Peach [Prunus persica (L.) Batsch] KNOPE1, a class 1 KNOX orthologue to Arabidopsis BREVIPEDICELLUS/KNAT1, is misexpressed during hyperplasia of leaf curl disease

Class 1 KNOTTED-like (KNOX) transcription factorscontrol cell meristematic identity. An investigation wascarried out to determine whether they maintain thisfunction in peach plants and might act in leaf curlinesscaused by the ascomycete Taphrina deformans.KNOPE1 function was assessed by overexpression inArabidopsis and by yeast two-hybrid assays withArabidopsis BELL proteins. Subsequently, KNOPE1mRNA and zeatin localization was monitored duringleaf curl disease. KNOPE1 and Arabidopsis BREVIPEDICELLUS(BP) proteins fell into the same phyleticgroup and recognized the same BELL factors.35S:KNOPE1 Arabidopsis lines exhibited altered traitsresembling those of BP-overexpressing lines. In peachshoot apical meristem, KNOPE1 was expressed in theperipheral and central zones but not in leaf primordia,identically to the BP expression pattern. These resultsstrongly suggest that KNOPE1 must be down-regulatedfor leaf initiation and that it can control cellmeristem identity equally as well as all class 1 KNOXgenes. Leaves attacked by T. deformans share histologicalalterations with class 1 KNOX-overexpressingleaves, including cell proliferation and loss of celldifferentiation. Both KNOPE1 and a cytokinin synthesisISOPENTENYLTRANSFERASE gene were found tobe up-regulated in infected curled leaves. At earlydisease stages, KNOPE1 was uniquely triggered in thepalisade cells interacting with subepidermal mycelium,while zeatin vascular localization was unaltered comparedwith healthy leaves. Subsequently, when myceliumcolonization and asci development occurred, bothKNOPE1 and zeatin signals were scattered in sectorsof cell disorders. These results suggest that KNOPE1misexpression and de novo zeatin synthesis of hostorigin might participate in hyperplasia of leaf curldisease.