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自交系的杂合f1杂交后代比其纯合亲本更具活力的现象。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，因为几乎所有商业种植的玉米都是杂交品种。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地吸收水分和养分以及将其分配到植物的能量输送部分至关重要。它们起源于根深处的一种特殊的细胞类型，韧皮部极中柱鞘细胞。本研究的总体目标是全面了解谷物根系侧根形成过程中杂种优势表现的分子和遗传机制。在初步工作中，我们筛选了160个重组自交系，这些自交系是由B73和Mo17的4个杂交和7轮自交（IBM RIL syn4）产生的。选择侧根密度高的3个品系和侧根密度低的3个品系与其亲本自交系B73和Mo17回交，得到部分杂合的f1杂交种。本文拟对这些侧根密度差异较大的重组自交系及其杂种进行研究。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确定自交系和杂交系在基础分生组织中的细胞差异。同时，在生长素依赖性细胞分裂的背景下，生长素最大值和DNA含量（2n/4n）将被确定。在此基础上，利用激光捕获显微解剖技术（LCM）结合RNA-Seq技术对重组自交系及其杂交种韧皮部中柱鞘细胞的转录组进行了研究。鉴定单亲、非加性和等位基因特异性基因表达模式将有助于探索侧根形成过程中杂种优势的系统发育和功能基础。此外，高连接基因共表达模块和枢纽基因将通过网络分析工具WGCNA进行鉴定。RNA原位杂交将提供更多的见解，选择枢纽基因的功能在侧根杂种优势的表现。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群体的基因型中进行研究。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.