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• Background and Aims Rice is one of the few crops able to withstand periods of partial or even complete submergence. One of the adaptive traits of rice is the constitutive presence and further development of aerenchyma which enables oxygen to be transported to submerged organs. The development of lysigenous aerenchyma is promoted by ethylene accumulating within the submerged plant tissues, although other signalling mechanisms may also co-exist. In this study, aerenchyma development was analysed in two rice (Oryza sativa) varieties, 'FRI3A' and 'Arborio Precoce', which show opposite traits in flooding response in terms of internode elongation and survival. • Methods The growth and survival of rice varieties under submergence was investigated in the leaf sheath of 'F RI3A' and 'Arborio Precoce'. The possible involvement of ethylene and reactive oxygen species (ROS) was evaluated in relation to aerenchyma formation. Cell viability and DNA fragmentation were determined by FDA/FM4-64 staining and TUNEL assay, respectively. Ethylene production was monitored by gas chromatography and by analysing ACO gene expression. ROS production was measured by using Amplex Red assay kit and the fluorescent dye DCFH₂-DA. The expression of APX1 was also evaluated. AVG and DPI solutions were used to test the effect of inhibiting ethylene biosynthesis and ROS production, respectively. • Key Results Both the varieties displayed constitutive lysigenous aerenchyma formation, which was further enhanced when submerged. 'Arborio Precoce', which is characterized by fast elongation when submerged, showed active ethylene biosynthetic machinery associated with increased aerenchymatous areas. 'FRI3A', which harbours the Subi A gene that limits growth during oxygen deprivation, did not show any increase in ethylene production after submersion but still displayed increased aerenchyma. Hydrogen peroxide levels increased in 'FRI' but not in 'Arborio Precoce'. • Conclusions While ethylene controls aerenchyma formation in the fast-elongating 'Arborio Precoce' variety, in 'FRI' ROS accumulation plays an important role.

Anthocyanins are water-soluble polyphenolic compounds with a high nutraceutical value. Despite the fact that cultivated tomato varieties do not accumulate anthocyanins in the fruit, the biosynthetic pathway can be activated in the vegetative organs by several environmental stimuli. Little is known about the molecular mechanisms regulating anthocyanin synthesis in tomato. Here, we carried out a molecular and functional characterization of two genes, SlAN2 and SlANT1, encoding two R2R3-MYB transcription factors. We show that both can induce ectopic anthocyanin synthesis in transgenic tomato lines, including the fruit. However, only SlAN2 acts as a positive regulator of anthocyanin synthesis in vegetative tissues under high light or low temperature conditions.

Increasing evidence shows that sugars can act as signals affecting plant metabolism and development. Some of the effects of sugars on plant growth and development suggest an interaction of sugar signals with hormonal regulation. We investigated the effects of sugars on the induction of alpha-amylase by gibberellic acid in barley embryos and aleurone layers. Our results show that sugar and hormonal signaling interact in the regulation of gibberellic acid-induced gene expression in barley grains. The induction of alpha-amylase by gibberellic acid in the aleurone layer is unaffected by the presence of sugars, but repression by carbohydrates is effective in the embryo. alpha-Amylase expression in the embryo is localized to the scutellar epithelium and is hormone and sugar modulated. The effects of glucose are independent from the effects of sugars on gibberellin biosynthesis. They are not due to an osmotic effect, they are independent of abscisic acid, and only hexokinase-phosphorylatable glucose analogs are able to trigger gene repression. Overall, the results suggest the existence of an interaction between the hormonal and metabolic regulation of alpha-amylase genes in barley grains

Sugars act as signaling molecules, whose signal transduction pathways may lead to the activation or inactivation of gene expression. Whole-genome transcript profiling reveals that the flavonoid and anthocyanin biosynthetic pathways are strongly up-regulated following sucrose (Suc) treatment. Besides mRNA accumulation, Suc affects both flavonoid and anthocyanin contents. We investigated the effects of sugars (Suc, glucose, and fructose) on genes coding for flavonoid and anthocyanin biosynthetic enzymes in Arabidopsis (Arabidopsis thaliana). The results indicate that the sugar-dependent up-regulation of the anthocyanin synthesis pathway is Suc specific. An altered induction of several anthocyanin biosynthetic genes, consistent with in vivo sugar modulation of mRNA accumulation, is observed in the phosphoglucomutase Arabidopsis mutant accumulating high levels of soluble sugars.

The metabolism of carbohydrates was investigated in rice (Oryza sativa L.) seedlings grown under anoxia. Two phases can be recognized in the utilization of carbohydrates: during the first days of germination under anoxia, the metabolism of sugars is mainly degradative, whereas after the induction of alpha-amylase (EC 3.2.1.1) has taken place, the increased presence of glucose and sucrose indicates that both starch degradation and sucrose synthesis operate. The analysis of the enzymes involved in carbohydrate metabolism indicates that anoxic rice seedlings possess a set of enzymes that allow the efficient metabolism of starch and sucrose to fructose-6-phosphate. We propose that cytosolic sucrose metabolism in anoxic rice seedlings takes place mainly through a sucrose synthase (EC 2.4.1.13) pathway with nucleoside diphosphate kinase (EC 2.7.4.6), allowing the cycling of urydilates needed for the operation of this pathway

We investigated sugar uptake and transport in rice (Oryza sativa) embryo during grain germination. Endogenous sugar levels, accumulation of starch granules, and gene expression of a rice sucrose transporter (OsSUT1) were examined using rice embryos germinated with or without exogenous sugar supply. Starch granules remarkably accumulated in the cells around vascular bundles as a consequence of the sugar taken up by the embryos, indicating that the taken-up sugars are transiently converted into starch. In situ detection for OsSUT1 mRNA indicated its localization in the phloem companion cells. Furthermore, northern-blot and in situ hybridization analyses showed that OsSUT1 expression is not detectable in embryos subjected to sugar starvation conditions, whereas its expression is enhanced by an increased endogenous sugar level. Overall results indicate that the expression of companion cell-specific sucrose transporter, OsSUT1 is regulated by the endogenous sugar status as well as light exposure.

