Schneeberger R, Tsiantis M, Freeling M, Langdale J A
Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720-3102, USA.
Development. 1998 Aug;125(15):2857-65. doi: 10.1242/dev.125.15.2857.
Leaves of higher plants are produced in a sequential manner through the differentiation of cells that are derived from the shoot apical meristem. Current evidence suggests that this transition from meristematic to leaf cell fate requires the down-regulation of knotted1-like homeobox (knox) gene expression. If knox gene expression is not repressed, overall leaf shape and cellular differentiation within the leaf are perturbed. In order to identify genes that are required for the aquisition of leaf cell fates, we have genetically screened for recessive mutations that confer phenotypes similar to dominant mutations (e.g. Knotted1 and Rough sheath1) that result in the ectopic expression of class I knox genes. Independently derived mutations at the rough sheath2 (rs2) locus condition a range of pleiotropic leaf, node and internode phenotypes that are sensitive to genetic background and environment. Phenotypes include dwarfism, leaf twisting, disorganized differentiation of the blade-sheath boundary, aberrant vascular patterning and the generation of semi-bladeless leaves. knox genes are initially repressed in rs2 mutants as leaf founder cells are recruited in the meristem. However, this repression is often incomplete and is not maintained as the leaf progresses through developement. Expression studies indicate that three knox genes are ectopically or over-expressed in developing primordia and in mature leaves. We therefore propose that the rs2 gene product acts to repress knox gene expression (either directly or indirectly) and that rs2 gene action is essential for the elaboration of normal leaf morphology.
高等植物的叶片是通过源自茎尖分生组织的细胞分化依次产生的。目前的证据表明，从分生组织细胞命运向叶细胞命运的转变需要下调类knotted1同源异型盒（knox）基因的表达。如果knox基因的表达没有被抑制，叶片的整体形状和叶片内的细胞分化就会受到干扰。为了鉴定获得叶细胞命运所需的基因，我们对隐性突变进行了遗传筛选，这些突变赋予的表型类似于导致I类knox基因异位表达的显性突变（如Knotted1和Rough sheath1）。在粗糙叶鞘2（rs2）位点独立产生的突变会导致一系列多效性的叶片、节和节间表型，这些表型对遗传背景和环境敏感。表型包括矮化、叶片扭曲、叶片-叶鞘边界的分化紊乱、异常的维管束模式以及半无叶叶片的产生。当叶原基在分生组织中被招募时，knox基因最初在rs2突变体中受到抑制。然而，这种抑制往往是不完全的，并且随着叶片发育进程不会持续存在。表达研究表明，三个knox基因在发育中的原基和成熟叶片中异位表达或过度表达。因此，我们提出rs2基因产物起到抑制knox基因表达（直接或间接）的作用，并且rs2基因的作用对于正常叶片形态的形成至关重要。
