euAP2a, a Key Gene that Regulates Flowering Time with a Novel Mechanism in Peach (Prunus persica)

Spring frost is a growing risk to temperate fruit production as warmer winter conditions can lead to earlier bloom, increasing the chance of damaging cold temperatures. One strategy to minimize the impacts of frost is to breed late-flowering cultivars to avoid the frost risk period. In this study, we analyzed Late-Flowering Peach (LFP) germplasm and showed its floral buds require longer chilling and warming periods during dormancy than the control cultivar, ‘John Boy’ (JB). We identified a 983-bp deletion in an AP2 gene, dubbed euAP2a, present only in LFP but not in 14 other peach genomes analyzed. This mutation eliminates an miR172 binding site, possibly allowing the euAP2a transcript to accumulate preferentially during chilling. These findings together with an early report that a deletion in the same euAP2a causes increasing floral petals, a morphological mark that also occurs in LFP, implies that the 983-bp deletion may contribute to the late-flowering phenotype. Furthermore, RNAseq data revealed that that two chilling- and three warm-responsive co-expression modules, which were collectively composed of 2,931 genes, were differentially activated at four of 13 dormancy stages. This activation was concurrent with a transient, stage-specific down-regulation of euAP2a. However, the mutated euAP2a in LFP did not exhibit the periodic downregulation events observed in JB and the concurrent activation of the five modules, leading to potential loss of activation of two chilling-responsive modules and an 8–12-day delay of three warm-responsive modules, which corresponds to the longer chilling requirement and delayed flowering time in the LFP buds. These findings support euAP2a as a potential regulator to control both floral development and bloom time in peach. Our findings provide important insight into the mechanisms underlying flowering time in peach, as well as a novel regulatory pathway that may operate in other plants. The results provide new insights to facilitate the breeding of new cultivars with late-flowering frost-avoidance traits. Overall design: Comparison of RNA profiles between 2 peach genotypes with different flowering time

春季霜冻已成为温带水果生产的潜在风险，因为较温暖的冬季条件可能导致花期提前，增加遭受损害性低温的可能性。为最小化霜冻的影响，一种策略是培育晚开花品种以规避霜冻风险期。在本研究中，我们分析了晚开花桃（LFP）种质，并显示其花蕾在休眠期间所需的低温和升温周期长于对照品种‘约翰男孩’（JB）。我们在LFP中发现了一个仅在LFP中存在而不在其他14个桃基因组中发现的AP2基因（称为euAP2a）的983碱基缺失。这种突变消除了miR172的结合位点，可能允许euAP2a转录本在低温期间优先积累。这些发现与早期报告的同一euAP2a基因的缺失会导致花瓣数量增加的事实相一致，这一形态特征也出现在LFP中，暗示了983碱基缺失可能有助于晚开花表型。此外，RNA测序数据揭示，在13个休眠阶段的四个阶段中，由2,931个基因共同组成的两个低温响应和三个温暖响应共表达模块被不同激活。这种激活与euAP2a的短暂、阶段特异性下调同时发生。然而，LFP中突变的euAP2a没有表现出在JB中观察到的周期性下调事件，以及五个模块的并发激活，导致潜在的两个低温响应模块的激活丧失和三个温暖响应模块的8-12天延迟，这与LFP芽的较长低温需求和延迟开花时间相对应。这些发现支持euAP2a作为控制桃花发育和开花时间的一个潜在调控因子。我们的研究结果为揭示桃开花时间的机制提供了重要见解，以及可能在其他植物中发挥作用的新的调控途径。这些结果为培育具有晚开花避霜特性的新品种提供了新的见解。总体设计：比较不同开花时间桃基因型的RNA谱。