Response of rice (Oryza sativa) with root surface iron plaque under aluminium stress

BACKGROUND AND AIMS: Rice (Oryza sativa) is an aquatic plant with a characteristic of forming iron plaque on its root surfaces. It is considered to be the most Al-tolerant species among the cereal crops. The objective of this study was to determine the effects of root surface iron plaque on Al translocation, accumulation and the change of physiological responses under Al stress in rice in the presence of iron plaque. METHODS: The japonica variety rice, Koshihikari, was used in this study and was grown hydroponically in a growth chamber. Iron plaque was induced by exposing the rice roots to 30 mg L(-1) ferrous iron either as Fe(II)-EDTA in nutrient solution (6 d, Method I) or as FeSO(4) in water solution (12 h, Method II). Organic acid in root exudates was retained in the anion-exchange resin and eluted with 2 m HCl, then analysed by high-performance liquid chromatography (HPLC) after proper pre-treatment. Fe and Al in iron plaque were extracted with DCB (dithionite-citrate-bicarbonate) solution. KEY RESULTS AND CONCLUSIONS: Both methods (I and II) could induce the formation of iron plaque on rice root surfaces. The amounts of DCB-extractable Fe and Al on root surfaces were much higher in the presence of iron plaque than in the absence of iron plaque. Al contents in root tips were significantly decreased with iron plaque; translocation of Al from roots to shoots was significantly reduced with iron plaque. Al-induced secretion of citrate was observed and iron plaque could greatly depress this citrate secretion. These results suggested that iron plaque on rice root surfaces can be a sink to sequester Al onto the root surfaces and Fe ions can pre-saturate Al-binding sites in root tips, which protects the rice root tips from suffering Al stress to a certain extent.     

Interaction between iron plaque and root border cells ameliorates aluminum toxicity of Oryza sativa differing in aluminum tolerance

Root border cells (RBCs), which are generated during plant growth and surround the root cap, and iron plaque (IP), ubiquitously formed on the root surfaces of rice, are known to alleviate aluminum (Al) toxicity. To verify the interactive effects of IP and RBCs on ameliorating Al toxicity, two rice cultivars differing in Al resistance were used to compare Al tolerance between cultivars. Additionally, root elongation, Al uptake and RBCs viability were measured as indicators of the effects of Al. The amounts of DCB-extractable Fe and Al on the root surfaces were much higher in the presence of IP than the absence. IP presence significantly decreased Al-induced inhibition of root elongation and Al contents in roots and root tips. The removal of RBCs from the root tips caused a more severe inhibition of root elongation and a higher Al accumulation in rice roots and root tips. Furthermore, root growth inhibition and Al contents in roots and root tips were significantly lower in roots with a combination of IP and RBCs than in roots with IP or RBCs only. The formation of IP on the root surface maintained higher RBCs viability and depressed mucilage exudation in an Al-tolerant rice cultivar. The results suggest that both IP and RBCs ameliorate Al toxicity, and IP has a greater capacity for Al resistance. The combination of IP and RBCs exhibited a synergistic effect associated with Al resistance.     

Ethylene is involved in the autochemotropism of Phycomyces

The sporangiophore of the fungus Phycomyces blakesleeanus has the property of growing away from a barrier which is few mm from the growing zone of the sporangiophore (avoidance or autochemotropic response). A model has been published (Cohen, R.J., Jan, N.Y., Matricon, J., Delbrück, M.: J. Gen. Physiol. 66, 67–95 (1975)). To explain the avoidance response which postulates that the sporangiophore emits and readsorbs a volatile growth-promoting effector (gas X) and that the barrier modifies the effector distribution by acting as an aerodynamic obstacle, causing a higher concentration of gas X on the side of the sporangiophore closer to the barrier. From this model we deduced three properties of the gas X. Of the several gases tested (N₂, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆) only ethylene (C₂H₄) had all these three properties, a finding which suggests that it has a role in the avoidance response (autochemotropism).Abbreviation Spph Sporangiophore     

Hydrogen peroxide is involved in the regulation of rice (Oryza sativa L.) tolerance to salt stress

In the present study, we investigated the salt tolerance mechanism of two rice cultivars (Zhenghan-2 and Yujing-6), which show different tolerance to drought and disease. NaCl induced higher extent of lipid peroxide and ion leakage in Yujing-6 roots than those in Zhenghan-2 roots. H₂O₂ accumulation in Zhenghan-2 roots was lower than that in Yujing-6 roots under salt stress. Comparatively, NaCl treatment did not increase O₂ ^? contents in both rice roots, however, O₂ ^? level in Yujing-6 roots was higher than that in Zhenghan-2 roots under both control and salt stress conditions. Ascorbate peroxidases (APX) activity increased more significantly in Zhenghan-2 roots than that in Yujing-6 roots. The activity of catalase (CAT), peroxidase (POD), superoxide dismutase (SOD), and glucose-6-phosphate dehydrogenase (G6PDH) was similarly enhanced in both rice roots under salt stress; however, they showed higher levels in Zhenghan-2 roots than in Yujing-6 roots. Exogenous H₂O₂ could enhance APX, CAT, POD, SOD and G6PDH activities in a concentration-dependent manner in both rice roots. Diphenylene iodonium (DPI), a plasma membrane (PM) NADPH oxidase inhibitor, which counteracted the NaCl-induced H₂O₂ accumulation, markedly decreased the activity of above enzymes. Moreover, ion leakage increased dramatically in Zhenghan-2 roots and reached to the similar level of Yujing-6 roots under NaCl+DPI treatment. Taken together, H₂O₂, which is mainly generated from PM NADPH oxidase, is involved in Zhenghan-2 rice tolerance to salt stress by enhancing the cellular antioxidant level.     

