Detection and validation of QTLs for green stem disorder of soybean (Glycine max (L.) Merr.).

In mechanically harvested soybean, green stem disorder (GSD) is an undesirable trait that causes green-stained seeds, which are graded lower in Japan. To obtain DNA markers for reduced GSD, we conducted a quantitative trait locus (QTL) analysis for 2 years using F4 and F5 lines from a cross between 'Suzuotome' (less GSD) and 'Fukuyutaka' (more GSD). We validated the effect of a detected QTL for GSD by first identifying F4 or F5 plants in which one or more markers in the QTL region were heterozygous. The F5 or F6 progeny of each plant was used to form a pair consisting of two groups in which the QTL region was homozygous for either the 'Suzuotome' or 'Fukuyutaka' allele in a similar genetic background, and the two groups within each pair were compared for GSD. Over 3 years of testing, the 'Suzuotome' allele of a QTL on chromosome 6 was found to reduce the level of GSD. This novel QTL was mapped to the region around DNA marker W06_0130, and was not closely linked to QTLs for important agronomic traits including yield components. Using this marker, the low level of GSD from 'Suzuotome' could be conferred to 'Fukuyutaka' or other high-GSD cultivars.

Heat stress during grain filling regulates seed germination through alterations of DNA methylation in barley (Hordeum vulgare L.).

KEY MESSAGE: Alterations in DNA methylation levels of ROS, GA and ABA related gene promoters cause transcriptional changes upon imbibition to induce seed germination in barley seeds exposed to heat stress during grain filling. Environmental changes, especially changes in temperature, during seed development affect germination in several plant species. We have previously shown that heat stress during rice grain filling alters DNA methylation, an epigenetic mark important for gene silencing, regulates transcript levels of phytohormone metabolism genes, and delays seed germination. However, whether this phenomenon is present in other plant species remained to be elucidated. In this study, we compared seeds germination of barley (Hordeum vulgare L.) plants grown at 15 °C (control) or 25 °C (heat stress) during grain filling. Heat stress during grain filling significantly promoted seed germination in comparison with the control. The phytohormone gibberellic acid (GA) and reactive oxygen species produced by NADPH oxidases promote seed germination, whereas phytohormone abscisic acid (ABA) suppresses seed germination. We found that in heat-stressed seeds, genes related to ABA biosynthesis (HvNCED1 and 2) were significantly suppressed, whereas genes related to ABA catabolism (HvABA8'OH) and GA biosynthesis (HvHA20ox, HvGA3ox), and NADPH oxidase (HvRboh) genes were significantly upregulated after imbibition. Using MeDIP-qPCR, we showed that the promoters of HvNCED were hyper-methylated, and those of HvABA8'OH1, HvABA8'OH3, HvGA3ox2, and HvRbohF2 were hypo-methylated in heat treated seeds. Taken together, our data suggest that heat stress during grain filling affects DNA methylation of germination-related genes and promotes seed germination in barley.

Mechanisms of Maturation and Germination in Crop Seeds Exposed to Environmental Stresses with a Focus on Nutrients, Water Status, and Reactive Oxygen Species.

Environmental stresses can reduce crop yield and quality considerably. Plants protect cell metabolism in response to abiotic stresses at all stages of their life cycle, including seed production. As the production of vigorous seeds is important to both yield and crop growth, we analyzed causes of yield loss and reduced grain quality in staple crops exposed to environmental stresses such as drought and temperature extremes, with a focus on the remobilization of nutrients and water status during seed filling. Because water is one of the factors that limit seed development, seeds must have mechanisms that allow them to withstand water loss during seed maturation. In addition, analysis of the effects of reactive oxygen species (ROS) on transcription regulation and signaling should help to elucidate the regulation of seed dormancy and germination. In this review, we focus on nutrient remobilization, water mobility, plant hormones (gibberellins, abscisic acid, and ethylene), and ROS in sink and source organs and describe how rice, wheat, barley, soybean, and cowpea plants control seed maturation and germination under environmental stresses.

