THE KNOTTED-LIKE KNOPE1 GENE REGULATES STEM ELONGATION AND LIGNIFICATION DURING PRIMARY GROWTH OF PEACH STEM (PRUNUS PERSICA L. BATSCH).

The KNOTTED-like genes (KNOX) encode homeodomain transcription factors and regulate several processes of plant organ development. The peach (Prunus persica L. Batsch) KNOPE1 was assessed to link to a QTL for the internode length in the Peach x Ferganensis population. The KNOPE1 expression decreased progressively from stem primary (elongation) to secondary growth (radial expansion) of adult shoots. During primary growth, the KNOPE1 mRNA was localised in the cortex and in the procambium/metaphloem zones, whereas it was undetected in incipient phloem and xylem fibres. KNOPE1 over-expression in the arabidopsis bp4 loss-of-function background (35S:KNOPE1/bp genotype) restored the rachis length, suggesting, together with the QTL association, a role for KNOPE1 in peach shoot elongation. Several lignin biosynthesis genes were up-regulated in the bp4 internodes but repressed in the 35S:KNOPE1/bp lines similarly to the wild type. Moreover, the lignin deposition pattern of 35S:KNOPE1/bp and the wild type internodes were the same. The KNOPE1 protein was found to recognize in vitro one of the typical KNOX-DNA binding sites that recurred in peach and arabidopsis lignin genes. KNOPE1 expression 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 that KNOPE1 prevents cell lignification by repressing lignin genes during peach stem primary growth