Molecular dissection of heterosis manifestation during early maize root development.

Heterosis is of paramount agronomic importance and has been successfully exploited in maize hybrid breeding for decades. Nevertheless, the molecular basis of heterosis remains elusive. Heterosis is not only observed in adult traits like yield or plant height, but is already detected during embryo and seedling development. Hence, the maize (Zea mays L.) primary root which is the first organ that emerges after germination is a suitable model to study heterosis manifestation. Various seedling root traits including primary root length and lateral root density display heterosis. Microarray studies suggest organ specific patterns of nonadditive gene expression in maize hybrids. Moreover, such experiments support the notion that global expression trends in maize primary roots are conserved between different hybrids. Furthermore, nonadditive expression patterns of specific genes such as a SUPEROXIDE DISMUTASE 2 might contribute to the early manifestation of heterosis. Proteome profiling experiments of maize hybrid primary roots revealed nonadditive accumulation patterns that were distinct from the corresponding RNA profiles underscoring the importance of posttranscriptional processes such as protein modifications that might be related to heterosis. Finally, analysis of selected metabolites imply that a subtle regulation of particular biochemical pathways such as the phenylpropanoid pathway in hybrids might contribute to the manifestation of heterosis in maize primary roots. In the future, recently developed molecular tools will facilitate the analysis of the molecular principles underlying heterosis in maize roots.

Analysis of nonadditive protein accumulation in young primary roots of a maize (Zea mays L.) F(1)-hybrid compared to its parental inbred lines.

Heterosis describes the superior performance of heterozygous F(1)-hybrids compared to their homozygous parental inbred lines. Heterosis is already manifested during early maize (Zea mays L.) primary root development. In this study, the most abundant soluble proteins have been investigated before the phenotypic manifestation of heterosis in 3.5-day-old primary roots in the flint inbred line UH002, the dent inbred line UH301 and the corresponding hybrid UH301 x UH002. In CBB-stained 2-DE gels, 150 of 304 detected proteins (49%) were accumulated in a nonadditive fashion in the hybrid compared to the average of their parental inbred lines (Student's t-test: p < 0.05). Remarkably, expression of 51% (76/150) of the nonadditively accumulated proteins exceeded the high parent or was below the low parent. ESI-MS/MS identified 75 of the 76 proteins that belonged to these expression classes. The most abundant functional classes among the 75 proteins that were encoded by 60 different genes were metabolism (58%) and disease and defense (19%). Nonadditive protein accumulation in primary roots of maize hybrids might be associated with heterosis manifestation. Identification of these proteins could therefore contribute to the better understanding of the molecular basis of heterosis.

Comparison of maize (Zea mays L.) F1-hybrid and parental inbred line primary root transcriptomes suggests organ-specific patterns of nonadditive gene expression and conserved expression trends.

The phenomenon of heterosis describes the increased agronomic performance of heterozygous F(1) plants compared to their homozygous parental inbred plants. Heterosis is manifested during the early stages of root development in maize. The goal of this study was to identify nonadditive gene expression in primary roots of maize hybrids compared to the average expression levels of their parental inbred lines. To achieve this goal a two-step strategy was used. First, a microarray preselection of nonadditively expressed candidate genes was performed. Subsequently, gene expression levels in a subset of genes were determined via high-throughput quantitative real-time (qRT)-PCR experiments. Initial microarray experiments identified 1941 distinct microarray features that displayed nonadditive gene expression in at least 1 of the 12 analyzed hybrids compared to the midparent value of their parental inbred lines. Most nonadditively expressed genes were expressed between the parental values (>89%). Comparison of these 1941 genes with nonadditively expressed genes identified in maize shoot apical meristems via the same experimental procedure in the same genotypes revealed significantly less overlap than expected by pure chance. This finding suggests organ-specific patterns of nonadditively expressed genes. qRT-PCR analyses of 64 of the 1941 genes in four different hybrids revealed conserved patterns of nonadditively expressed genes in different hybrids. Subsequently, 22 of the 64 genes that displayed nonadditive expression in all four hybrids were analyzed in 12 hybrids that were generated from four inbred lines. Among those genes a superoxide dismutase 2 was expressed significantly above the midparent value in all 12 hybrids and might thus play a protective role in heterosis-related antioxidative defense in the primary root of maize hybrids. The findings of this study are consistent with the hypothesis that both global expression trends and the consistent differential expression of specific genes contribute to the organ-specific manifestation of heterosis.

Towards the molecular basis of heterosis.

Heterosis describes the superior performance of heterozygous hybrid plants over their homozygous parental inbred lines. Despite the rediscovery of this phenomenon a century ago and its paramount agronomic importance, the genetic and molecular basis of heterosis remains enigmatic. Recently, various pioneer studies described differences in genome organization and gene expression of hybrids and their parental inbred lines. At the genomic level, a significant loss of colinearity at many loci between different inbred lines of maize was observed. At the level of gene expression, complex transcriptional networks specific for different developmental stages and tissues were monitored in maize (Zea mays), rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana). Integration of this complex expression data might contribute to improve our understanding of the molecular basis of heterosis.

Proteomic dissection of plant development.

Plant development is controlled by complex endogenous genetic programs and responses to environmental cues. Proteome analyses have recently been introduced to plant biology to identify proteins instrumental in these developmental processes. To date most plant proteome studies have been employed to generate reference maps of the most abundant soluble proteins of plant organs at a defined developmental stage. However, proteomics is now also utilized for genetic studies comparing the proteomes of different plant genotypes, for physiological studies analyzing the influences of exogenous signals on a particular plant organ, and developmental studies investigating proteome changes during development. Technical advances are now beginning to allow a proteomic dissection of individual cell types, thus greatly increasing the information revealed by proteome analyses.

Manifestation of heterosis during early maize (Zea mays L.) root development.

Heterosis is typically detected in adult hybrid plants as increased yield or vigor compared to their parental inbred lines. Only little is known about the manifestation of heterosis during early postembryonic development. Objective of this study was to identify heterotic traits during early maize root development. Four German inbred lines of the flint (UH002 and UH005) and dent (UH250 and UH301) pool and the 12 reciprocal hybrids generated from these inbred lines were subjected to a morphological and histological analysis during early root development. Primary root length and width were measured daily in a time course between 3 and 7 days after germination (DAG) and displayed average midparent heterosis (MPH) of 17-25% and 1-7%, respectively. Longitudinal size of cortical cells in primary roots was determined 5 DAG and displayed on average 24% MPH thus demonstrating that enlarged primary roots of hybrids can mainly be attributed to elongated cortical cells. The number of seminal roots determined 14 DAG showed on average 18% MPH. Lateral root density of all tested hybrids was determined 5 DAG. This root trait showed the highest degree of heterosis with an average MPH value of 51%. This study demonstrated that heterosis is already manifesting during the very early stages of root development a few days after germination. The young root system is therefore a suitable model for subsequent molecular studies of the early stages of heterosis manifestation during seedling development.