Heat shock protein 70 is associated with duration of cell proliferation in early pod development of soybean.

Pod is an important organ for seed production in soybean. Pod size varies among soybean cultivars, but the mechanism is largely unknown. Here we reveal one of the factors for pod size regulation. We investigate pod size differences between two cultivars. The longer pod of 'Tachinagaha' is due to more cell number than in the short pod of 'Iyodaizu'. POD SIZE OF SOYBEAN 8 (GmPSS8), a member of the heat shock protein 70 (HSP70) family, is identified as a candidate gene for determining pod length in a major QTL for pod length. Expression of GmPSS8 in pods is higher in 'Tachinagaha' than 'Iyodaizu' and is highest in early pod development. The difference in expression is the result of an in/del polymorphism　which includes an enhancer motif. Treatment with an HSP70 inhibitor reduces pod length and cell number in the pod. Additionally, shorter pods in Arabidopsis hsp70-1/-4 double mutant are rescued by overexpression of GmPSS8. Our results identify GmPSS8 as a target gene for pod length, which regulates cell number during early pod development through regulation of transcription in soybean. Our findings provide the mechanisms of pod development and suggest possible strategies enhancing yield potential in soybean.

Reactive oxygen species are involved in gibberellin/abscisic acid signaling in barley aleurone cells.

Reactive oxygen species (ROS) act as signal molecules for a variety of processes in plants. However, many questions about the roles of ROS in plants remain to be clarified. Here, we report the role of ROS in gibberellin (GA) and abscisic acid (ABA) signaling in barley (Hordeum vulgare) aleurone cells. The production of hydrogen peroxide (H2O2), a type of ROS, was induced by GA in aleurone cells but suppressed by ABA. Furthermore, exogenous H2O2 appeared to promote the induction of α-amylases by GA. In contrast, antioxidants suppressed the induction of α-amylases. Therefore, H2O2 seems to function in GA and ABA signaling, and in regulation of α-amylase production, in aleurone cells. To identify the target of H2O2 in GA and ABA signaling, we analyzed the interrelationships between H2O2 and DELLA proteins Slender1 (SLN1), GA-regulated Myb transcription factor (GAmyb), and ABA-responsive protein kinase (PKABA) and their roles in GA and ABA signaling in aleurone cells. In the presence of GA, exogenous H2O2 had little effect on the degradation of SLN1, the primary transcriptional repressor mediating GA signaling, but it promoted the production of the mRNA encoding GAMyb, which acts downstream of SLN1 and involves induction of α-amylase mRNA. Additionally, H2O2 suppressed the production of PKABA mRNA, which is induced by ABA:PKABA represses the production of GAMyb mRNA. From these observations, we concluded that H2O2 released the repression of GAMyb mRNA by PKABA and consequently promoted the production of α-amylase mRNA, thus suggesting that the H2O2 generated by GA in aleurone cells is a signal molecule that antagonizes ABA signaling.

Regulation of soybean seed germination through ethylene production in response to reactive oxygen species.

