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

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.

Hydrogen peroxide mediates the expression of ascorbate-related genes in response to methanol stimulation in Oncidium

“…In several model systems of plants, the oxidative burst and accumulation of H 2 O 2 appear to be mediated by the activation of a membrane-bound NADPH oxidase complex (Zhang et al, 2007;Königshofer et al, 2008;Wen et al, 2008). In the Oncidium system, H 2 O 2 induction has been identified as a signal to induce AsA-related genes during vegetative growth (Shen et al, 2009 OgMDHAR, are essential for scavenging ROS. OgSOD can function in dismutating superoxide (which was produced by NADPH oxidase) into H 2 O 2 (Mittler et al, 2004).…”

“…The activities of PG, PME, GalUAR, GMP, GalDH, GalLDH and APX were assayed following the method described by Shen et al (2009). The activities of MDHAR and superoxide dismutase (SOD; EC: 1.15.1.1) were measured following modified methods of Eltayeb et al (2007) and Paoletti et al (1986), respectively.…”

The signal network of ascorbate homeostasis

“…9 In addition to that, the transgenic Arabidopsis overexpressing OgPME (PECTIN METHYLESTERASE) displayed a delayed flowering time is associated with the higher ascorbate level than wild-type plant. 10 Therefore, methanol is potential to affect the phase transition because the elevated ascorbate level at the bolting period. Similarly, pectin amount is also less at the bolting period and proposed that hydrogen peroxide generated from the digestion and demethylation of pectin in cell wall is not sufficient to elicit the expressions of ascorbate-related genes.…”

“…15 The late-flowering phenotype of vtc1 in short day photoperiod was associated with the increasing ABA that is known to act antagonistically to GA. 13,16 inducing the expression level of ascorbaterelated genes, including Smirnoff-Wheeler pathway and galacturonate pathway biosynthesis such as ASCORBATE PEROXIDASE, ASCORBATE OXIDASE and MONODEHYDROASCORBATE REDUCTASE in defense system. 10 Subsequently, the expression level of OgLEAFY which controls the development of inflorescence is downregulated by high ascorbate level. Oncidium containing higher concentration of endogenous ascorbate displayed a longer vegetative stage (LV) (Fig.…”

“…Likewise the exogenous ascorbate treatment would abolish the bolting of Oncidium. 10 Thus, increased ascorbate at vegetative stage not only functions on scavenging the hydrogen peroxide but also limits the phase transition. 12,13 At the bolting period, the ascorbate level in pseudobulb is obviously lower than that at vegetative stage.…”