamic and fast evolving nature to the maize alpha-zein super gene family, which might be a common feature for other large gene families.

Developmental analysis of maize endosperm proteome suggests a pivotal role for pyruvate orthophosphate dikinase

Although the morphological steps of maize (Zea mays) endosperm development are well described, very little is known concerning the coordinated accumulation of the numerous proteins involved. Here, we present a proteomic study of maize endosperm development. The accumulation pattern of 409 proteins at seven developmental stages was examined. Hierarchical clustering analysis allowed four main developmental profiles to be recognized. Comprehensive investigation of the functions associated with clusters resulted in a consistent picture of the developmental coordination of cellular processes. Early stages, devoted to cellularization, cell division, and cell wall deposition, corresponded to maximal expression of actin, tubulins, and cell organization proteins, of respiration metabolism (glycolysis and tricarboxylic acid cycle), and of protection against reactive oxygen species. An important protein turnover, which is likely associated with the switch from growth and differentiation to storage, was also suggested from the high amount of proteases. A relative increase of abundance of the glycolytic enzymes compared to tricarboxylic acid enzymes is consistent with the recent demonstration of anoxic conditions during starch accumulation in the endosperm. The specific late-stage accumulation of the pyruvate orthophosphate dikinase may suggest a critical role of this enzyme in the starch-protein balance through inorganic pyrophosphate-dependent restriction of ADP-glucose synthesis in addition to its usually reported influence on the alanine-aromatic amino acid synthesis balance.

Effect of the opaque and floury mutations on the accumulation of dry matter and protein fractions in maize endosperm

Grains of nine opaque (o) and floury (fl) mutants of maize (Oh43o1, Oh43o2, B79o5, B37o7, W22o10, W22o11, W22o13, Oh43fl1 and Oh43fl2) were examined for the weight proportions of their component tissues and the content of eight nitrogen fractions in their endosperms. A linear regression was found connecting the amounts (mg per endosperm) of zeins and true proteins (crude proteins minus non-protein nitrogen) for the non-opaque2 mutants. The data points connecting zeins to true proteins present in the mature endosperms of six wild-type (+) inbred lines and their o2 versions were located outside (+) or within (o2) the 95% confidence range of the regression line. The data obtained from the developing and mature endosperms of the W22o7 inbred line (Di Fonzo et al., Plant Sci. Lett., 1979, 77) and the floury portion of mature endosperms of three other wild-type inbred lines fell practically on the regression line. The effects of genotype and environmental factors upon the relative accumulation rate of zeins were assessed from the present results and the data taken from the literature concerning the quantitative interdependence between zeins and true proteins in immature and mature endosperms.

Vacuolar H+-translocating inorganic pyrophosphatase (Vpp1) marks partial aleurone cell fate in cereal endosperm development

Cereal endosperm is a model system for cell fate determination in plants. In wild-type plants the outermost endosperm cells adopt aleurone cell fate, while all underlying cells display starchy endosperm cell fate. Mutant analysis showed that cell fate is determined by position rather than lineage. To further characterise the precise cell fate of the outermost cells, we performed a differential screen and isolated the novel marker gene Vpp1 . It encodes a vacuolar H+-translocating inorganic pyrophosphatase (V-PPase) and is mainly expressed in kernels, leaves and tassels. In kernels, its expression is restricted to the aleurone layer with the maximum of expression shifting from the adaxial to the abaxial side during early stages. Together with three other marker genes Vpp1 was then used to analyse the cell fate of the outermost cells in Dap3 , Dap7 , cr4 and dek1 mutants, all of which have aberrant aleurone layers. In the Dap3 and Dap7 mutants the Vpp1 and Ltp2 markers but not the A1 and Zein markers were expressed in patches without aleurone indicating that the outermost cells had some but not all features of aleurone cells and did not simply adopt starchy endosperm cell fate. A similar result was obtained in the cr4 mutant, although Ltp2 expression was less generalised. In other Dap7 patches characterised by multiple aleurone-like cell layers the expression of Vpp1 and Ltp2 confirmed the aleurone cell fate of the cells in the additional cell layers. The analysis of dek1 mutants confirmed the starchy endosperm cell fate of the majority but not all outermost cells. Based on these data we propose a model suggesting a stepwise commitment to aleurone cell fate. Sequential steps are marked by the expression of Vpp1 , the expression of Ltp2 , the acquisition of a regular shape and thick walls and finally pigmentation coupled with A1 expression.

The nitrogen-induced recovery of alpha-zein gene expression in in vitro cultured opaque2 maize endosperms depends on the genetic background

The effect of nitrogen nutrition on the accumulation of seed storage proteins has been studied in vitro by cultivating on agar media maize (Zea mays L.) endosperm explants from seeds at 10 days after pollination. The experiments were performed on various genetic backgrounds bearing different opaque2 (o2) mutant alleles and on the corresponding wild-type lines. In the seed of the o2 genotypes the high molecular weight alpha-zein polypeptides (zHs), whose transcription is Opaque2 (O2) regulated, are absent or extremely reduced. The endosperms were incubated on basal agar medium with amino acid supply. In these growth conditions, fresh and dry weights increased in both wild-type and o2 endosperms, irrespective of the genetic background. In 4 out of the 5 o2 mutant genotypes analysed we detected an accumulation of the zHs similar to the corresponding wild-type explants or seeds. However, in one of these mutants, Mo17o2R, the addition of amino acids to the culture media had no effect on the zH accumulation. We showed that the Mo17o2R behaviour is not due to a negative regulation but to the absence of putative transcription factor(s) able to regulate the zH transcription occurring in the other o2 mutants.