BACKGROUND AND AIMS: Despite their toxicity, reactive oxygen species (ROS) play important roles in plant cell signalling pathways, such as mediating responses to stress or infection and in programmed cell death, at lower levels. Although studies have indicated that hydrogen peroxide (H(2)O(2)) promotes seed germination of several plants such as Arabidopsis, barley, wheat, rice and sunflower, the role of H(2)O(2) in soybean seed germination is not well known. The aim of this study therefore was to investigate the relationships between ROS, plant hormones and soybean seed germination. METHODS: An examination was made of soybean seed germination, the expression of genes related to ethylene biosynthesis, endogenous ethylene contents, and the number and area of cells in the root tip, using N-acetylcysteine, an antioxidant, to counteract the effect of ROS. KEY RESULTS: H(2)O(2) promoted germination, which N-acetylcysteine suppressed, suggesting that ROS are involved in the regulation of soybean germination. H(2)O(2) was produced in the embryonic axis after imbibition. N-Acetylcysteine suppressed the expression of genes related to ethylene biosynthesis and the production of endogenous ethylene. Interestingly, ethephon, which is converted to ethylene, and H(2)O(2) reversed the suppression of seed germination by N-acetylcysteine. Furthermore, morphological analysis revealed that N-acetylcysteine suppressed cell elongation at the root tip, and this suppression was also reversed by ethephon or H(2)O(2) treatments, as was the case in germination. CONCLUSIONS: In soybean seeds, ROS produced in the embryonic axis after imbibition induce the production of endogenous ethylene, which promotes cell elongation in the root tip. This appears to be how ROS regulate soybean seed germination.

Regulation of reactive oxygen species and phytohormones in osmotic stress tolerance during seed germination in indica rice.

Climate change due to global warming is now affecting agricultural production worldwide. In rice, one of the most important crops, water limitation due to irregular rainfall in rainfed lowlands during crop growth limits yield. Dry direct-sowing has been proposed as a water-efficient approach to cope with water stress during rice growth, but poor seedling establishment due to drought during germination and emergence is a problem. Here, we germinated indica rice cultivars Rc348 (drought tolerant) and Rc10 (drought sensitive) under osmotic stress induced by PEG to elucidate mechanisms of germination under drought. Rc348 had higher germination rate and germination index under severe osmotic stress of -1.5 MPa, above those of Rc10. Rc348 showed up-regulated GA biosynthesis, down-regulated ABA catabolism, and up-regulated α-amylase gene expression in imbibed seeds under PEG treatment compared to that of Rc10. During germination, reactive oxygen species (ROS) play important roles in antagonism between gibberellic acid (GA) and abscisic acid (ABA). Embryo of Rc348 treated with PEG had significantly greater expression of NADPH oxidase genes and higher endogenous ROS levels, together with significantly increased endogenous GA1, GA4 and ABA contents compared to that of Rc10. In aleurone layers treated with exogenous GA, expression of α-amylase genes was higher in Rc348 than in Rc10, and expression of NADPH oxidase genes was enhanced with significantly higher ROS content in Rc348, suggesting higher sensitivity of GA to ROS production and starch degradation in aleurone cells of Rc348. These results suggest that the osmotic stress tolerance of Rc348 is due to enhancement of ROS production, GA biosynthesis, and GA sensitivity, resulting in a higher germination rate under osmotic stress.

Health assessment of rice cultivated and harvested from plasma-irradiated seeds.

This study provides the health effects assessment of rice cultivated from plasma-irradiated seeds. The rice (Oryza sativa L.) cultivated from seeds with plasma irradiation showed a growth improvement (slope-ratios of with plasma to without plasma were 1.066, 1.042, and 1.255 for tiller, and earing, and ripening periods, respectively) and an 4% increase in yield. The cultivated rice was used for repeated oral administrations to mice for 4-week period. Distilled water and rice cultivated from seeds without plasma irradiation were also used as control. The weights of the lung, kidney, liver, and spleen, with corresponding average values of 0.22 g, 0.72 g, 2.1 g, and 0.17 g for w/ plasma group and 0.22 g, 0.68 g, 2.16 g, and 0.14 g for w/o plasma group, respectively, showing no effect due to the administration of rice cultivated from plasma-irradiated seeds. Nutritional status, liver function, kidney function, and lipid, neutral fat profiles, and glucose metabolism have no significant difference between with and without plasma groups. These results show no obvious subacute effects were observed on rice grains cultivated and harvested from the mother plant that experienced growth improvement by plasma irradiation. This study provides a new finding that there is no apparent adverse health effect on the grains harvested from the plasma-irradiated seeds.

