No abstract

Hybrid vigour (heterosis) has been used for decades in crop industries, especially in the production of maize and rice. Hybrid varieties usually exceed their parents in plant biomass and seed yield. But the molecular basis of hybrid vigour is not fully understood. In this project, we studied heterosis at early stages of seedling development in Arabidopsis hybrids derived from crossing Ler and C24 accessions. We found that early heterosis is associated with non-additive gene expression that resulted from earlier changes in gene expression in the hybrids relative to the parents. The non-additively expressed genes are involved in metabolic pathways, including photosynthesis, critical for plant growth. The early increased expression levels of genes involved in energy production in hybrids is associated with heterosis in the young seedlings that could be essential for biomass heterosis at later developmental stages of the plant.

Arabidopsis thaliana hybrids have similar properties to hybrid crops, with greater biomass relative to the parents. We asked whether the greater biomass was due to increased photosynthetic efficiency per unit leaf area or to overall increased leaf area and increased total photosynthate per plant. We found that photosynthetic parameters (electron transport rate, CO2 assimilation rate, chlorophyll content, and chloroplast number) were unchanged on a leaf unit area and unit fresh weight basis between parents and hybrids, indicating that heterosis is not a result of increased photosynthetic efficiency. To investigate the possibility of increased leaf area producing more photosynthate per plant, we studied C24×Landsberg erecta (Ler) hybrids in detail. These hybrids have earlier germination and leaf growth than the parents, leading to a larger leaf area at any point in development of the plant. The developing leaves of the hybrids are significantly larger than those of the parents, with consequent greater production of photosynthate and an increased contribution to heterosis. The set of leaves contributing to heterosis changes as the plant develops; the four most recently emerged leaves make the greatest contribution. As a leaf matures, its contribution to heterosis attenuates. While photosynthesis per unit leaf area is unchanged at any stage of development in the hybrid, leaf area is greater and the amount of photosynthate per plant is increased.