The peach (Prunus persicaL. Batsch) genome harbours 10KNOXgenes, which are differentially expressed in stem development, and the class 1KNOPE1regulates elongation and lignification during primary growth

TheKNOTTED-like (KNOX) genes encode homeodomain transcription factors and regulate several processes of plant organ development. The peach (Prunus persicaL. Batsch) genome was found to contain 10KNOXmembers (KNOPE genes); six of them were experimentally located on thePrunusreference map and the class 1KNOPE1was found to link to a quantitative trait locus (QTL) for the internode length in the peach×Ferganensis population. All theKNOPEgenes were differentially transcribed in the internodes of growing shoots; theKNOPE1mRNA abundance decreased progressively from primary (elongation) to secondary growth (radial expansion). During primary growth, theKNOPE1mRNA was localized in the cortex and in the procambium/metaphloem zones, whereas it was undetected in incipient phloem and xylem fibres.KNOPE1overexpression in theArabidopsis bp4loss-of-function background (35S:KNOPE1/bpgenotype) restored the rachis length, suggesting, together with the QTL association, a role forKNOPE1in peach shoot elongation. Several lignin biosynthesis genes were up-regulated in thebp4internodes but repressed in the35S:KNOPE1/bplines similarly to the wild type. Moreover, the lignin deposition pattern of the35S:KNOPE1/bpand the wild-type internodes were the same. The KNOPE1 protein was found to recognizein vitroone of the typical KNOX DNA-binding sites that recurred in peach andArabidopsislignin genes.KNOPE1expression was inversely correlated with that of lignin genes and lignin deposition along the peach shoot stems and was down-regulated in lignifying vascular tissues. These data strongly support thatKNOPE1prevents cell lignification by repressing lignin genes during peach stem primary growth.