Detection of NO3- introduced in plasma-irradiated dry lettuce seeds using liquid chromatography-electrospray ionization quantum mass spectrometry (LC-ESI QMS).

Discharge plasma irradiates seeds with reactive oxygen and nitrogen species (RONS). However, RONS introduced in seeds by plasma irradiation have not been successfully detected thus far. This study provides experimental evidence that nitrate ion NO3- is introduced in lettuce seeds as RONS upon irradiation with atmospheric-pressure air dielectric barrier discharge plasma. Plasma irradiation for 5 min promotes seed germination. The components of the plasma-irradiated seeds were examined using electrospray ionization quantum mass spectrometry (ESI QMS), which revealed that the plasma irradiation introduced an ion with a mass of 62 m/z in detectable amounts. This ion was identified as NO3- by liquid chromatography (LC), multiple wavelength detector (MWD), and LC-ESI QMS. A one-dimensional simulation at electron temperature Te = 1 eV, electron density Ne = 1013/m3, and gas temperature Tg = 300 K indicated the introduction of NO3-, involving nitric oxide NO. NO3- is one of the most important ions that trigger signal transduction for germination when introduced in seeds. The scanning electron microscopy (SEM) images revealed that there was no change on the surface of the seeds after plasma irradiation. Plasma irradiation is an effective method of introducing NO3- in seeds in a dry process without causing damage.

The ethylene signal mediates induction of GmATG8i in soybean plants under starvation stress.

In higher plants, autophagy-related genes (ATGs) appear to play important roles in development, senescence, and starvation responses. Hormone signals underlying starvation-induced gene expression are involved in the expression of ATGs. An effect of starvation stress on the expression of ATGs and ethylene-related genes in young seedlings of soybean (Glycine max [L.] Merr. cv. Fukuyutaka) was analyzed. Reverse transcription-polymerase chain reaction (RT-PCR) showed that the expression levels of GmATG8i and GmATG4 increase in a starvation medium, but at a null or marginal level in a sucrose/nitrate-rich medium. The expression of GmACC synthase and GmERF are also upregulated in the starvation medium. In addition, immunoblot revealed that ethylene insensitive 3 (Ein3), an ethylene-induced transcription factor are accumulated in seedlings subjected to severe starvation stress. These results indicate that starvation stress stimulates the expression of GmATG8i and ethylene signal-related genes. Since the ethylene signal is involved in senescence and various environmental stresses, it is possible that starvation stress-induced autophagy is partly mediated by the ethylene signaling.

Mechanism of delayed seed germination caused by high temperature during grain filling in rice (Oryza sativa L.).

High temperature during grain filling considerably reduces yield and quality in rice (Oryza sativa L.); however, how high temperature affects seed germination of the next generation is not yet well understood. Here, we report that seeds from plants exposed to high temperature during the grain filling stage germinated significantly later than seeds from unstressed plants. This delay remained even after dormancy release treatments, suggesting that it was not due to primary seed dormancy determined during grain filling. In imbibed embryos of heat-stressed seeds, expression of abscisic acid (ABA) biosynthesis genes (OsNCEDs) was higher than in those of control seeds, whereas that of ABA catabolism genes (OsABA8'OHs) was lower. In the aleurone layer, despite no change in GA signaling as evidenced by no effect of heat stress on OsGAMYB gene expression, the transcripts of α-amylase genes OsAmy1C, OsAmy3B, and OsAmy3E were significantly down-regulated in heat-stressed seeds in comparison with controls. Changes in promoter methylation levels were consistent with transcriptional changes of ABA catabolism-related and α-amylase genes. These data suggest that high temperature during grain filling results in DNA methylation of ABA catabolism-related and α-amylase gene promoters, delaying germination of heat-stressed seeds.

Effect of Seedling Nitrogen Condition on Subsequent Vegetative Growth Stages and Its Relationship to the Expression of Nitrogen Transporter Genes in Rice.

