Ascorbate Oxidase-Dependent Changes in the Redox State of the Apoplast Modulate Gene Transcript Accumulation Leading to Modified Hormone Signaling and Orchestration of Defense Processes in Tobacco  

Abstract

The role of the redox state of the apoplast in hormone responses, signaling cascades, and gene expression was studied in transgenic tobacco (Nicotiana tabacum) plants with modified cell wall-localized ascorbate oxidase (AO). High AO activity specifically decreased the ascorbic acid (AA) content of the apoplast and altered plant growth responses triggered by hormones. Auxin stimulated shoot growth only when the apoplastic AA pool was reduced in wild-type or AO antisense lines. Oxidation of apoplastic AA in AO sense lines was associated with loss of the auxin response, higher mitogen-activated protein kinase activities, and susceptibility to a virulent strain of the pathogen Pseudomonas syringae. The total leaf glutathione pool, the ratio of reduced glutathione to glutathione disulfide, and glutathione reductase activities were similar in the leaves of all lines. However, AO sense leaves exhibited significantly lower dehydroascorbate reductase and ascorbate peroxidase activities than wild-type and antisense leaves. The abundance of mRNAs encoding antioxidant enzymes was similar in all lines. However, the day/night rhythms in the abundance of transcripts encoding the three catalase isoforms were changed in response to the AA content of the apoplast. Other transcripts influenced by AO included photorespiratory genes and a plasma membrane Ca 21 channel-associated gene. We conclude that the redox state of the apoplast modulates plant growth and defense responses by regulating signal transduction cascades and gene expression patterns. Hence, AO activity, which modulates the redox state of the apoplastic AA pool, strongly influences the responses of plant cells to external and internal stimuli.

A Key Role for Apoplastic H2O2 in Norway Spruce Phenolic Metabolism

“…2). Furthermore, the results are in accordance with those obtained in transgenic tobacco with a modified apoplastic redox state (Pignocchi et al, 2006) and are supported by the phenolic analyses presented here. The observations corroborate that the detected effects were due to H 2 O 2 scavenging.…”

“…Interestingly, in spruce cells, the modulation of the longer term redox status of the apoplast had a profound influence, not only on apoplastic metabolism but on whole cellular metabolism. Our results are in accordance with those obtained with transgenic tobacco with modified ascorbate oxidase levels in the apoplast: the oxidation state of the apoplast had an effect on gene expression and hormone signaling, affecting growth and susceptibility to pathogens (Pignocchi et al, 2006). It is tempting to speculate that H 2 O 2 present at micromolar concentrations in the culture medium of lignin-forming cultures (Kärkönen and Fry, 2006;Kärkönen et al, 2009) is perceived by plasma membrane-located receptors (e.g.…”

Ascorbate oxidase: The unexpected involvement of a ‘wasteful enzyme’ in the symbioses with nitrogen-fixing bacteria and arbuscular mycorrhizal fungi

“…It is especially noteworthy that AO is potentially able to regulate not only oxygen, but also AsA availability. Studies by the Foyer group [12] highlighted that controlling AsA extracellular levels may have a regulatory role in intercellular communication, which is probably essential in meristem tissues.…”

“…Gene expression analysis showed that the AO gene is induced by light [10] and [11]. Further studies on the effects of AO overexpression in tobacco [12] revealed that the enzyme affects the apoplast redox state, activating a signal transduction mechanism affecting the expression of relevant genes involved in plant growth and defense responses.…”

Auxin-Responsive Genes AIR12 Code for a New Family of Plasma Membrane b-Type Cytochromes Specific to Flowering Plants

“…Maintenance of the apoplastic ascorbate pool depends on transport systems of the plasma membrane (PM; Horemans et al, 2000). The redox state of the ascorbate in the apoplast is relatively flexible and typically more oxidized than in the symplast (Cordoba-Pedregosa et al, 2003de Pinto and De Gara, 2004;Padu et al, 2005;Pignocchi et al, 2006). Ascorbate oxidation can be effected enzymatically by ascorbate oxidase or ascorbate peroxidase, and nonenzymatically by direct interaction with ROS (including ozone; Sandermann, 2008), transition metals (e.g.…”