Expression analysis of key auxin synthesis, transport, and metabolism genes in different young dwarfing apple trees

Abstract

The control of scion vigor by dwarfing apple rootstocks is most convincingly elucidated by the shoot–root–shoot signaling of auxins and other hormones. To identify auxin and auxin-related genes that may play roles in the composite tree’s auxin metabolism and dwarfing mechanism, the concentrations of IAA and the expression level of key auxin synthesis, transport, and metabolism genes were measured in leaves, phloem, and roots from the dwarfing Fuji/M9 and the vigorous Fuji/MM106. The results showed that the indole-3-acetic acid (IAA) content was lower in the dwarfing Fuji/M9 than in the vigorous Fuji/MM106. The IAA content in the Fuji/M9 rootstock’s phloem was higher than that of its scion phloem. The expression level of MdYUCCA10a gene was significantly lower in the leaves and roots of Fuji/M9 than in that of the Fuji/MM106. The phloem and roots of the Fuji/M9 rootstock showed low expression levels of MdPIN1b and MdPIN8a. The auxin-conjugated genes MdGH3-5b and MdGH3-9a showed lower expression levels in the Fuji/M9 than in the Fuji/MM106. However, the Fuji/M9 showed higher levels of the auxin-conjugate hydrolase genes MdIAR3c and MdILL6c. The low expression level of auxin synthesis gene MdYUCCA10a in Fuji/M9 probably induced the low auxin level. The lower expression levels of auxin transport genes MdPIN1b and MdPIN8a in the M9 rootstock were suggested to probably contribute to auxin accumulation in Fuji/M9 rootstock phloem. The low amount of auxin transported from the shoots along with the root auxin synthesis deficiencies reduced the root growth and then decreased the supply of root-produced substances to the shoots in Fuji/M9.