Nitrogen (N) deficiency is one of the most common problems in soils, limiting crop growth and production. However, the effects of N limitation in seedlings on vegetative growth remain poorly understood. Here, we show that N limitation in rice seedlings restricted vegetative growth but not yield. Aboveground parts were affected mainly during the period of tillering, but belowground parts were sensitive throughout vegetative growth, especially during panicle development. At the tillering stage, N-limited plants had a significantly lower N content in shoots, but not in roots. On the other hand, N content in roots during the panicle development stage was significantly lower in N-limited plants. This distinct response was driven by significant changes in expression of N transporter genes during growth. Under N limitation, N translocation from roots to shoots was greatly sped up by systemic expression of N transporter genes to obtain balanced growth. N limitation during the seedling stage did not reduce any yield components. We conclude that the N condition during the seedling stage affects physiological responses such as N translocation through the expression of N transporter genes.

Reactive oxygen species induced by heat stress during grain filling of rice (Oryza sativa L.) are involved in occurrence of grain chalkiness.

Heat stress during grain filling increases rice grain chalkiness due to increased activity of α-amylase, which hydrolyzes starch. In rice and barley seeds, reactive oxygen species (ROS) produced after imbibition induce α-amylase activity via regulation of gibberellin (GA) and abscisic acid (ABA) levels during seed germination. Here, we examined whether ROS is involved in induction of grain chalkiness by α-amylase in developing rice grains under heat stress. To elucidate the role of ROS in grain chalkiness, we grew post-anthesis rice plants (Oryza sativa L. cv. Koshihikari) under control (25°C) or heat stress (30°C) conditions with or without antioxidant (dithiothreitol) treatment. The developing grains were analyzed for expression of NADPH oxidases, GA biosynthesis genes (OsGA3ox1, OsGA20ox1), ABA catabolism genes (OsABA8'OH1, OsABA8'OH2) and an α-amylase gene (OsAmy3E), endogenous H2O2 content and the grain quality. In grains exposed to heat stress, the expression of NADPH oxidase genes (especially, OsRbohB, OsRbohD, OsRbohF and OsRbohI) and the ROS content increased. Heat stress also increased the expression of OsGA3ox1, OsGA20ox1, OsABA8'OH1, OsABA8'OH2 and OsAmy3E. On the other hand, dithiothreitol treatment reduced the effects of heat stress on the expression of these genes and significantly reduced grain chalkiness induced by heat stress. These results suggest that, similar to cereal seed germination mechanism, ROS produced under heat stress is involved in α-amylase induction in maturating rice grains through GA/ABA metabolism, and consequently caused grain chalkiness.

Effects of light quality on pod elongation in soybean (Glycine max (L.) Merr.) and cowpea (Vigna unguiculata (L.) Walp.).

Soybean pods are located at the nodes, where they are in the shadow, whereas cowpea pods are located outside of the leaves and are exposed to sunlight. To compare the effects of light quality on pod growth in soybean and cowpea, we measured the length of pods treated with white, blue, red or far-red light. In both species, pods elongated faster during the dark period than during the light period in all light treatments except red light treatment in cowpea. Red light significantly suppressed pod elongation in soybean during the dark and light periods. On the other hand, the elongation of cowpea pods treated with red light markedly promoted during the light period. These results suggested that the difference in the pod set sites between soybean and cowpea might account for the difference in their red light responses for pod growth.

The Interrelationship between Abscisic Acid and Reactive Oxygen Species Plays a Key Role in Barley Seed Dormancy and Germination.

