Zeatin accumulation and misexpression of a class I knox gene are intimately linked in the epiphyllous response of the interspecific hybrid EMB-2 (Helianthus annuus x H. tuberosus)

Epiphylly, occurring in a somaclonal variant(EMB-2) of the interspecific hybrid Helianthus annuus· H. tuberosus, was used to investigate molecular andcyto-physiological mechanisms that underlie cellular fatechange. EMB-2 plants are characterized by profuseproliferation of shoot- and embryo-like structures onsome leaves. We addressed the putative relationshipbetween cytokinins and knox genes in EMB-2 plants. Aclass I knox gene, HtKNOT1, was isolated from H. tuberosus.A high level of HtKNOT1 transcripts was detectedin EMB-2 epiphyllous leaves compared to nonepiphyllous(NEP) ones. In addition, epiphylly was relatedto a localized increases in zeatin and N-glycosylatedcytokinins. As ectopic morphogenesis proceeded,HtKNOT1 transcripts and zeatin co-localized andshowed different patterns in ectopic shoot comparedwith embryo-like structures, consistent with the differentialrole of both cytokinin and knox genes in the twomorphogenetic events. Notably, a massive shoot/embryoregeneration was induced in EMB-2 NEP leaves byin vitro zeatin treatment. These results clearly indicatethat localized cytokinin accumulation and ectopicexpression of HtKNOT1 are closely linked in the epiphyllyof EMB-2 plants.