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Abstract

Heterosis refers to the superior performance of F1 hybrids over their respective parental inbred lines. Although the genetic
and expression basis of heterosis have been previously investigated, the metabolic basis for this phenomenon is poorly
understood. In a preliminary morphological study in Brassica juncea, we observed significant heterosis at the 50% flowering
stage, wherein both the growth and reproduction of F1 reciprocal hybrids were greater than that of their parents. To identify
the possible metabolic causes or consequences of this heterosis, we carried out targeted LC-MS analysis of 48 primary
(amino acids and sugars) and secondary metabolites (phytohormones, glucosinolates, flavonoids, and phenolic esters) in five
developmental tissues at 50% flowering in hybrids and inbred parents. Principal component analysis (PCA) of metabolites
clearly separated inbred lines from their hybrids, particularly in the bud tissues. In general, secondary metabolites displayed
more negative heterosis values in comparison to primary metabolites. The tested primary and secondary metabolites
displayed both additive and non-additive modes of inheritance in F1 hybrids, wherein the number of metabolites showing an
additive mode of inheritance were higher in buds and siliques (52.77–97.14%) compared to leaf tissues (47.37–80%). Partial
least regression (PLS) analysis further showed that primary metabolites, in general, displayed higher association with
morphological parameters in F1 hybrids. Overall, our results are consistent with a resource-cost model for heterosis in B.
juncea, where metabolite allocation in hybrids appears to favor growth, at the expense of secondary metabolism.