A molecular framework underlying the compound leaf pattern of Medicago truncatula

通讯作者：Jianghua Chen

提出问题：How the relevant morphogenetic activity is established and modulated to form a proper pattern of leaflets is a central question.

主要发现：Here we show that the trifoliate leaf pattern of the model leguminous plant Medicago truncatula is controlled by the BEL1-like homeodomain protein PINNATE-LIKE PENTAFOLIATA1 (PINNA1).

结果：We identify PINNA1 as a determinacy factor during leaf morphogenesis that directly represses transcription of the LEAFY (LFY) orthologue SINGLE LEAFLET1 (SGL1), which encodes an indeterminacy factor key to the morphogenetic activity maintenance. PINNA1 functions alone in the terminal leaflet region and synergizes with another determinacy factor, the C2H2 zinc finger protein PALMATE-LIKE PENTAFOLIATA1 (PALM1), in the lateral leaflet regions to define the spatiotemporal expression of SGL1, leading to an elaborate control of morphogenetic activity.

结论：This study reveals a framework for trifoliate leaf-pattern formation and sheds light on mechanisms generating diverse leaf forms.

作者在前期的研究中筛选到了一个palmate-like pentafoliata1 (palm1)苜蓿突变体（Chen et al., 2010, PNAS），野生型的三叶变为掌状五叶（下图）。进一步的研究分析发现该突变体的突变基因为一个C2H2锌指蛋白PALM1，该蛋白能够抑制LEAFY同源基因SGL1的转录，从而影响苜蓿的复叶发育。本文中，作者为了鉴定其它参与苜蓿复叶发育的调控子，通过Tnt1逆转座子插入突变筛到了一个pinnate-like pentafoliata1-1（pinna1-1），该突变体特征为原本的三叶变为了羽状五叶（下图）。进一步的研究发现显示该突变体的突变基因是一个BLH蛋白PINNA1。

原位杂交试验显示PINNA1基因主要在顶生小叶和侧生小叶原基中表达，而在SAM中不表达。亚细胞定位显示PINNA1蛋白主要定位于细胞核，说明其是一个转录调控因子。接着，作者研究了其与SGL1以及PALM1之间的关系，最终提出了一个model（下图）。

Fig. 6 | Model for PINNA1, PALM1 and SGL1 action in the pattern formation of compound leaf in M. truncatula. a, SGL1 facilitates the initiation of LL primordia (green arrows). PINNA1 acts alone in the TL primordium, whereas PALM1 has a master regulator role (bold line) and PINNA1 has a secondary regulator role in the LL primordia, in repression of SGL1 expression. b, Summary of mature-leaf phenotype results from loss of gene functions (labelled arrows). Terminal and lateral leaflet regions are coloured in grey and purple, respectively.

 通讯作者 

**陈江华**