Gerttula Suzanne, Zinkgraf Matthew, Muday Gloria K, Lewis Daniel R, Ibatullin Farid M, Brumer Harry, Hart Foster, Mansfield Shawn D, Filkov Vladimir, Groover Andrew
US Forest Service, Pacific Southwest Research Station, Davis, California 95618.
Wake Forest University, Winston-Salem, North Carolina 27106.
Plant Cell. 2015 Oct;27(10):2800-13. doi: 10.1105/tpc.15.00531. Epub 2015 Sep 26.
Angiosperm trees reorient their woody stems by asymmetrically producing a specialized xylem tissue, tension wood, which exerts a strong contractile force resulting in negative gravitropism of the stem. Here, we show, in Populus trees, that initial gravity perception and response occurs in specialized cells through sedimentation of starch-filled amyloplasts and relocalization of the auxin transport protein, PIN3. Gibberellic acid treatment stimulates the rate of tension wood formation and gravibending and enhances tissue-specific expression of an auxin-responsive reporter. Gravibending, maturation of contractile fibers, and gibberellic acid (GA) stimulation of tension wood formation are all sensitive to transcript levels of the Class I KNOX homeodomain transcription factor-encoding gene ARBORKNOX2 (ARK2). We generated genome-wide transcriptomes for trees in which gene expression was perturbed by gravistimulation, GA treatment, and modulation of ARK2 expression. These data were employed in computational analyses to model the transcriptional networks underlying wood formation, including identification and dissection of gene coexpression modules associated with wood phenotypes, GA response, and ARK2 binding to genes within modules. We propose a model for gravitropism in the woody stem in which the peripheral location of PIN3-expressing cells relative to the cambium results in auxin transport toward the cambium in the top of the stem, triggering tension wood formation, while transport away from the cambium in the bottom of the stem triggers opposite wood formation.
被子植物树木通过不对称地产生一种特殊的木质部组织——张力木，来重新定向其木质茎干。张力木会施加强大的收缩力，导致茎干产生负向重力性。在此，我们在杨树中发现，最初的重力感知和反应发生在特化细胞中，通过充满淀粉的造粉体沉降以及生长素转运蛋白PIN3的重新定位来实现。赤霉素处理会刺激张力木的形成速率和重力弯曲，并增强生长素响应报告基因的组织特异性表达。重力弯曲、收缩纤维的成熟以及赤霉素（GA）对张力木形成的刺激，都对I类KNOX同源结构域转录因子编码基因ARBORKNOX2（ARK2）的转录水平敏感。我们生成了树木的全基因组转录组，这些树木的基因表达受到重力刺激、GA处理以及ARK2表达调控的干扰。这些数据被用于计算分析，以模拟木材形成背后的转录网络，包括识别和剖析与木材表型、GA反应以及ARK2与模块内基因结合相关的基因共表达模块。我们提出了一个木质茎干中重力性的模型，其中表达PIN3的细胞相对于形成层的外周位置导致生长素向茎干顶部的形成层运输，触发张力木的形成，而向茎干底部的形成层外运输则触发相反的木材形成。
