Auxin is required for pollination-induced ovary growth in Dendrobium orchids

In Dendrobium and other orchids the ovule becomes mature long after pollination, whereas the ovary starts growing within two days of pollination. The signalling pathway that induces rapid ovary growth after pollination has remained elusive. We placed the auxin antagonist α-(p-chlorophenoxy) isobutyric acid (PCIB) or the auxin transport inhibitor 2,3,5-triiodobenzoic acid (TIBA) on the stigma, before pollination. Both treatments nullified pollination-induced ovary growth. The ovaries also did not grow after similar stigma treatment with 1-methylcyclopropene (1-MCP), AgNO₃ (both inhibitors of ethylene action), aminooxyacetic acid (AOA) or CoCl₂ (which both inhibit ethylene synthesis), before pollination. Pollination could be replaced by placement of the auxin naphthylacetic acid (NAA) on the stigma. All mentioned inhibitors nullified the effect of NAA, indicating that if auxin is the initiator of ovary growth, it acts through ethylene. The results suggest that the pollination effect on ovary growth requires auxin (at least auxin transport and maybe also auxin signalling), and both ethylene synthesis and ethylene action.