Silicon ameliorates manganese toxicity by regulating manganese transport and antioxidant reactions in rice (Oryza sativa L.)

Background and aims

This study aimed to investigate the roles of silicon (Si) in ameliorating manganese (Mn) toxicity in two rice (Oryza sativa L.) cultivars: i.e. cv. Xinxiangyou 640 (XXY), a Mn-sensitive cultivar and cv. Zhuliangyou 99 (ZLY), a Mn-tolerant cultivar.

Methods

Plants were cultured in nutrient solution containing normal Mn (6.7 μM) or high Mn (2.0 mM), both with or without Si supply at 1.5 mM Si.

Results

Plant growth was severely inhibited by high Mn in cv. XXY, but was enhanced by Si supply. In cv. XXY, Si-enhanced tolerance resulted from a restriction of Mn transport, whereas in cv. ZLY Mn uptake was depressed. In cv. XXY, high Mn significantly increased superoxide dismutase (SOD), catalase and ascorbate peroxidase activities but decreased non-protein thiols and glutathione concentrations, leading to accumulation of H₂O₂ and malondialdehyde. The addition of Si significantly counteracted high Mn-elevated malondialdehyde and H₂O₂ concentrations and enhanced plant growth. In cv. ZLY, high Mn considerably raised SOD activities and glutathione concentrations, thus leading to relatively low oxidative damage.

Conclusions

Si-enhanced Mn tolerance was attributed mainly to restricted Mn transport in cv. XXY but to depressed Mn uptake in cv. ZLY. Silicon mainly influenced non-enzymatic antioxidants in these two rice cultivars under high Mn stress.     

Direct evidence showing the effect of root surface iron plaque on arsenite and arsenate uptake into rice (Oryza sativa) roots

The present study aimed to investigate the effects of root surface iron plaque on the uptake kinetics of arsenite and arsenate by excised roots of rice (Oryza sativa) seedlings. The results demonstrated that the presence of iron plaque enhanced arsenite and decreased arsenate uptake. Arsenite and arsenate uptake kinetics were adequately fitted by the Michaelis-Menten function in the absence of plaque, but produced poor fits to this function in the presence of plaque. Phosphate in the uptake solution did not have a significant effect on arsenite uptake irrespective of the presence of iron plaque; however phosphate had a significant effect on arsenate uptake. Without iron plaque, phosphate inhibited arsenate uptake. The presence of iron plaque diminished the effect of phosphate on arsenate uptake, possibly through a combined effect of arsenate desorption from iron plaque.     

Cytokinin inhibits lateral root initiation but stimulates lateral root elongation in rice (Oryza sativa)

Research in lateral root (LR) development mainly focuses on the role of auxin. This article reports the effect of cytokinins (kinetin and trans-zeatin) on LR formation in rice (Oryza sativa L.). Our results showed that cytokinin has an inhibitory effect on LR initiation and stimulatory effect on LR elongation. Both KIN and ZEA at a concentration of 1 microM and above completely inhibited lateral root primordium (LRP) formation. The inhibitory effect of cytokinin on LR initiation required a continuous presence of KIN or ZEA in the growth solution. Cytokinin did not show any inhibitory effect on LR emergence from the seminal root once LRPs had been formed. The LRPs that developed in cytokinin-free solution can emerge normally in the solution containing inhibitory concentration (1 microM) of KIN and ZEA. The KIN and ZEA treatment dramatically stimulated LR elongation at all the concentrations tested. Maximum LR elongation was observed at a concentration of 0.01 microM KIN and 0.001 microM ZEA. The epidermal cell length increased significantly in LRs of cytokinin treated seedlings compared to those of untreated control. This result indicates that the stimulation of LR elongation by cytokinin is due to increased cell length. Exogenously applied auxin counteracted the effect of cytokinin on LR initiation and LR elongation, suggesting that cytokinin acts on LR elongation through an auxin dependent pathway.     

ES5 is involved in the regulation of phosphatidylserine synthesis and impacts on early senescence in rice (Oryza sativa L.)