An adequate carbohydrate supply contributes to the survival of seeds under conditions of limited oxygen availability. The amount of soluble, readily fermentable carbohydrates in dry cereal seeds is usually very limited, with starch representing the main storage compound. Starch breakdown during the germination of cereal seeds is the result of the action of hydrolytic enzymes and only through the concerted action of alpha-amylase (EC 3.2.1.1), beta-amylase (EC 3.2.1.2), debranching enzyme (EC 3.2.1.41), and alpha-glucosidase (EC 3.2.1.20) can starch be hydrolyzed completely. We present here data concerning the complete set of starch-degrading enzymes in three cereals, rice (Oryza sativa L.), which is tolerant to anaerobiosis, and wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.), which are unable to germinate under anoxia. Among the cereal seeds tested under anoxia, only rice is able to degrade nonboiled, soluble starch, reflecting the ability to degrade the starch granules in vivo. This is explained by the presence of the complete set of enzymes needed to degrade starch completely either as the result of de novo synthesis (alpha-amylase, beta-amylase) or activation of preexisting, inactive forms of the enzyme (debranching enzyme, alpha-glucosidase). These enzymes are either absent or inactive in wheat and barley seeds kept under anaerobic conditions

The great majority of seeds from higher plants fail to germinate under anoxic conditions. While the metabolic pathways operating during the aerobic process of seed germination are well described, there is only limited evidence concerning metabolic pathways operating under conditions of limited oxygen availability. This is despite the importance of carbohydrate availability for plant tolerance to anaerobic conditions. We review and discuss carbohydrate metabolism in cereal seeds and seedlings subjected to anaerobic conditions. Analysis of recent results reveals that, among cereal seeds, only rice is able to degrade the starchy endosperm. This is achieved by the concerted action of the complete set of amylolytic enzymes. In anoxic wheat and barley seeds, the enzymes involved in starch degradation are either absent or present as inactive forms. Differences are also observed in the metabolism of soluble carbohydrates and in status of related enzymes, with rice showing anoxia-enhanced activities of sucrose synthase, fructokinase. glucose-6P dehydrogenase, and nucleoside diphosphate kinase. The possible occurrence of a sucrose synthase pathway for sucrose utilization under anoxia is discussed.

Exogenous sucrose (Suc) greatly enhances anoxia tolerance of Arabidopsis (Arabidopsis thaliana) seedlings. We used the Affymetrix ATH1 GeneChip containing more than 22,500 probe sets to explore the anaerobic transcriptome of Arabidopsis seedlings kept under anoxia for 6 h in presence or absence of exogenous Suc. Functional clustering was performed using the MapMan software. Besides the expected induction of genes encoding enzymes involved in Suc metabolism and alcoholic fermentation, a large number of genes not related to these pathways were affected by anoxia. Addition of exogenous Suc mitigated the effects of anoxia on auxin responsive genes that are repressed under oxygen deprivation. Anoxia-induced Suc synthases showed a lower induction in presence of exogenous Suc, suggesting that induction of these genes might be related to an anoxia-dependent sugar starvation. Anoxic induction of genes coding for heat shock proteins was much stronger in presence of exogenous Suc. Interestingly, a short heat treatment enhanced anoxia tolerance, suggesting that heat shock proteins may play a role in survival to low oxygen. These results provide insight into the effects of Suc on the anoxic transcriptome and provide a list of candidate genes that enhance anoxia tolerance of Suc-treated seedlings.

Issue Title: Waterlogging & Salinity Tolerance. Invited papers in honour of Hank Greenway Production of α-amylase during the germination of rice grains is thought to play an important role for tolerance to anoxia of these cereal grains. Under aerobic conditions α-amylases production is enhanced in response to gibberellins produced by the embryos, but the role of these hormones is less clear under anoxia. In this paper we analysed α-amylase gene expression in a rice mutant (Tan-ginbozu) severely impaired in gibberellin biosynthesis. Expression of α-amylase genes others than the gibberellin-induced Amy1A gene is observed. The expression of the Amy3D gene, which does dot require gibberellins to be induced, is high under anoxia in the Tan-ginbozu mutant suggesting that germination under anoxia can proceed thanks to the activity of the α-amylase isoform encoded by the Amy3D gene. Amy3D gene expression is repressed in the presence of high levels of soluble carbohydrates, indicating that the anaerobic expression of this gene can be triggered by a lower carbohydrate content of rice grains kept under anoxia. Germination under anoxia of Tan-ginbozu grains can proceed even in absence of exogenously-added gibberellic acid. Overall, results indicate that gibberellins are not required for the anaerobic germination of rice grains.[PUBLICATION ABSTRACT]