UC San Diego

Heterosis, or hybrid vigor, refers to the phenomenon in which F1 offspring of two genetically distinct parental lines exhibit phenotypes outside the range of its parents. Agricultural breeding programs commonly utilize heterosis to enhance yield, disease resistance, and other desirable traits in both crops and livestock, despite the labor-intensive field tests required for hybrid breeding programs. Despite its importance, the mechanisms by which genetic variation between two parents combine to produce non-additive phenotypes in hybrids remain unclear. To identify molecular components that may contribute to heterosis, we analyzed paired proteomic and transcriptomic data from leaf tissue of maize hybrids and their inbred parents. Expression levels of plastid proteins involved in translation and photosynthesis were increased in seedling leaf tissue of hybrids relative to mid-parent and were positively correlated with heterosis levels in adult plants. Conversely, levels of proteins involved in stress responses were reduced in seedling leaf tissue of hybrids relative to mid-parent and were negatively correlated with heterosis levels in adult plants. An ethylene biosynthesis mutant copied the hybrid proteome, indicating that the most of the altered protein levels in hybrids are downstream of the reduction in ethylene biosynthesis. Protein expression patterns across the developmental gradient of hybrid leaves were altered in comparison to the inbred parents. Proteins involved in photosynthesis and translation were reduced in the basal zone of the leaf but increased in the mature zone. This suggests that hybrids have more precisely controlled expression patterns across the developmental gradient which could contribute to their greater fitness. The same expression patterns were observed in the ethylene mutant, indicating that reduced ethylene biosynthesis largely mediates the developmental differences between hybrids and inbreds.