Effect of bending and the joint tree training system on the expression levels of GA3- and GA2-oxidases during flower bud development in ‘Kiyo’ Japanese plum

• Branch bending and joint tree training system influence the expression level of GA3ox and endogenous hormones contents.
• Bending and joint tree training accelerated flower buds differentiation in ‘Kiyo’ Japanese plum.
• The differentiation of meristematic apices of ‘Kiyo’ Japanese plum was examined using histological technique.

We investigated the changes in flower bud development and in the expression levels of GA3- and GA2-oxidases in 6-year-old ‘Kiyo’ Japanese plum trees. Shoot tips were sampled from May to September 2014 from trees grown using the joint tree training system (joint trees) and stand-alone, bent trees (bent trees) were compared to the upright shoots and horizontal shoots of stand-alone, non-bent trees (control trees). The expression level of the GA biosynthetic gene PslGA3ox of joint trees, bent trees, and the horizontal shoots of control trees was lower than in the upright shoots of control trees during the undifferentiation meristem apex and the beginning of differentiation stages (May–July) of flower bud development. The observed differences in the expression of the GA deactivation gene PslGA2ox were not significant. Flower bud development became visible earlier in instances of low levels of PslGA3ox expression; however, no significance was observed when we compared joint trees, bent trees, and the horizontal shoots of control trees. Moreover, low expression levels of PslGA3ox were related to low levels of active GA1, GA3, GA4, and IAA. We suggest that the joint tree training system and branch bending inhibits the expression of PslGA3ox, resulting in a decrease in active GAs. It is possible that PslGA3ox is involved in the control of flower bud development in ‘Kiyo’ Japanese plum.