Immanen Juha, Nieminen Kaisa, Smolander Olli-Pekka, Kojima Mikiko, Alonso Serra Juan, Koskinen Patrik, Zhang Jing, Elo Annakaisa, Mähönen Ari Pekka, Street Nathaniel, Bhalerao Rishikesh P, Paulin Lars, Auvinen Petri, Sakakibara Hitoshi, Helariutta Ykä
Institute of Biotechnology, University of Helsinki, 00014 Helsinki, Finland; Department of Biosciences, University of Helsinki, 00014 Helsinki, Finland.
Green Technology, Natural Resources Institute Finland (Luke), Jokiniemenkuja 1, 01301 Vantaa, Finland.
Curr Biol. 2016 Aug 8;26(15):1990-1997. doi: 10.1016/j.cub.2016.05.053. Epub 2016 Jul 14.
Despite the crucial roles of phytohormones in plant development, comparison of the exact distribution profiles of different hormones within plant meristems has thus far remained scarce. Vascular cambium, a wide lateral meristem with an extensive developmental zonation, provides an optimal system for hormonal and genetic profiling. By taking advantage of this spatial resolution, we show here that two major phytohormones, cytokinin and auxin, display different yet partially overlapping distribution profiles across the cambium. In contrast to auxin, which has its highest concentration in the actively dividing cambial cells, cytokinins peak in the developing phloem tissue of a Populus trichocarpa stem. Gene expression patterns of cytokinin biosynthetic and signaling genes coincided with this hormonal gradient. To explore the functional significance of cytokinin signaling for cambial development, we engineered transgenic Populus tremula × tremuloides trees with an elevated cytokinin biosynthesis level. Confirming that cytokinins function as major regulators of cambial activity, these trees displayed stimulated cambial cell division activity resulting in dramatically increased (up to 80% in dry weight) production of the lignocellulosic trunk biomass. To connect the increased growth to hormonal status, we analyzed the hormone distribution and genome-wide gene expression profiles in unprecedentedly high resolution across the cambial zone. Interestingly, in addition to showing an elevated cambial cytokinin content and signaling level, the cambial auxin concentration and auxin-responsive gene expression were also increased in the transgenic trees. Our results indicate that cytokinin signaling specifies meristematic activity through a graded distribution that influences the amplitude of the cambial auxin gradient.
尽管植物激素在植物发育中起着关键作用，但迄今为止，对不同激素在植物分生组织内的确切分布模式的比较仍然很少。维管形成层是一种具有广泛发育分区的宽阔侧生分生组织，为激素和基因分析提供了一个理想的系统。利用这种空间分辨率，我们在此表明，两种主要的植物激素，细胞分裂素和生长素，在形成层上显示出不同但部分重叠的分布模式。与在活跃分裂的形成层细胞中浓度最高的生长素不同，细胞分裂素在毛果杨茎的发育中的韧皮部组织中达到峰值。细胞分裂素生物合成和信号转导基因的基因表达模式与这种激素梯度一致。为了探索细胞分裂素信号传导对形成层发育的功能意义，我们构建了细胞分裂素生物合成水平升高的转基因欧洲山杨×美洲黑杨树木。这些树木证实了细胞分裂素作为形成层活性的主要调节因子发挥作用，显示出形成层细胞分裂活性受到刺激，导致木质纤维素树干生物量产量大幅增加（干重增加高达80%）。为了将增加的生长与激素状态联系起来，我们以前所未有的高分辨率分析了形成层区域的激素分布和全基因组基因表达谱。有趣的是，除了显示形成层细胞分裂素含量和信号水平升高外，转基因树木中的形成层生长素浓度和生长素响应基因表达也增加了。我们的结果表明，细胞分裂素信号传导通过影响形成层生长素梯度幅度的分级分布来确定分生组织活性。
