Genotype-dependent regulation of transcriptomic networks during heterosis manifestation in the competence phase of maize pericycle cells

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Heterosis describes the phenomenon that heterozygous F1-hybrid progeny of genetically diverse inbred lines are more vigorous than their homozygous parents. Understanding the molecular and genetic basis of heterosis manifestation is a key to global food security because almost all commercially grown maize is hybrid based. In maize, lateral root density displays the highest degree of heterosis during early root development. Highly branched lateral roots are critical for anchorage and for the optimal uptake of water and nutrients from soil and for their allocation to the energy-delivering, above-ground parts of the plant. They are initiated from a specific cell-type deep inside the root, the phloem pole pericycle cells.The overall objective of this proposal is to obtain a comprehensive understanding of the molecular and genetic mechanisms involved in heterosis manifestation during lateral root initiation in cereal root systems.In preliminary work we screened a panel of 160 recombinant inbred lines generated by four crosses of the inbred lines B73 and Mo17 and seven rounds of selfing (IBM RIL syn4) for lateral root density. Among those, we selected three lines with high and three lines with low lateral root density and backcrossed them to their parental inbred lines B73 and Mo17 thus generating partially heterozygous F1-hybrids. These recombinant inbred lines with contrasting lateral root density and their hybrids will be studied in this proposal. First, detailed histological and cytological experiments will pinpoint the cellular differences between the inbred lines and their hybrids in the basal meristem by light microscopy and transmission electron microscopy. In parallel, auxin maxima and DNA content (2n/4n) of the basal meristem will be determined in the context of the auxin-dependent cell division.Based on these results, the transcriptome of phloem pole pericycle cells from the basal meristem of these recombinant inbred lines and their hybrids will be studied by laser capture microdissection (LCM) of these cells coupled with RNA-Seq. Identification of single parent-, non-additive-, and allele-specific gene expression patterns will allow to explore the phylogenetic and functional basis of heterosis during lateral root initiation. Moreover, highly connective gene coexpression modules and hub genes will be identified by the network analysis tool WGCNA. RNA in situ hybridization will provide additional insights into the function of selected hub genes in the manifestation of lateral root heterosis. Moreover, to substantiate these findings, expression patterns of these genes will be studied in genotypes of other heterotic groups. Mutant analyses of such genes will complement this study in the second funding period.