Dimas Mendes Ribeiro

Sensitivity to ethylene as a major component in the germination of seeds of Stylosanthes humilis

Physiological dormancy of scarified seeds of Townsville stylo ( Stylosanthes humilis H.B.K.) is broken by ethylene. When the biosynthesis of this gas was impaired by 2-aminoethoxyvinylglycine (AVG) plus Co 2+ , the response to ethylene at very low concentrations was appreciable in non-dormant seeds and nil in the dormant ones. Complete inhibition of germination of non-dormant seeds occurred only when they were treated with AVG plus Co 2+ under an ethylene-free atmosphere, a condition in which no trace of the gas in the atmosphere of Erlenmeyer flasks could be detected. Injection of ethylene into that system triggered germination of both dormant and non-dormant seeds, demonstrating a requirement for the gas. Non-dormant seeds were at least 50-fold more sensitive to ethylene than the dormant ones. Perception of ethylene occurred within a very short time (at most 15 min), since exposure of both dormant and non-dormant seeds to the gas, at a steeply declining concentration, sufficed to cause substantial germination.

Germination of salt-stressed seeds as related to the ethylene biosynthesis ability in three Stylosanthes species☆

Stylosanthes, a genus of tropical forage legume, is known to exhibit good persistence in saline soils, yet mechanisms for regulation of seed germination under salt stress are poorly understood. This study was carried out to evaluate the mode of action of salt stress on seed germination of Stylosanthes. 1-Aminocyclopropane-1-carboxylic acid (ACC) increased ethylene biosynthesis and germination of NaCl-inhibited seeds in a dose-dependent manner. Contents of ACC and germination of Stylosanthes humilis seeds increased following transfer from NaCl solution to deionised water, but not after transfer to l-α-(2-aminoethoxyvinyl)-glycine (AVG) solution, an inhibitor of ethylene biosynthesis. Ethylene biosynthesis was much larger in NaCl-treated seeds of Stylosanthes guianensis than in seeds of S. humilis and Stylosanthes capitata, a fact which was reflected in higher germination rates. S. guianensis seedlings also displayed higher growth and survival rates than S. humilis and S. capitata under salt stress. Moreover, smaller ACC levels, as well as reduced ethylene biosynthesis of S. capitata seeds were accompanied by lower germination under salt stress. In addition, S. capitata seedlings treated with NaCl solutions exhibited relatively lower growth and survival rates in comparison with S. humilis and S. guianensis. Thus, different abilities to synthesize ethylene by S. guianensis, S. humilis and S. capitata seeds explain the differences in tolerance to salt stress of the three species.

Dormancy breakage of Stylosanthes humilis seeds by aluminium.

Physiological dormancy of scarified seeds of Townsville stylo ( Stylosanthes humilis HBK) was released by acidic aluminium (Al 3+ ) solution. Antiethylenic substances inhibited germination of low-pH-stimulated dormant seeds, with a correspondingly low ethylene production and low activity of 1-aminocyclopropane-1-carboxylate (ACC) oxidase in seeds. On the other hand, antiethylenic substances did not decrease the germination of Al 3+ -stimulated seeds, but ACC oxidase activity and ethylene production by the seeds was decreased to a large extent. These data provide evidence that dormancy breakage by Al 3+ differs from that caused by low pH and is not associated with ethylene production. Similarly to Al 3+ action, methyl viologen (MV), a reactive oxygen species-generating compound, broke dormancy of Townsville stylo seeds. Sodium selenate and N -acetyl cysteine, antioxidant compounds, largely decreased germination of MV- and Al 3+ -stimulated dormant seeds. Altogether these data point to oxidative radicals constituting key molecules in the chain of events triggered by Al 3+ leading to dormancy breakage.

Growth inhibition by selenium is associated with changes in primary metabolism and nutrient levels in Arabidopsis thaliana.

