背景回顾：Leaves of seed plants provide an attractive system to study the development and evolution of form. Leaves show varying degrees of margin complexity ranging from simple, as in Arabidopsis thaliana, to fully dissected into leaflets in the closely related species Cardamine hirsuta. Leaflet formation requires actions of Class I KNOTTED1-LIKE HOMEOBOX (KNOX1) and REDUCED COMPLEXITY (RCO) homeobox genes, which are expressed in the leaves of C. hirsuta but not A. thaliana. Evolutionary studies indicate that diversification of KNOX1 and RCO genes was repeatedly associated with increased leaf complexity. 

提出问题：However, whether this gene combination represents a developmentally favored avenue for leaflet formation remains unknown, and the cell-level events through which the combined action of these genes drives leaflet formation are also poorly understood. 

主要发现：Here we show, through a genetic screen, that when a C. hirsuta RCO transgene is expressed in A. thaliana, then ectopic KNOX1 expression in leaves represents a preferred developmental path for leaflet formation. Using time-lapse growth analysis, we demonstrate that KNOX1 expression in the basal domain of leaves leads to prolonged and anisotropic cell growth. This KNOX1 action, in synergy with local growth repression by RCO, is instrumental in generating rachises and petiolules, the linear geometrical elements, that bear leaflets in complex leaves. 

结论：Our results show how the combination of cell-level growth analyses and genetics can help us understand how evolutionary modifications in expression of developmentally important genes are translated into diverse leaf shapes.

 摘 要 

种子植物的叶片为研究器官发育和演化提供了一个很好的系统。叶片会表现出不同程度的边缘复杂性，比如说拟南芥的简单叶形，再到其近缘物种碎米芥深裂以至形成小叶的叶片。小叶的形成需要KNOX1和RCO基因，其在碎米芥中表达，但是在拟南芥中不表达。演化研究表明，KNOX1和RCO基因的分化与叶片复杂性的增加相关。但是，该基因组合是否代表是发育上有利于小叶形成的，以及这些基因的联合作用如何在细胞水平上作用于小叶的形成，这些问题都还不清楚。本文中，作者通过遗传筛查，发现当碎米芥的RCO基因在拟南芥中表达时，叶片中异位的KNOX1基因表达代表了小叶形成的优选发育路径。利用延时摄影生长分析，作者发现叶片基部区域的KNOX1基因表达，会导致细胞的伸长生长和各向异性生长。这种KNOX1作用和RCO介导的局部生长抑制，一起协同作用于叶轴和小叶的产生，即复杂叶片中用于承载小叶的线性几何元素。本文的研究结合了细胞水平上的生长分析和遗传学，从而揭示了发育上重要基因的表达修饰会导致多样叶片形状的产生。

 Miltos Tsiantis