Seed dormancy is one of the adaptive responses in the plant life cycle and an important agronomic trait. Reactive oxygen species (ROS) release seed dormancy and promote seed germination in several cereal crops; however, the key regulatory mechanism of ROS-mediated seed dormancy and germination remains controversial. Here, we focused on the relationship between hydrogen peroxide (a ROS) and abscisic acid (ABA) in dormant and non-dormant barley seeds. The hydrogen peroxide (H2O2) level produced in barley seed embryos after imbibition was higher in non-dormant seeds than in dormant seeds. H2O2 regulated the ABA content in the embryos through ABA-8'-hydroxylase, an ABA catabolic enzyme. Moreover, compared with non-dormant seeds, in dormant seeds the activity of NADPH oxidase, which produces ROS, was lower, whereas the activity of catalase, which is a H2O2 scavenging enzyme, was higher, as was the expression of HvCAT2. Furthermore, precocious germination of isolated immature embryos was suppressed by the transient introduction of HvCAT2 driven by the maize (Zea mays) ubiquitin promoter. HvCAT2 expression was regulated through an ABA-responsive transcription factor (HvABI5) induced by ABA. These results suggest that the changing of balance between ABA and ROS is active in barley seed embryos after imbibition and regulates barley seed dormancy and germination.

Role of reactive oxygen species produced by NADPH oxidase in gibberellin biosynthesis during barley seed germination.

NADPH oxidase catalyzes the production of the superoxide anion (O2(-)), a reactive oxygen species (ROS), and regulates the germination of barley (Hordeum vulgare L.). Diphenyleneiodonium (DPI) chloride, an NADPH oxidase inhibitor, delayed barley germination, and exogenous H2O2 (an ROS) partially rescued it. Six enzymes, ent-copalyl diphosphate synthase (CPS), ent-kaurene synthase (KS), ent-kaurene oxidase (KO), ent-kaurenoic acid oxidase (KAO), GA20-oxidase (GA20ox) and GA3-oxidase (GA3ox), catalyze the transformation of trans-geranylgeranyl diphosphate to active gibberellin, which promotes germination. Exogenous H2O2 promoted the expressions of HvKAO1 and HvGA3ox1 in barley embryos. These results suggest that ROS produced by NADPH oxidase are involved in gibberellin biosynthesis through the regulation of HvKAO1 and HvGA3ox1.

Nitrogen redistribution and its relationship with the expression of GmATG8c during seed filling in soybean.

It is well known that some nitrogen in the vegetative organs is redistributed to the seeds during seed filling in soybean (Glycine max [L.] Merrill). This redistribution is considered to affect the seed yield of soybean. However, it is still not clear when the nitrogen moves from the vegetative part to the seeds, and the relationship between nitrogen redistribution and leaf senescence has not been clarified. The soybean variety Fukuyutaka was grown in the experimental field of Saga University, Japan from 22 July to 31 October, 2014. After the first flower stage (R1), the plant samples were collected weekly and were separated into leaf, petiole, stem, podshell and seed. The nitrogen concentrations in each plant part were determined. Fresh leaf samples were provided for the determination of soluble protein and autophagy gene GmATG8c expression. The nitrogen that accumulated in the vegetative parts reached its highest level at 60days after sowing (DAS), then began to decrease at 73DAS (R6). This decrease is considered to be the consequence of nitrogen redistribution from the vegetative parts to the seeds. The movement of nitrogen from the vegetative parts to the seeds was estimated to occur at around 73DAS (R6). At this stage, leaf SPAD values, leaf nitrogen, and soluble protein concentrations began to decrease simultaneously, suggesting the onset of leaf senescence. Furthermore, the expression of the autophagy gene GmATG8c in the leaves increased dramatically from 73 to 85DAS, which is the duration of nitrogen redistribution. The results suggest that the nitrogen redistribution from the vegetative parts to the seeds could be one of the initiating factors of leaf senescence, and the autophagy gene GmATG8c was associated with this process.

A Role for Reactive Oxygen Species Produced by NADPH Oxidases in the Embryo and Aleurone Cells in Barley Seed Germination.

