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Plant and Cell Physiology Advance Access published online on March 22, 2009
Plant and Cell Physiology, doi:10.1093/pcp/pcp045
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© The Author 2009. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Decreased L-ascorbate content mediating bolting is mainly regulated by the galacturonate pathway in Oncidium

Shen Chin-Hui, Krishnamurthy Ramanarayan and Yeh Kai-Wun

Shen, Chin-Hui; Plant Biology, Life Science; No. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan (R.O.C). Krishnamurthy, Ramanarayan; Plant Biology, Life Science; No. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan (R.O.C). Yeh, Kai-Wun; Plant Biology, Life Science; No. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan (R.O.C).

Corresponding author: Prof. Yeh, Kai-Wun; Address: No. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan (R.O.C); TEL: 886-2-33662536; FAX: 886-2-23918940; e-mail: ykwbppp{at}ntu.edu.tw


  Abstract

We investigated the alteration in L-ascorbate (AsA, reduced form) content and expression pattern of its related genes during the phase transition in Oncidium orchid. During vegetative growth, a high H2O2 level was associated with a high content of the reduced form of AsA. At bolting period, AsA content and H2O2 level were greatly reduced in parallel with increased expression of OgLEAFY, the gene encoding for a key transcription factor integrating different flowering-inductive pathways. This observation suggests that reduced AsA content, consumed in scavenging H2O2, is a prerequisite for mediating the phase transition in Oncidium. A survey of the AsA biosynthetic pathway revealed that the gene expression and enzymatic activities of relevant genes of the galacturonate (GalUA) pathway, such as Polygalacturonase (OgPG), Pectin methylesterase (OgPME) and Galacturonate reductase (OgGalUAR), were markedly decreased at bolting period, as compared with those at the vegetative stage. However, the genes involved in the Smirnoff-Wheeler pathway retained a similar expression level between the two growth stages. The data suggested that OgPME of the GalUA pathway was the pivotal gene in regulating AsA biosynthesis at bolting period. Further elucidation by overexpressing OgPME in Arabidopsis demonstrated a considerable increase of AsA content, as well as a resulting delayed-flowering phenotype. Our results strongly imply that the reduced level of AsA, regulating bolting for phase transition, resulting in part from the consumption by scavenging H2O2, was mainly caused by the down-regulation of the GalUA pathway, not the Smirnoff-Wheeler pathway.

Keywords: L-ascorbic acid (AsA) - Bolting period - GalUA pathway - H2O2 - Pectin methylesterase (PME) - Smirnoff-Wheeler pathway

(Received February 17, 2009; Accepted March 17, 2009)
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