Anoxia Effects on Plant Physiology - Plants and Crops Science
Abstract
The response of plants to anaerobiosis is characterized by a dramatic change in gene expression. The availability of molecular tools for genome-wide analyses of Arabidopsis thaliana seedlings transcriptome opens new opportunities for the study of plant responses to anoxia. Anoxia exerts a dramatic effect on the transcriptome. In this article, we analyze the results obtained by transcript profiling of Arabidopsis seedlings under anoxia and discuss the results in relation to the existing literature on the anaerobic response of higher plants.
INTRODUCTION
Plants are aerobic organisms requiring oxygen for their life. However, plants can experience a lower oxygen availability (hypoxia) or total absence of oxygen (anoxia) owing to flooding of the soil or as a consequence of the anatomical structure of some tissues. Plants cannot survive for long periods of time under hypoxia or anoxia, but some species can avoid or withstand anaerobiosis.
The physiological mechanism(s) allowing some plant species to survive in oxygen-deprived environments are still largely unknown. In this review, we describe some classical aspects of the physiology of plants under limited oxygen availability, highlighting new evidence of gene modulation by anoxia arising from recent microarray experiments using Arabidopsis seedlings.
ANOXIA REPRESSES LIPID METABOLISM, ENHANCES SUCROSE SYNTHASES-DRIVEN SUCROSE UTILIZATION, AND ACTIVATES ALCOHOLIC FERMENTATION
Genes coding for enzymes of the lipid degradation pathway are repressed by anoxia, mostly at the level of lipases, while the genes encoding b-oxidation and the glyoxylate cycle genes are unaffected or moderately induced by anoxia. Repression of lipases may avoid the buildup of fatty acids, which cannot be metabolized because of the inactivity of b-oxidation in the absence of molecular oxygen.
The microarray results highlight the strong induction of two sucrose synthase genes, which, together with the repression of neutral invertase (At1g35580) and the activation of an invertase inhibitor, suggests that a sucrose-synthase pathway predominates for sucrose utilization under anoxia, as previously proposed for rice seedlings. The effects of anoxia on starch synthesis genes are limited, and the pathway of starch degradation is not induced under anoxia in Arabidopsis thaliana.
Several glycolytic genes are strongly induced by anoxia. Among these genes, we find those involved in alcoholic fermentation such as alcohol dehydrogenase, pyruvate decarboxylase, and pyruvate decarboxylase. The concerted action of pyruvate decarboxylase and alcohol dehydrogenase may be unable to consume the pyruvate accumulating as a consequence of the inactivity of the Krebs cycle. The induction of an alanine aminotransferase allows the conversion of the excess pyruvate to alanine. The production of alanine is indeed relevant in rice roots, reaching up to 1.2% of the dry weight after 24 hr under anoxia.
REDOX GENES: ANOXIA INDUCES STRONGLY A NON-SYMBIOTIC HEMOGLOBIN AND ACTIVATES THE ASCORBATE–GLUTATHIONE CYCLE
Although the activation of the alcoholic fermentation will likely mitigate the excessive buildup of Nicotinamide adenine dinucleotide reduced (NADH), activation of other genes able to compensate for the redox imbalance could be of importance. Non-symbiotic hemoglobin (Ahb1, At2g16060) is the most induced gene in the group of redox genes, while a class 2 nonsymbiotic hemoglobin (At3g10520) is repressed.
Although the role of Ahb1 in the physiology of plants is still largely unknown, it has been proposed that Ahb1 may be functioning as an NADH-dependent nitric oxide (NO) oxidizing factor, producing nitrate that is, in turn, converted back to NO by nitrate reductase. Remarkably, nitrate reductase1 (NR1, At1g77760) is induced by anoxia.
