Abstract
Reorientation of cucumber seedlings induces re-localization of CsPIN1 auxin efflux carriers in endodermal cells of the transition zone between hypocotyl and roots. This study examined whether the re-localization of CsPIN1 was due to the graviresponse. Immunohistochemical analysis indicated that, when cucumber seedlings were grown entirely under microgravity conditions in space, CsPIN1 in endodermal cells was mainly localized to the cell side parallel to the minor axis of the elliptic cross-section of the transition zone. However, when cucumber seeds were germinated in microgravity for 24 h and then exposed to 1g centrifugation in a direction crosswise to the seedling axis for 2 h in space, CsPIN1 was re-localized to the bottom of endodermal cells of the transition zone. These results reveal that the localization of CsPIN1 in endodermal cells changes in response to gravity. Furthermore, our results suggest that the endodermal cell layer becomes a canal by which auxin is laterally transported from the upper to the lower flank in response to gravity. The graviresponse-regulated re-localization of CsPIN1 could be responsible for the decrease in auxin level, and thus for the suppression of peg formation, on the upper side of the transition zone in horizontally placed seedlings of cucumber.
Highlights
Plants respond to gravity by changing their growth orientation and morphology.[1,2] The formation of a specialized protuberance, the peg, in cucurbitaceous seedlings is a unique gravimorphogenesis.[2,3]When cucumber seeds (Cucumis sativus L.) are placed in a horizontal position and allowed to germinate, a peg forms on the lower side of the transition zone between the hypocotyl and the root
We previously reported that, when 24-h-old cucumber seedlings grown vertically were placed in a horizontal position, the localization of CsPIN1 in the endodermal cell layer of the transition zone changed.[6]
Previously, we reported that CsPIN1 in the endodermal cells relocalize to the bottom side upon reorientation of the seedlings, which is observable on the upper endodermal cells in the transition zone between the hypocotyl and the root of the horizontally placed seedlings of cucumber.[6]
Summary
Plants respond to gravity by changing their growth orientation and morphology.[1,2] The formation of a specialized protuberance, the peg, in cucurbitaceous seedlings is a unique gravimorphogenesis.[2,3]. When cucumber seeds (Cucumis sativus L.) are placed in a horizontal position and allowed to germinate, a peg forms on the lower side of the transition zone between the hypocotyl and the root. Cucumber seedlings have the potential to develop a peg on each side of the transition zone as, when seeds are placed before germination in a vertical position with the radicles pointing down or under microgravity conditions, a peg develops on each side.[4] peg formation on the upper side of the transition zone is suppressed in response to gravity when the seedlings are grown in a horizontal position on the ground.[4]
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