Expression of almond Knotted1 homologue (PdKn1) anticipates adventitious shoot initiation

The transcription factor encoded by the gene Knotted1 is a nuclear homeodomain protein, regulating meristematic cells at the shoot apical meristem. It has been demonstrated that Knotted1 (KN1) expression specifies stem cell fate in adventitious shoot induction in herbaceous plants. This gene may thus potentially identify the initiation of meristem development in adventitious shoot induction in difficult-to-regenerate plants such as Prunoideae. We isolated an almond (Prunus dulcis Mill.) KN1-type gene using degenerate primers targeting the most conserved regions of Knotted1 gene. The 5′ and 3′ ends of the isolated sequence were obtained by rapid amplification of cDNA ends-polymerase chain reaction (RACE-PCR), and the gene was named P. dulcis Knotted1-like (PdKn1). PdKn1 transcripts were detected by reverse transcription (RT)-PCR mainly in shoot apical and axillary meristems. The RT-PCR and RT-quantitative PCR PdKn1 expression in almond leaf explants was found to anticipate the organization of adventitious shoot meristems. Apricot RNA isolated from induced leaf explants cross-hybridized with the almond probe PdKn1 in Northern blotting. We suggest that almond PdKn1 may be a useful marker to study the adventitious regeneration system by revealing the potential organogenic conditions, not only in almond but also in other Prunoideae.