Fig. 5.

A model for seedling growth heterosis triggered by complementary high-parent–dominant expression between growth-related key genes involved in the cell cycle and photosynthesis. At the early stages of seedling growth (3–8 d after sowing [DAS]), regulatory network hub genes involved in photosynthesis had higher expression levels in parent Per-1 than in parent Col-0, and had expression levels similar to Per-1 in the F₁ hybrid mainly during 3–5 DAS (the key period for establishing seedling cotyledon photosynthesis capability). In contrast, the regulatory network hub genes involved in the cell cycle had higher expression levels in Col-0 than in Per-1, and in temporal complementarity had expression patterns similar to Col-0 in the hybrid mainly during 6–8 DAS (the key period for cell division-driven true leaf initiation in the shoot apex). Thus, the hybrid spatiotemporally integrates one parent’s higher expression of cell cycle pathway genes and the other parent’s higher expression of photosynthesis pathway genes to realize enhanced growth vigor triggered by the complementary capacities of these two fundamental biological pathways (MP: midparent, the average level of both parents).