Besides the important role of fermentative metabolism, the AHB/NR1-dependent NADH utilization can be useful to regenerate nicotinamide adenine dinucleotide (NAD) under anoxia. Hypoxia, as well as reoxygenation, affects the production of reactive oxygen species (ROS), and the production of antioxidant molecules is likely of importance for survival. The ascorbate–glutathione cycle (Halliwell-Asada pathway) may operate to reduce hydrogen peroxide (H2O2). The genes involved in this pathway are modulated by anoxia, with induction of ascorbate peroxidase, monodehydroascorbate reductase (At3g09940), dehydroascorbate reductase, and glutathione reductase
LACK OF OXYGEN AFFECTS HORMONE SYNTHESIS AND SIGNALING
A low-oxygen environment induces the production of ethylene in several plant species, where it plays a role in petiole/internodes elongation and/or in aerenchyma and adventitious root formation. Ethylene production under anoxia is prevented by the lack of oxygen, required for the conversion of 1-aminocyclopropane1-carboxylic acid (ACC) to ethylene, while production of this hormone is observed under hypoxia.
Loretietal. show that none of the 11 genes coding for 1-aminocyclopropane-1-carboxylic acid synthase (ACS) or ACS-like proteins are induced by anoxia in Arabidopsis. On the contrary, a gene coding for an ACC oxidase (Aco, At2g19590) is induced by anoxia. Remarkably, a number of genes involved in ethylene sensing and signal transduction are induced by anoxia. Flooding induces an AtEtr1 homologue in Rumex, and our results show that the Etrgene is induced by anoxia.
Induction of Etr2 by hypoxia in Arabidopsis root cultures has also been reported by Kloketal. Eight genes coding for putative ethylene responsive elements are also induced by anoxia. Induction of these genes by anoxia, when ethylene cannot be synthesized, rules out the possible induction by hypoxia-triggered ethylene synthesis. As outlined by Vriezenetal, induction of elements involved in the synthesis, perception, and/or signaling pathway may enhance the response to ethylene, produced during flooding or after flooding, when oxygen is available and may drive a burst of ethylene production during postanoxia.
Anoxia plays a negative role on the physiology of auxin. Among the repressed genes, there are four genes coding for proteins involved in auxin transport. Repression of auxin transport is indeed of importance for flooding-dependent adventitious root formation, as proposed by Visser et al. A large group of genes encoding auxin-regulated proteins is downregulated by anoxia. Abscisic acid (ABA) signaling appears to be activated under anoxia.
ABA level increases in flooded tomato plants, although no increase in ABA levels was detected in Arabidopsis under hypoxia. It has been proposed that ABA may contribute to the induction of alcohol dehydrogenase (ADH) and, indeed, exogenous ABA does increase the Adh transcript level. However, ADH induction by hypoxia is retained in ABA-insensitive mutants, suggesting that distinct signaling pathways control the induction of ADH by ABA and hypoxia. ADH is also induced by cold stress, and the cold and ABA-inducible protein Kin1 is induced under anoxia. Gibberellins may play a role in petiole elongation under anoxia, as proposed by Rijndersetal. in Rumex species. The impact of anoxia on the physiology of gibberellins in Arabidopsis seedlings appears however to be limited.
Cytokinins (CK) may play a role in flooding tolerance. Transgenic Arabidopsis plants expressing an Agrobacterium CK biosynthetic gene controlled by the senescence-specific SAG12 promoter tolerate flooding and submergence better than wild-type plants. The SAG12 promoter is activated in Arabidopsis leaves from flooded plants, but the 6 hr anoxia treatment we used did not affect its expression, indicating that SAG12 is activated as a consequence of the flooding-induced senescence rather than by oxygen absence. A limited number of genes involved in CK physiology are modulated under anoxia. A small number of genes involved in brassinosteroids, jasmonate, and salicylate physiology are induced by anoxia. Involvement of these signaling molecules under anoxia has not been reported before, with the exception of the microarray study by Kloketal, showing the induction of jasmonate and brassinosteroid-related genes.
CONCLUSIONS
We reported an overview of the effects of anoxia on the Arabidopsis transcriptome. Some results, as expected, confirm the available knowledge about the anaerobic response of plants, with a clear induction of previously identified anaerobic proteins such as sucrose synthases and genes involved in alcoholic fermentation. Besides the classical view of anoxic metabolism through fermentation, it has to be emphasized that a large number of genes not related to this pathway are remarkably affected by anoxia. However, most of these genes encode proteins of unknown function and an effort to identify the function of these genes will hopefully lead to a more complete understanding of plant responses to anaerobiosis.