Although Selenium (Se) stress is relatively well known for causing growth inhibition, its effects on primary metabolism remain rather unclear. Here, we characterized both the modulation of the expression of specific genes and the metabolic adjustments in Arabidopsis thaliana in response to changes in Se level in the soil. Se treatment culminated with strong inhibition of both shoot and root growth. Notably, growth inhibition in Se-treated plants was associated with an incomplete mobilization of starch during the night. Minor changes in amino acids levels were observed in shoots and roots of plants treated with Se whereas the pool size of tricarboxylic acid (TCA) cycle intermediates in root was not altered in response to Se. By contrast, decreased levels of organic acids involved in the first part of the TCA cycle were observed in shoots of Se-treated plants. Furthermore, decreased expression levels of expansins and endotransglucosylases/endohydrolases (XHTs) genes were observed after Se treatment, coupled with a significant decrease in the levels of essential elements. Collectively, our results revealed an exquisite interaction between energy metabolism and Se-mediated control of growth in Arabidopsis thaliana to coordinate cell wall extension, starch turnover and the levels of a few essential nutrients.

Germination of dormant seeds of Stylosanthes humilis as promoted by ethylene accumulation in closed environments

Germination of scarified dormant seeds of Townsville stylo (Stylosanthes humilis) is very low in Petri dishes, but may be appreciable in sealed Erlenmeyer flasks if ethylene accumulates in their atmospheres. When ethylene in the flask atmosphere was fixed by a mercuric perchlorate solution, emanation of the gaseous regulator and germination of dormant seeds were drastically decreased. On the other hand, ethylene produced by dormant seeds and accumulated in the flask atmosphere led to a further enhancement of ethylene accumulation and a corresponding increased seed germination. Germination of both dormant and non-dormant seeds did not occur when ethylene biosynthesis was completely blocked.

Ethylene coordinates seed germination behavior in response to low soil pH in Stylosanthes humilis

AimsStylosanthes humilis is known to exhibit high persistence in acid soils, however, how low soil pH controls seed germination as well as root and hypocotyl growth remains unknown. This study was carried out to evaluate the hormonal and metabolic alterations induced by low soil pH on seed germination behavior of S. humilis.MethodsSeeds of S. humilis were sown in acid soil samples or sand soaked in buffer solution with pH ranging from 4.0 to 7.0. Concentrations of indole-3-acetic acid, ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC), primary metabolite profile and final seed germination were evaluated after four days.ResultsLow soil pH led to increased final seed germination, concomitantly with higher root penetration into the soil as well as higher ACC and ethylene production by seedlings. Treatment with the ethylene biosynthesis inhibitor L-α-(2-aminoethoxyvinyl)-glycine (AVG) greatly reduced final seed germination under acidic conditions. Final seed germination of seeds treated with AVG was increased by exogenous ethylene application in a dose-dependent manner. Furthermore, low soil pH promoted distinct changes in IAA concentrations, and in carbon and nitrogen metabolism in hypocotyl and roots.ConclusionsLow soil pH increases the final germination of S. humilis seeds through alterations in ethylene metabolism, allowing root penetration into the soil.

Action of ferric and aluminium ions on the dormancy breakage of Stylosanthes humilis seeds

Dormancy of scarified seeds of Stylosanthes humilis was broken by acidic Al3+ and Fe3+ solutions. Fe+3-stimulated seeds exhibited a high activity of 1-aminocyclopropane-1-carboxylate (ACC) oxidase and produced great amounts of ethylene, which showed correlated with the germination process. In addition, specific inhibitors of ethylene biosynthesis and action largely depressed the Fe3+-stimulated germination, leading to the conclusion that the ion broke dormancy by triggering ethylene production by the seeds. By contrast, inhibitors of ethylene biosynthesis and action did not impair germination of Al3+-stimulated dormant seeds. Moreover, ethylene production and activity of ACC oxidase of Al3+-treated seeds was substantially decreased by inhibitors of ethylene biosynthesis, but germination kept large. Together these data suggest that ethylene biosynthesis was not required in the chain of events triggered by Al3+ leading to dormancy breakage. Methyl viologen (MV), a reactive oxygen species-generating compound, broke dormancy of seeds to the same extent as Al3+ did. Germination of both Al3+- and MV-stimulated dormant seeds was inhibited by sodium selenate, an antioxidant compound; selenate, however had no effect on germination of Fe3+-stimulated seeds. Together these data indicate that the mechanisms underlying the germination of Al3+- and Fe3+-treated seeds are not the same.

