Enhanced gibberellin catabolism promotes somatic embryo induction from spinach apical root sections

Abstract

Gibberellins (GA) are essential for induction of somatic embryogenesis from spinach apical root sections. To elucidate the role of GA in this process, expression of genes encoding enzymes that catalyze the final step of GA20 oxidation to bioactive GA, GA3-oxidase (GA3-ox), and GA degradation, GA2-oxidase (GA2-ox), was assessed. In spinach, there is only one GA3-ox and three GA2-ox (GA2ox 1, GA2 ox2, GA2 ox3). Expression of these genes was tested in the explants cultivated on noninductive medium (NM), supplemented with 20 μM NAA, inductive medium (IM), containing 20 μM NAA + 5 μM GA3, or plant growth regulator (PGR)-free medium during a 28 d induction period. Root-tips isolated from seedlings and immediately frozen for RNA isolation were used as a control. In the explants cultivated on PGR-free medium, expression of GA3-ox increased gradually up to 10-fold and was constantly higher than in control. GA2-ox1, with an increase of up to 60-fold, was the most highly expressed GA2-ox, while the expression of GA2-ox2 and GA2-ox3 only slightly increased compared to the control. In the explants cultivated on NM, expression of GA3-ox decreased slightly until the 7th day of cultivation and then increased up to 2-fold until the end of the experiment, while the expression of GA2-ox1 and GA2-ox2 was only slightly higher than in control. However, explants cultivated on IM showed the constant and significant decrease of GA3-ox (down to 7-fold) and increase of GA2-ox2 (up to 20-fold) expression. Here, we propose that continuous decrease in GA3-ox and increase in GA2-ox2 expression were favorable conditions for somatic embryo induction.