Department of Horticulture, Plant and Soil Science Building, Michigan State University, East Lansing, Michigan, United States of America.
Michigan State University Extension, East Lansing, Michigan, United States of America.
PLoS One. 2021 Feb 19;16(2):e0245487. doi: 10.1371/journal.pone.0245487. eCollection 2021.
Many apple cultivars are subject to biennial fluctuations in flowering and fruiting. It is believed that this phenomenon is caused by a repressive effect of developing fruit on the initiation of flowers in the apex of proximal bourse shoots. However, the genetic pathways of floral initiation are incompletely described in apple, and the biological nature of floral repression by fruit is currently unknown. In this study, we characterized the transcriptional landscape of bourse shoot apices in the biennial cultivar, 'Honeycrisp', during the period of floral initiation, in trees bearing a high fruit load and in trees without fruit. Trees with high fruit load produced almost exclusively vegetative growth in the subsequent year, whereas the trees without fruit produced flowers on the majority of the potential flowering nodes. Using RNA-based sequence data, we documented gene expression at high resolution, identifying >11,000 transcripts that had not been previously annotated, and characterized expression profiles associated with vegetative growth and flowering. We also conducted a census of genes related to known flowering genes, organized the phylogenetic and syntenic relationships of these genes, and compared expression among homeologs. Several genes closely related to AP1, FT, FUL, LFY, and SPLs were more strongly expressed in apices from non-bearing, floral-determined trees, consistent with their presumed floral-promotive roles. In contrast, a homolog of TFL1 exhibited strong and persistent up-regulation only in apices from bearing, vegetative-determined trees, suggesting a role in floral repression. Additionally, we identified four GIBBERELLIC ACID (GA) 2 OXIDASE genes that were expressed to relatively high levels in apices from bearing trees. These results define the flowering-related transcriptional landscape in apple, and strongly support previous studies implicating both gibberellins and TFL1 as key components in repression of flowering by fruit.
许多苹果品种的开花和结果都存在两年一次的波动。人们认为，这种现象是由于发育中的果实对顶端近轴芽中花的起始的抑制作用造成的。然而，苹果中花起始的遗传途径尚未完全描述，果实对花的抑制的生物学性质目前尚不清楚。在这项研究中，我们在丰登品种“Honeycrisp”中描述了二年生芽顶端的转录景观，在承载高果实负荷的树木和无果实的树木中。承载高果实负荷的树木在次年几乎只产生营养生长，而无果实的树木则在大多数潜在的开花节点上产生花。使用基于 RNA 的序列数据，我们以高分辨率记录了基因表达，鉴定了 >11000 个以前未注释的转录本，并描述了与营养生长和开花相关的表达谱。我们还对与已知开花基因相关的基因进行了普查，组织了这些基因的系统发育和共线性关系，并比较了同系物之间的表达。几个与 AP1、FT、FUL、LFY 和 SPLs 密切相关的基因在无负荷、花决定的芽顶端表达更强，这与它们假定的花促进作用一致。相比之下，TFL1 的同源物仅在承载、营养决定的芽顶端中强烈且持续上调，表明其在花抑制中的作用。此外，我们鉴定了四个赤霉素 2 氧化酶基因，它们在承载树的芽顶端表达水平相对较高。这些结果定义了苹果中与开花相关的转录景观，并强烈支持先前的研究，表明赤霉素和 TFL1 都是果实抑制开花的关键组成部分。
