Possible Involvement of CS-ACS1 and Ethylene in Auxin-induced Peg Formation of Cucumber Seedlings

Abstract

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.

p-Chlorophenoxyisobutyric acid impairs auxin response for gravity-regulated peg formation in cucumber (Cucumis sativus) seedlings

“…When exogenous auxin is applied to whole seedlings during seed germination and early growth, the seedlings form a peg, not only on the lower side but also on the upper side of the transition zone, even when the seeds are placed in a horizontal position (Witztum and Gersani 1975;Kamada et al 2000). We have shown that the mRNAs of auxininducible genes such as CsIAA1 and CS-ACS1 are more abundant in the lower side than in the upper side of the transition zone when cucumber seedlings are grown in a horizontal position (Fujii et al , 2007Kamada et al 2000;Saito et al 2004Saito et al , 2005. Moreover, we have identified CsGRP1 as a gene whose mRNA is more abundant in the upper side than in the lower side of the transition zone in cucumber seedlings grown in a horizontal position, and we have revealed that auxin application decreases this mRNA accumulation (Shimizu et al 2006).…”

Differential accumulation of the mRNA of the auxin-repressed gene CsGRP1 and the auxin-induced peg formation during gravimorphogenesis of cucumber seedlings

“…It has been suggested that increases in the mRNA levels of auxin-inducible genes such as CsIAA1 and CS-ACS1 in the lower side of the transition zone in cucumber seedlings grown in a horizontal position could account for asymmetrical peg formation Kamada et al 2000;Saito et al 2005). However, Fig.…”

“…It has been described that the mRNAs of auxin-inducible genes such as CsIAA1 and CS-ACS1 accumulate more in the 1 mm (a-d, g), 0.1 mm (e, f, h, i). g direction of gravity lower side than in the upper side of the transition zone of cucumber seedlings grown in a horizontal position Kamada et al 2000;Saito et al 2005). Usually, the eVects of auxin application on the mRNA accumulation of auxin-inducible genes have been indicated using excised segments to exclude the eVects of endogenous auxin synthesized in apical meristem and/or cotyledons (Theologis et al 1985).…”

“…When cucumber seedlings are grown in a vertical position or under microgravity conditions, both sides of the transition zone maintain enough auxin to induce peg formation . Accordingly, we have found that auxin-inducible genes, such as CsIAA1 and CS-ACS1, which encode the Aux/IAA protein and ACC synthase, respectively, are expressed more abundantly in the lower side than the upper side of the transition zone when cucumber seedlings are grown in a horizontal position Kamada et al 2000;Saito et al 2005). However, to obtain a further insight into the mechanism underlying gravity-regulated morphogenesis in cucurbitaceous plants, it is necessary to identify factors diVerentially expressed in the upper side of the transition zone, which could suppress peg formation following auxin redistribution.…”

Gravistimulation Changes the Accumulation Pattern of the CsPIN1 Auxin Efflux Facilitator in the Endodermis of the Transition Zone in Cucumber Seedlings

“…This gravity response of Mock 92.6 6 6.5 7.4 6 6.5 0.0 6 0.0 10 25 M TIBA 92.6 6 6.5 7.4 6 6.5 0.0 6 0.0 10 24 M TIBA 3.5 6 3.0* 96.5 6 3.0* 0.0 6 0.0 10 23 M TIBA 2.4 6 4.1* 88.4 6 4.8* 9.3 6 8.5* CsPIN1 localization occurred on the upper side of the transition zone and was consistent with the findings of spaceflight experiments . Gravistimulation is suspected to induce asymmetric auxin distribution in the transition zone and influence the location of peg formation Kamada et al, 2000Kamada et al, , 2003Saito et al, 2004Saito et al, , 2005. The application of TIBA, an inhibitor of auxin efflux, induces peg formation on the upper side of the transition zone of cucumber seedlings grown horizontally .…”