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

杂种优势是指遗传多样性不同的自交系的杂合F1杂交后代比纯合子亲本更健壮的现象。了解杂种优势表现的分子和遗传基础是全球粮食安全的关键，因为几乎所有商业化种植的玉米都是以杂交为基础的。在玉米根系发育早期，侧根密度表现出最大程度的杂种优势。高度分枝的侧根对于锚定和从土壤中最佳吸收水分和养分以及将其分配到植物的能量输送地上部分至关重要。这项研究的总体目标是全面了解谷类作物根系侧根发生过程中表现杂种优势的分子和遗传机制。在初步工作中，我们筛选了由自交系B73和Mo17的4个杂交组合和7轮自交(IBM RIL Syn4)产生的160个重组自交系的侧根密度。其中，我们选择了三个侧根密度较高的品系和三个侧根密度较低的品系与其亲本自交系B73和Mo17回交，从而产生了部分杂合的F1杂交种。本研究将对侧根密度不同的重组自交系及其杂交种进行研究。首先，详细的组织学和细胞学实验将通过光学显微镜和透射电子显微镜来确定自交系及其杂交后代在基本分生组织中的细胞差异。同时，在生长素依赖的细胞分裂的背景下，生长素最大值和基本分生组织的DNA含量(2n/4n)将被确定。基于这些结果，这些重组自交系及其杂交后代的韧皮部极周细胞的转录组将被用激光捕获显微切割(LCM)结合RNA-Seq来研究。鉴定单亲、非加性和等位基因特异的基因表达模式将有助于探索侧根发生过程中杂种优势的系统发育和功能基础。此外，网络分析工具WGCNA将识别高度连接的基因共表达模块和HUB基因。RNA原位杂交将为所选HUB基因在侧根杂种优势表现中的功能提供更多的见解。此外，为了证实这些发现，这些基因的表达模式将在其他杂种优势群的基因类型中进行研究。对这些基因的突变分析将在第二个资助期补充这项研究。

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.