Low soil pH modulates ethylene biosynthesis and germination response of Stylosanthes humilis seeds

ABSTRACT The tropical forage legume Stylosanthes humilis is naturally distributed in the acidic soils of the tropics. However, data concerning the role of low soil pH in the control of S. humilis seed germination remains limited. Recently, we have demonstrated that acidic soil triggers increased ethylene production during germination of S. humilis seeds, concomitantly with higher root penetration into the soil. Our finding points an important role of low soil pH as a signal allowing penetration of root in the soil through interaction with the ethylene signalling pathway. Herein, we discuss how low soil pH induces changes on seed hormonal physiology.

Salt stress inhibits germination of Stylosanthes humilis seeds through abscisic acid accumulation and associated changes in ethylene production

In Stylosanthes humilis, salt stress tolerance is associated with ethylene production by the seeds, however, how salt stress controls seed germination and ethylene production is poorly understood. Here, we studied the hormonal and metabolic changes triggered by salt stress on germination of S. humilis seeds. Salt stress led to decreased seed germination and ethylene production, concomitantly with higher abscisic acid (ABA) production by seeds. Treatment with NaCl and ABA promoted distinct changes in energy metabolism, allowing seeds to adapt to salt stress conditions. Treatment with the ABA biosynthesis inhibitor fluridone or ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) reversed the effects of salt stress on seed germination and ethylene production. Moreover, ethylene concentration was decreased by increasing the pH of the salt solution. High pH, however, did not influence concentration of ABA in seeds under salt stress. We conclude that biosynthesis of ABA and ethylene in response to salt stress constitutes a point of convergence that provides flexibility to regulate energy metabolism and embryo growth potential of S. humilis seeds within a given pH condition.

Bundle sheath extensions in tomato affect leaf phenotypic plasticity in response to irradiance

Coordination between structural and physiological traits is key to plants’ responses to environmental fluctuations. In heterobaric leaves, bundle sheath extensions (BSEs) increase photosynthetic performance (light-saturated rates of photosynthesis, Amax) and water transport capacity (leaf hydraulic conductance, Kleaf). However, it is not clear how BSEs affect these and other leaf developmental and physiological parameters in response to environmental conditions. The obscuravenosa (obv) mutation, found in many commercial tomato varieties, leads to absence of BSEs. We examined structural and physiological traits of tomato heterobaric and homobaric (obv) near-isogenic lines (NILs) grown at two different irradiance levels. Kleaf, minor vein density and stomatal pore area index decreased with shading in heterobaric but not in homobaric leaves, which show similarly lower values in both conditions. Homobaric plants, on the other hand, showed increased Amax, leaf intercellular air spaces and mesophyll surface area exposed to intercellular airspace (Smes) in comparison with heterobaric plants when both were grown in the shade. BSEs further affected carbon isotope discrimination, a proxy for long-term water-use efficiency. BSEs confer plasticity in traits related to leaf structure and function in response to irradiance levels and might act as a hub integrating leaf structure, photosynthetic function and water supply and demand. Summary statement The presence of bundle sheath extension (BSEs) defines leaves as heterobaric, as opposed to homobaric leaves that lack them. Multiple functions have been proposed for BSEs, but their impact on different environmental conditions is still unclear. Here, we compared a tomato (Solanum lycopersicum) homobaric mutant lacking BSEs with its corresponding heterobaric wild-type, grown under two irradiance conditions. We show that the presence of BSEs differentially alters various physiological and anatomical parameters in response to growth irradiance. We propose that BSEs could act as hubs coordinating leaf plasticity in response to environmental factors. Article type Research article