Compound leaves are those that look as though they are made of several smaller leaves, or leaflets. They develop from simple leaf primordia that maintain organogenic potential around their edges, enabling the growth of leaflets. To investigate how primordia develop into complex compound leaves Blein and colleagues examined the expression of some members of a large, evolutionarily conserved gene family — the NO APICAL MERISTEM and CUP-SHAPED COTYLEDON3 (NAM/CUC3) family — that had previously been implicated in plant organ separation. They found that these genes are expressed in a narrow strip of cells at the distal boundary of the leaflet primordium in the five different species of eudicots with compound leaves that were studied. So, NAM/CUC3 expression seems to underlie a fundamental mechanism of leaflet development.
Next, the authors reduced NAM and/or CUC3 expression by virus-induced gene silencing or by upregulating a repressive microRNA. The resulting defects in leaf development suggested that the NAM/CUC3 genes act locally, and probably cell autonomously, at the primordium boundary to regulate leaflet separation and margin dissection, but also act at a distance to control leaflet formation. Another key finding of these gene silencing studies was that NAM/CUC3 genes form a feed-forward regulatory loop with class 1 KNOTTED-LIKE HOMEOBOX (KNOX1) and LEAFY (LFY)-like genes. However, the contribution of KNOX1 and LFY to leaf development depends on the species, whereas function of NAM/CUC3 gene products is more universal.
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