Paclobutrazol elevates auxin and abscisic acid, reduces gibberellins and zeatin and modulates their transporter genes in Marubakaido apple (Malus prunifolia Borkh. var. ringo Asami) rootstocks.

To examine the dwarfing mechanism in apples, one-year-old Marubakaido (Malus prunifolia Borkh.) (invigorating) apple rootstock stools were foliar-sprayed with 860 mg L-1 of paclobutrazol (PBZ) as a single application or without. M.9 apple rootstock (dwarf) was used as a positive control. The phytohormones were estimated in the shoot bark and sub-apical shoot and gene expression in the apices of terminal shoots. Evident responses to PBZ were observed a fortnight after treatment, as the shoot and internode lengths were suppressed significantly. Endogenous indole-3-acetic acid increased in the PBZ treatment, and the polar auxin transporter genes MdPIN1 and MdLAX1 and the biosynthesis gene MdYUCCA10a were upregulated along with the MdARF2 gene. Additionally, PBZ increased the abscisic acid (ABA) concentration and the biosynthesis-related gene MdNCED1 but repressed the degradation gene MdCYP707A1. The ABA transporter gene MdAITb-like was upregulated by PBZ. The concentrations of the gibberellins (GAs) GA1 and GA4 decreased in the PBZ-treated rootstocks. The GA transporter gene MdNFP3.1-like and the signaling gene MdGID1b-like were strongly downregulated by PBZ, whereas the catabolic gene MdGA2OX2 was upregulated. PBZ treatment significantly reduced trans-zeatin (tZ) levels and downregulated the cytokinin biosynthesis gene MdIPT6 but upregulated the MdCKX7 degradation gene. Additionally, PBZ upregulated the cytokinin-related transporter genes MdPUP7-like and MdPUP9-like. Collectively, our results show that the physiological and molecular effect of PBZ was observed within two weeks, and this was indicated by the modulation of phytohormonal levels as well as transporter and other gene expression in Marubakaido apple rootstocks.