Possible involvement of CS-ACS1 and ethylene in auxin-induced peg formation of cucumber seedlings

• Background and Aims: Cucumber (Cucumis sativus) seedlings develop a peg on the concave side of the gravitropically bending transition zone between the hypocotyl and the root after seed germination. Peg initiation occurs in response to auxin when its levels in the concave side of the transition zone exceed a particular threshold through the graviresponse. Ethylene also plays an important role in peg formation, but its relationship to auxin in this event is not understood. Here, the role ethylene plays in auxin-induced peg formation is studied. • Methods: Peg formation of cucumber seedlings exposed to ethylene at different stages of growth or during exogenous auxin treatment was observed. In addition, ethylene evolution from the concave and convex sides of the transition zone was compared and their transcription of CS-ACS (1-aminocyclopropane-1-carboxylic acid synthase) genes was analysed by RT-PCR and in situ hybridization. • Key Results: Seedlings treated with ethylene after peg initiation produced an enlarged peg, whereas ethylene treatment before peg initiation inhibited peg formation. Ethylene also promoted the development of the peg in the auxin-treated seedlings. Furthermore, the concave side of the transition zone at peg initiation produced more ethylene and CS-ACS1 mRNA than the convex side. • Conclusions: Since CS-ACS1 is an auxin-inducible gene, the greater abundance of auxin in the concave side of the transition zone causes peg initiation and increases CS-ACS1-mediated ethylene biosynthesis, which then facilitates peg development.