Hay Angela, Kaur Hardip, Phillips Andrew, Hedden Peter, Hake Sarah, Tsiantis Miltos
Plant and Microbial Biology Department, University of California, Berkeley 94720, USA.
Curr Biol. 2002 Sep 17;12(18):1557-65. doi: 10.1016/s0960-9822(02)01125-9.
The shoot apical meristem (SAM) is an indeterminate structure that gives rise to the aerial parts of higher plants. Leaves arise from the differentiation of cells at the flanks of the SAM. Current evidence suggests that the precise regulation of KNOTTED1-like homeobox (KNOX) transcription factors is central to the acquisition of leaf versus meristem identity in a wide spectrum of plant species. Factors required to repress KNOX gene expression in leaves have recently been identified. Additional factors such as the CHD3 chromatin remodeling factor PICKLE (PKL) act to restrict meristematic activity in Arabidopsis leaves without repressing KNOX gene expression. Less is known regarding downstream targets of KNOX function. Recent evidence, however, has suggested that growth regulators may mediate KNOX activity in a variety of plant species.
Here we show that reduced activity of the gibberellin (GA) growth regulator pathway promotes meristematic activity, both in the natural context of KNOX function in the SAM and upon ectopic KNOX expression in Arabidopsis leaves. We show that constitutive signaling through the GA pathway is detrimental to meristem maintenance. Furthermore, we provide evidence that one of the functions of the KNOX protein SHOOTMERISTEMLESS (STM) is to exclude transcription of the GA-biosynthesis gene AtGA20ox1 from the SAM. We also demonstrate that AtGA20ox1 transcript is reduced in the pkl mutant in a KNOX-independent manner. Moreover, we show a similar interaction between KNOX proteins and GA-biosynthesis gene expression in the tomato leaf and implicate this interaction in regulation of the dissected leaf form.
We suggest that repression of GA activity by KNOX transcription factors is a key component of meristem function. Transfer of the KNOX/GA regulatory module from the meristem to the leaf may have contributed to the generation of the diverse leaf morphologies observed in higher plants.
茎尖分生组织（SAM）是一种不确定的结构，可产生高等植物的地上部分。叶片由SAM两侧细胞的分化产生。目前的证据表明，对类KNOTTED1同源异型框（KNOX）转录因子的精确调控对于众多植物物种中叶片与分生组织特性的获得至关重要。最近已鉴定出叶片中抑制KNOX基因表达所需的因子。其他因子，如染色质重塑因子CHD3的PICKLE（PKL），在不抑制KNOX基因表达的情况下限制拟南芥叶片中的分生组织活性。关于KNOX功能的下游靶标了解较少。然而，最近的证据表明，生长调节剂可能在多种植物物种中介导KNOX活性。
我们在此表明，赤霉素（GA）生长调节剂途径活性的降低促进了分生组织活性，这在SAM中KNOX功能的自然背景下以及拟南芥叶片中异位KNOX表达时均如此。我们表明，通过GA途径的组成型信号传导对分生组织维持有害。此外，我们提供证据表明，KNOX蛋白无柄分生组织（STM）的功能之一是将GA生物合成基因AtGA20ox1的转录排除在SAM之外。我们还证明，AtGA20ox1转录本在pkl突变体中以KNOX非依赖的方式减少。此外，我们在番茄叶片中显示了KNOX蛋白与GA生物合成基因表达之间类似的相互作用，并表明这种相互作用参与了叶形的调控。
我们认为，KNOX转录因子对GA活性的抑制是分生组织功能的关键组成部分。KNOX/GA调控模块从分生组织转移到叶片可能有助于产生高等植物中观察到的多种叶片形态。
