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Plant and Cell Physiology Advance Access originally published online on March 22, 2009
Plant and Cell Physiology 2009 50(5):935-946; 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 email: journals.permissions@oxfordjournals.org

Decreased L-Ascorbate Content Mediating Bolting is Mainly Regulated by the Galacturonate Pathway in Oncidium

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

Plant Biology, Life Science, No. 1, Sec. 4, Roosevelt Road, Taipei, 10617 Taiwan

*Corresponding author: E-mail, ykwbppp{at}ntu.edu.tw; Fax, + 886-2-23918940.


  Abstract

We investigated the alteration in L-ascorbate (AsA, reduced form) content and the expression pattern of its related genes during the phase transition in Oncidium orchid. During the vegetative growth, a high H2O2 level was associated with a high content of the reduced form of AsA. During the bolting period, the AsA content and H2O2 level were greatly reduced in parallel with increased expression of OgLEAFY, the gene encoding a key transcription factor integrating different flowering-inducing pathways. This observation suggests that reduced AsA content, due to it having been 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 the products of relevant genes of the galacturonate (GalUA) pathway, such as polygalacturonase (OgPG), pectin methylesterase (OgPME) and galacturonate reductase (OgGalUAR), were markedly decreased during the bolting period, as compared with during the vegetative stage. However, the genes whose products were involved in the Smirnoff–Wheeler pathway retained a similar expression level in the two growth stages. The data suggested that OgPME of the GalUA pathway was the pivotal gene in regulating AsA biosynthesis during the bolting period. Further elucidation by overexpressing OgPME in Arabidopsis demonstrated a considerable increase in 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 its 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

Abbreviations: AIR, alcohol-insoluble residue; AO, ascor-bate oxidase; APX, ascorbate peroxidase; AsA, L-ascorbate; B, bolting period; CaMV, cauliflower mosaic virus; DAB, 3,3'-diaminobenzidene; DAS, days after sowing; DTT, dithiothreitol; GalDH, L-galactose dehydrogenase; GalL, L-galactono-1,4-lactone; GalLDH, GalL dehydrogenase; GalUA, D-galacturonate; GalUAR, galacturonate reductase; GMP, GDP-D-mannose pyrophosphorylase; PG, polygalactu-ronase; PME, pectin methylesterase; PVPP, polyvinylpoly-pyrrolidone; R, reproductive stage; RACE, rapid amplification of cDNA ends; ROS, reactive oxygen species; TCA, tricarboxylic acid; V, vegetative stage

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