Leaf senescence, which affects plant growth and yield in rice, is an ideal target for crop improvement and remarkable advances have been made to identify the mechanism underlying this process. We have characterized an early senile mutant es5 (early leaf senescence 5) in rice exhibiting leaf yellowing phenotype after the 4-leaf stage. This phenotype was confirmed by the higher accumulation of reactive oxygen species (ROS) and malondialdehyde (MDA), the disintegration of chloroplasts, reduction in chlorophyll content and photosynthetic rate and up-regulation of senescence-associated genes (SAGs) like Osh36, OsI57, and OsI85. Positional cloning revealed that the es5 phenotype is the result of one base substitution in ES5, encoding phosphatidylserine synthase (PSS) family protein, which is involved in the base-exchange type reaction to synthesize the minor membrane phospholipid phosphatidylserine. Functional complementation of ES5 in the es5 plants completely restored the wild-type phenotype. Ultra-high-performance liquid chromatography (UHPLC) analysis showed that es5 plants had increased levels of phosphatidylserine (PS) and decreased level of phosphatidylcholine (PC). These results provide evidence about the role of PS in rice leaf senescence.

     

Photosynthesis and root characteristics of rice (Oryza sativa L.) in floating culture

To address the issue of water eutrophication and to use water more effectively, we conducted experiments on rice (Oryza sativa L.) grown in floating culture. From 2009 to 2011, we compared the photosynthesis and root characteristics of the rice, hybrid line Zhuliangyou 02, grown under a conventional tillage and in a floating culture in Huaihua, the home of hybrid rice. Rice in the floating culture showed a higher net photosynthetic rate and stomatal conductance than that under the conventional tillage. The activities of phosphoenolpyruvate carboxylase and NADP-malic enzyme were 32 and 28% higher, respectively, in rice in the floating culture than under the conventional tillage. Rice in the floating culture also showed significantly greater number of roots, root activity, and antioxidant enzyme activity than that under the conventional tillage. Compared with rice under the conventional tillage, rice in the floating culture had 18 and 24% higher tiller number and effective panicle number, respectively. These results suggested that the floating culture system can promote rice production through enhancing root absorption, increasing effective panicle number, and improving the photosynthetic rate. In addition, rice cultivated in the floating culture could remove excess nutrients from water, which addresses the problems of a lack of arable land and water pollution.     

Shoot tolerance mechanisms to iron toxicity in rice (Oryza sativa L.)

Iron toxicity frequently affects lowland rice and leads to oxidative stress via the Fenton reaction. Tolerance mechanisms were investigated in contrasting genotypes: the intolerant IR29 and the tolerant recombinant inbred line FL483. Seedlings were exposed to 1000 mg L^‐1 ferrous iron, and the regulation of genes involved in three hypothetical tolerance mechanisms was investigated (I) Iron uptake, partitioning and storage. The iron concentration and speciation in different plant tissues did not differ significantly between genotypes. Sub‐cellular iron partitioning genes such as vacuolar iron transporters or ferritin showed no genotypic differences. (II) Antioxidant biosynthesis. Only one gene involved in carotenoid biosynthesis showed genotypic differences, but carotenoids are unlikely to scavenge the reactive oxygen species (ROS) involved in Fe toxicity, i.e. H₂O₂ and hydroxyl radicals. (III) Enzymatic activities for ROS scavenging and antioxidants turnover. In shoots, glutathione‐S‐transferase and ascorbate oxidase genes showed genotypic differences, and consistently, the tolerant FL483 had lower dehydroascorbate reductase and higher ascorbate oxidase activity, suggesting that high rates ascorbate reduction confer sensitivity. This hypothesis was confirmed by application of exogenous reduced ascorbate or L‐galactono‐1,4‐lactone, which increased lipid peroxidation under iron toxic conditions. Our results demonstrate in planta pro‐oxidant activity of reduced ascorbate in the presence of iron.     

Ethylene perception is involved in female cucumber flower development

It is well established that ethylene promotes female flower development in cucumber. However, little is known about how the gaseous hormone selectively affects female flowers, and what mechanism it uses. Previously, we found organ‐specific DNA damage in the primordial anther of female cucumber flowers. This finding led to a hypothesis that ethylene might promote female flower development via the organ‐specific induction of DNA damage in primordial anthers. In this study, we tested this hypothesis first by demonstrating ethylene induction of DNA damage via the ethylene signaling pathway using cucumber protoplasts. Then, using representative component genes of the ethylene signaling pathway as probes, we found that one of the ethylene receptors, CsETR1, was temporally and spatially downregulated in the stamens of stage‐6 female cucumber flowers, especially along with the increase of the nodes. Furthermore, by constructing transgenic Arabidopsis plants with organ‐specific expression of antisense CsETR1 under the control of an AP3 promoter to downregulate ETR1 expression in the stamens, we generated Arabidopsis ‘female flowers’, in which the abnormal stamens mimic those of female cucumber flowers. Our data suggest that ethylene perception is involved in the arrest of stamen development in female cucumber flowers through the induction of DNA damage. This opens up a novel perspective and approach to solve the half‐century‐long puzzle of how gaseous ethylene selectively promotes female flowers in the monoecious cucumber plant.