The Genome Center, University of California, Davis, Davis, CA 95616, USA.
Department of Plant Sciences, University of California, Davis, Davis, CA 95616, USA.
G3 (Bethesda). 2021 Jul 14;11(7). doi: 10.1093/g3journal/jkab097.
Tipburn is an important physiological disorder of lettuce, Lactuca sativa L., related to calcium deficiency that can result in leaf necrosis and unmarketable crops. The major quantitative trait locus (QTL), qTPB5.2, can account for up to 70% of the phenotypic variance for tipburn incidence in the field. This QTL was genetically dissected to identify candidate genes for tipburn by creating lines with recombination events within the QTL and assessing their resistance to tipburn. By comparing lines with contrasting haplotypes, the genetic region was narrowed down to ∼877 Kb that was associated with a reduction of tipburn by ∼60%. Analysis of the lettuce reference genome sequence revealed 12 genes in this region, one of which is a calcium transporter with a single nucleotide polymorphism in an exon between haplotypes with contrasting phenotypes. RNA-seq analysis of recombinants revealed two genes that were differentially expressed between contrasting haplotypes consistent with the tipburn phenotype. One encodes a Teosinte branched1/Cycloidea/Proliferating Cell factor transcription factor; however, differential expression of the calcium transporter was detected. The phenotypic data indicated that there is a second region outside of the ∼877 Kb region but within the QTL, at which a haplotype from the susceptible parent decreased tipburn by 10-20%. A recombinant line was identified with beneficial haplotypes in each region from both parents that showed greater tipburn resistance than the resistant parent; this line could be used as the foundation for breeding cultivars with more resistance than is currently available.
顶尖烧边是生菜（Lactuca sativa L.）的一种重要生理失调症，与缺钙有关，可导致叶片坏死和作物不可销售。主要的数量性状位点（QTL）qTPB5.2 可以解释田间顶尖烧边发病率的 70%的表型方差。通过在 QTL 内创建具有重组事件的系，并评估其对顶尖烧边的抗性，对该 QTL 进行了遗传剖析，以鉴定顶尖烧边的候选基因。通过比较具有不同单倍型的系，将遗传区域缩小到与顶尖烧边减少约 60%相关的 877 Kb 左右。对生菜参考基因组序列的分析表明，该区域有 12 个基因，其中一个是钙转运蛋白，在具有不同表型的单倍型之间的外显子中存在一个单核苷酸多态性。重组体的 RNA-seq 分析揭示了两个在不同单倍型之间差异表达的基因，与顶尖烧边表型一致。其中一个编码 Teosinte branched1/Cycloidea/Proliferating Cell factor 转录因子；然而，钙转运蛋白的差异表达被检测到。表型数据表明，在 QTL 内但在约 877 Kb 区域之外，还有第二个区域，来自易感亲本的一个单倍型可使顶尖烧边减少 10-20%。鉴定出一个重组系，其在来自双亲的每个区域都具有有利的单倍型，比抗性亲本表现出更强的顶尖烧边抗性；该系可作为培育比目前更具抗性的品种的基础。