Specific combinations of zein genes and genetic backgrounds influence the transcription of the heavy-chain zein genes in maize opaque-2 endosperms

The transcript levels of heavy-chain zein genes (zH1 and zH2) and the occurrence of the zH polypeptides in different opaque-2 (o2) lines were investigated by RNA-blot analyses and by sodium dodecylsulfate-polyacrylamide gel electrophoresis or two-dimensional gel electrophoresis protein fractionations. Four mutant alleles o2R, o2T, o2It, and o2-676 introgressed into different genetic backgrounds (GBs) were considered. The mono-dimensional gel electrophoresis zein pattern can be either conserved or different among the various GBs carrying the same o2 allele. Likewise, in the identical GB carrying different o2 alleles, the zein pattern can be either conserved or differentially affected by the different mutant allele. Zein protein analysis of reciprocal crosses between lines with different o2 alleles or the same o2 showed in some case a more than additive zH pattern in respect to the o2 parent lines. Electrophoretic mobility shift assay approaches, with O2-binding oligonucleotide and endosperm extracts from the above o2 lines, failed to reveal o2-specific retarded band in any of the o2 extracts. The results suggest that the promoter of some zH1 and zH2 contains motif(s) that can respond to factors other than O2.

Symbiotic root nodules of the actinorhizal plant Datisca glomerata express Rubisco activase mRNA

N2-fixing symbiotic root nodules of the actinorhizal host Datisca glomerata express Dgrca (D. glomerata Rubisco activase) mRNA, a transcript usually associated with photosynthetic organs or tissues. In northern blots a mature, 1700-nucleotide Dgrca mRNA was detected in green plant organs (leaves, flowers, and developing fruits) and in nodules but was not detected in roots. A second message of 3000 nucleotides was observed only in nodules. Both size classes of transcripts were polyadenylated. The larger transcript was 2- to 5-fold more abundant than the mature mRNA; it was hybridized to an intronic probe, indicating that a stable, incompletely spliced transcript was accumulating. Treatment with light on excised nodules did not alter the relative abundance of the two species. In in situ hybridizations the Dgrca message was expressed intensely in the nuclei of infected cells. The Dgrca transcripts also accumulated at lower levels in uninfected cortical cells adjacent to the periderm and the vascular cylinder. mRNA encoding the large subunit of Rubisco (DgrbcL) was abundant in mature infected cells and in the amyloplast-rich sheath of uninfected cortical cells lying between the infected cells and nodule periderm. The proteins Rubisco activase, Rubisco, and the 33-kD O2-evolving complex subunit did not accumulate to detectable levels, indicating that a functional photosynthetic apparatus was not prevalent in nodule tissue. Signals or factors required for the transcription of Dgrca appeared to be present in nodules, but efficient splicing and translation of the message were not observed in Frankia-infected tissue where transcript accumulation was highest.

Molecular analysis of wild-type and mutant alleles at the Opaque-2 regulatory locus of maize reveals different mutations and types of O2 products

The expression of the various members of the zein multigene family in maize endosperm is controlled by different regulatory loci. One of these loci, Opaque-2, coding for a bZIP transcriptional factor, controls the expression of a subset of zein genes. Analysis of genomic DNA from plants carrying wild-type (O2) or mutant o2 alleles shows specific DNA restriction patterns that correlate with transcript types and their various gene products. Northern and western analyses show the presence in different wild types of a 1.7 kb transcript coding for different sizes of normal O2 proteins that migrate as doublets in the 68-72 kDa range. Among the various o2 mutants analysed we showed the occurrence of various null-transcript alleles, the presence of alleles with a normal size transcript which, however, produce a different-sized o2 protein, and a mutant producing both a normal size transcript and a longer transcript, but generating only a single o2 product migrating around 40 kDa. Analysis of other mutations (o7, fl2) known to affect zein polypeptide synthesis shows no interference of these mutations in the expression of the O2 gene products. The overall results indicate the occurrence of micro heterogeneity in the O2 wild-type genes and a broad spectrum of o2 mutations, both producing different sizes of O2 or o2 proteins. A nomenclature of the O2 and o2 genes based on the RFLP, transcripts and products of the various alleles is presented.

Antiparallel expression of the sense and antisense transcripts of maize alpha-tubulin genes

In all eukaryotes alpha- and beta-tubulins are encoded by small families of closely related genes and are highly conserved. In Zea mays, at least six different alpha-tubulin coding sequences are known. We describe the isolation from scutellar nodes of the maize inbred line W22 of a clone (CTM5) coding for an alpha-tubulin. On the basis of the 3' end nucleotide sequence, this clone can be assigned to the already reported tua4 gene. Northern analysis demonstrates that CTM5 encodes a 1.5 kb transcript, which is expressed in different tissues of the seed and of the seedling. In order to define the spatial and temporal expression of alpha-tubulin genes, in situ hybridization experiments were performed on these tissues. Unexpectedly, a specific signal was detected with both antisense and sense RNA strands. Temporal and spatial distribution of the two RNAs, however, shows that high levels of the two transcripts are always discordant. In tissues where sense transcripts are highly abundant (embryos at various developmental stages, root tips, pollen grains), the antisense transcripts are expressed in relatively small amounts, while in pericarp, coleoptile, leaves, and scutellar node, where antisense transcripts accumulate, the sense transcript only reaches a very low level. Northern analysis using single-stranded DNA probes confirmed the presence of an antisense transcript of 1.5 kb, prompting speculation about the role of this transcript in the regulation of the expression of alpha-tubulin genes.