Phytohormone profiles of sterile Douglas-fir mutants and the responses to stem-injected gibberellins

Significant differences in phytohormone profiles existed among the normal tree and two sterile mutant trees in interior Douglas-fir. Stem injection of gibberellin A4 and A7 induced male cone formation in one of the two genotypes of sterile trees. Two sterile genotypes of interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) were analyzed. The phenotype of these mutant trees is characterized by compact internodes, stunted vegetative growth and an inability to initiate male or female cone buds. Long shoots were sampled at the stage prior to cone bud initiation in both sterile and wild type trees. Multiple phytohormones and their selected metabolites were analyzed using high-performance liquid chromatography–electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS). Compared with concentrations in wild-type samples, levels of total quantifiable cytokinins were about half (51.9%) in samples of sterile trees. In particular, levels of both trans-zeatin riboside and isopentenyl adenosine were lower. Concentrations of abscisic acid (ABA)-related metabolites, especially ABA-glucose ester, were higher (1.4-fold) in sterile trees. Total quantifiable gibberellins were about one-third (28.3%) lower in the sterile trees: concentrations of GA34 were significantly lower. There were no significant differences in concentrations of indole-3-acetic acid (IAA) or its conjugated form, IAA-aspartate. After stem injection of GA4+7, male cone formation was induced in one of the sterile genotypes. Gibberellin injection also caused the vegetative growth phenotype to change. Branches expanded to approximately five times their previous size in years past and their internodes showed more normal spacing. With their unique phytohormone profiles, these sterile mutants of Douglas-fir could provide a valuable research system for further exploration of molecular and physiological regulations during conifer reproduction.