Reactive oxygen species (ROS) promote the germination of several seeds, and antioxidants suppress it. However, questions remain regarding the role and production mechanism of ROS in seed germination. Here, we focused on NADPH oxidases, which produce ROS. After imbibition, NADPH oxidase mRNAs were expressed in the embryo and in aleurone cells of barley seed; these expression sites were consistent with the sites of ROS production in the seed after imbibition. To clarify the role of NADPH oxidases in barley seed germination, we examined gibberellic acid (GA) / abscisic acid (ABA) metabolism and signaling in barley seeds treated with diphenylene iodonium chloride (DPI), an NADPH oxidase inhibitor. DPI significantly suppressed germination, and suppressed GA biosynthesis and ABA catabolism in embryos. GA, but not ABA, induced NADPH oxidase activity in aleurone cells. Additionally, DPI suppressed the early induction of α-amylase by GA in aleurone cells. These results suggest that ROS produced by NADPH oxidases promote GA biosynthesis in embryos, that GA induces and activates NADPH oxidases in aleurone cells, and that ROS produced by NADPH oxidases induce α-amylase in aleurone cells. We conclude that the ROS generated by NADPH oxidases regulate barley seed germination through GA / ABA metabolism and signaling in embryo and aleurone cells.

Programmed cell death in barley aleurone cells is not directly stimulated by reactive oxygen species produced in response to gibberellin.

The cereal aleurone layer is a secretory tissue that produces enzymes to hydrolyze the starchy endosperm during germination. We recently demonstrated that reactive oxygen species (ROS), produced in response to gibberellins (GA), promoted GAMyb expression, which induces α-amylase expression in barley aleurone cells. On the other hand, ROS levels increase during programmed cell death (PCD) in barley aleurone cells, and GAMyb is involved in PCD of these cells. In this study, we investigated whether the ROS produced in response to GA regulate PCD directly by using mutants of Slender1 (SLN1), a DELLA protein that negatively regulates GA signaling. The wild-type, the sln1c mutant (which exhibits gibberellin-type signaling even in the absence of GA), and the Sln1d mutant (which is gibberellin-insensitive with respect to α-amylase production) all produced ROS in response to GA, suggesting that ROS production in aleurone cells in response to GA is independent of GA signaling through this DELLA protein. Exogenous GA promoted PCD in the wild-type. PCD in sln1c was induced even without exogenous GA (and so without induction of ROS), whereas PCD in Sln1d was not induced in the presence of exogenous GA, even though the ROS content increased significantly in response to GA. These results suggest that PCD in barley aleurone cells is not directly stimulated by ROS produced in response to GA but is regulated by GA signaling through DELLA protein.

Assimilate translocation and expression of sucrose transporter, OsSUT1, contribute to high-performance ripening under heat stress in the heat-tolerant rice cultivar Genkitsukushi.

High temperature reduces the grain quality of rice, a situation likely to become more frequent because of global warming. We studied the effects of high-temperature stress on grain quality of heat-tolerant cultivar 'Genkitsukushi' and heat-sensitive cultivar 'Tsukushiroman'. When day/night temperatures were 31/26 °C from heading until maturity, the grain quality of 'Genkitsukushi' was rated at the first inspection grade (high quality), whereas 'Tsukushiroman' showed a remarkable increase in the percentage of white immature kernels (low quality). Nonstructural carbohydrate content in the stem of 'Genkitsukushi' the early maturation was significantly higher than in 'Tsukushiroman' and greatly decreased under high temperature. From 14 to 21 days after heading, the expression of the sucrose transporter gene, OsSUT1, was higher in the stem of 'Genkitsukushi' grown under high temperature than in 'Tsukushiroman'. In addition, the expression of OsSUT1 in the grains of 'Genkitsukushi' was significantly higher than in 'Tsukushiroman' during the ripening period. These results indicate that sugar transport functions more effectively in 'Genkitsukushi' than in 'Tsukushiroman', and that the effectiveness of sugar transport contributes to maintaining high grain quality in 'Genkitsukushi' under high-temperature conditions.