Auxin Receptor TRANSPORT INHIBITOR RESPONSE1 Promotes Tulip Plant Growth and Bulb Swelling

Tulips are mainly asexually propagated by bulbs that are modified underground stems. Hormones, including auxin, play pivotal roles during plant development. In this study, we investigated the roles of the auxin receptor TRANSPORT INHIBITOR RESPONSE1 (TgTIR1) in tulip bulb growth. TgTIR1 promoter activity analysis showed that it is most abundant in fast growing tissues including hypocotyl, cotyledons, and lateral root in Arabidopsis. Expression of TgTIR1 in Arabidopsis activated lateral organ boundary domain (LBD) transcription factors to promote the outgrowth of axillary branches and lateral roots. TgTIR1 transgenic plants also exhibited increased primary root length, plant height and leaf width. Moreover, downregulation of endogenous TgTIR1 through virus-induced gene silencing (VIGS) inhibited the growth of tulip plants and daughter bulbs. Exogenous auxin promoted while the auxin transport inhibitor 1-N-naphthylphthalamic acid (NPA) repressed tulip daughter bulb growth. This study offered evidence that the auxin pathway was involved in lateral root initiation and tulip bulb growth.