The genes with homéoboîtes TALE and l' activity of the méristème:

SHOOT MERISTEMLESS (STM), the founding member of the KNAT family, is required for the initiation and the self-maintenance of the SAM. BREVIPEDICELLUS (BP) or KNAT1 contributes to SAM maintenance with STM and controls the inflorescence patterning with PENNYWISE. We are studying the role of the two other close members, KNAT2 and KNAT6. Using an inducible transgenic line that overexpressed KNAT2, we revealed a link between KNAT genes and carpel development and a link between cytokinin and KNAT2. We showed that KNAT6 contributes with STM to the maintenance of the shoot apical meristem and to organ separation. KNAT2 does not have such a function. More recently, we showed that BP and PNY restrict KNAT6 and KNAT2 expression to promote correct inflorescence development. Currently, we are examining the interaction of KNAT2 and KNAT6 with other key meristem genes.

Dynamics of the operation of the méristème:

miRNAs, boundary domains and architecture of Arabidopsis
In Arabidopsis, the boundary domain that separates organ primordia is determined by the redundant action of the CUC1, 2 and 3 genes. In addition, the CUC genes promote meristem formation during normal development and during in vitro culture. Two of the CUC (CUC1 and CUC2) genes are regulated by a microRNA, miR164. miRNAs are small single stranded RNAs that post-transcriptionnally regulate gene expression. We have analysed the role of miRNAs during cell differentiation in the meristem. We have analysed the role of miR164 in the regulation of the CUC1 and CUC2 genes which underlines the importance of miRNA regulation during plant development et reveals novel roles for the CUC genes.
- We showed that miR164-mediated degradation of CUC1 and CUC2 messengers is required to constrain boundary enlargement around the organ primordia (Laufs et al., 2004).
- Lines expressing a miR164 resistant version of the CUC2 gene present a dramatic modification of the phyllotaxy. Surprisingly, the phyllotaxy measured directly in the meristem of these lines is normal and we could correlate the changes of the phyllotaxy with modifications of the cell proliferation during the growth and differentiation of the stem. These results indicate that the phyllotaxy has not only to be properly set up in the meristem but needs to be further maintained during stem growth and differentiation (Peaucelle et al., 2007).
- We have shown the miR164/CUC regulatory unit acts also during later stages of leaf development. miR164 overexpresssion leads to the formation of leaves with entire margins whereas on the contrary, the inactivation of MIR164A led to the formation of leaves presenting exaggerated lobes. A similar phenotype of enhanced lobes is observed in plants expressing a miR164-resistant CUC2 gene. Altogether, these results suggest a model in which the final level of CUC2 activity, but not of CUC1, balanced by miR164 will determine the level of growth repression in the leaf sinus, thus determining - - Finally, in collaboration with the group of K. Theres, whithin the trilateralGENOSOME project we have shown that the miR164/CUC balance controles branching in Arabidopsis (Raman et al., 2008).
Altogether, these observations indicate a central role for the miR164/CUC regulatory unit in the controle of the aerial architecture of Arabidopsis as it is involved in organ position control, organ shape and branching.

Boundary genes and leaf shape
Whereas Arabidopsis has simple leaves, many other species develop leaves dissected into small units called leaflets. We have identified the NAM/CUC3 genes of 5 representative Eudicots, columbine, tomato, potato, pea and cardamine and showed that in all these species they are expressed in relation with the dissection into leaflets or the incision of the leaf margin to form lobes or serrations. When the expression levels of the NAM/CUC3 genes are lowered, the incisions are reduced leading to fusion between leaflets or smoothening of the margins. Fewer leaflets were also observed when the NAM/CUC3 were silenced. Thus, we proposed a general framework for leaf development in which all the levels of incision rely on NAM/CUC3 genes. Furthermore, leaflet formation also involves NAM/CUC3 genes via the formation of a positive feed-back loop between these genes and species-specific factors (Blein et al., 2008). These observations led also to the general hypothesis that the boundary domains, in addition to their previously recognised role in the separation between outgrowing units, may also have a rôle in the stimulation of the outgrowth of these structures.

Construction of new tools.
In order to follow differentiation in a living structure or to allow inducible gene expression in specific domains, we have developed a collection of inducible lines expressing GFP in different meristem subdomains (Deveaux et al., 2003). This is based on the ethanol switch that we used previously to dissect the functions of UFO during flower development or to analyse miR164 function (Laufs et al., 2003, 2004). The system of inducible reporter expression under the control of specific promoters, combined with the observation of living meristems (Grandjean et al., 2004), allow us to follow in vivo cell differentiation in the meristem.

Our group is also involved in an ANR-Génoplante funded project called Régénéome aiming at the development of Laser-Assisted-Microdissection