HIMyb3, a putative regulatory factor in hop (Humulus lupulus L.), shows diverse biological effects in heterologous transgenotes

A hop-specific cDNA library from glandular tissue-enriched hop cones was screened for Myb transcription factors. cDNA encoding for R2R3 Myb, designated HIMyb3, was cloned and characterized. According to the amino acid (aa) sequence, HIMyb3 shows the highest homology to GhMyb5 from cotton and is unrelated to the previously characterized HIMyb1 from the hop. Southern blot analyses indicated that HIMyb3 is a unique gene, which was detected in various Humulus lupulus cultivars, but not in Humulus japonicus. Reverse transcription and real-time PCR revealed the highest levels of HIMyb3 mRNA in hop cones at a late stage of maturation and in colored petiole epidermis, while the lowest levels were observed in hop flowers. Two alternative open reading frames starting in the N-terminal domain of HIMyb3, encoding for proteins having 269 and 265 amino acids with apparent molecular masses of 30.3 and 29.9 kDa, respectively, were analyzed as transgenes that were overexpressed in Arabidopsis thaliana, Nicotiana benthamiana, and Petunia hybrida plants. Transformation with the longer 269 aa variant designated I-HIMyb3 led to a flowering delay and to a strong inhibition of seed germination in A. thaliana. Nearly complete flower sterility, dwarfing, and leaf curling of P. hybrida and N. benthamiana I-HIMyb3 transgenotes were noted. On the contrary, the shorter 265-aa-encoding s-HIMyb3 transgene led in A. thaliana to the stimulation of initial seed germination, to fast initiation of the lateral roots, and to quite specific branching phenotypes with many long lateral stems formed at angles near 90 degrees. Limited plant sterility but growth stimulation and rather branched phenotypes were evident for s-HIMyb3 transgenotes of P. hybrida and N. benthamiana. It was found that both HIMyb3 transgenes interfere in the accumulation and composition of flavonol glycosides and phenolic acids in transformed plants. These effects on heterologous transgenotes suggest that the HIMyb3 gene may influence hop morphogenesis, as well as metabolome composition during lupulin gland maturation.