Irradiance-regulated biomass allocation in Raphanus sativus plants depends on gibberellin biosynthesis.

Gibberellin has been proposed to increase leaf elongation in radish (Raphanus sativus L.) plants, which is associated with decreased tuber growth. Since light intensity can control growth through interaction with gibberellin, investigation of the effect of gibberellin levels on the growth of radish plants would be a step forward towards unraveling factors that underlie biomass accumulation and allocation in response to irradiance levels. Here, we report that the gibberellin biosynthesis inhibitor paclobutrazol (PAC) decreased petiole elongation, but not lamina growth of radish plants grown under full sunlight. However, shading promoted an increase in shoot elongation, while in plants treated with PAC the petiole and leaf lamina fail to elongate. Plants treated with PAC allocated proportionally more biomass to their tubers and less to shoot compared to control under shade. Moreover, PAC decreased the abundance of transcripts encoding cell wall expansion proteins in leaf lamina and petiole of plants grown under shade, which was positively correlated with sugar consumption by the tuber, thereby increasing the mass fraction and concentrations of minerals for tuber. Thus, allocation of biomass during the growth of radish plants and nutritional quality of tubers depend on gibberellin and light intensity.

Selenium uptake and grain nutritional quality are affected by nitrogen fertilization in rice (Oryza sativa L.).

KEY MESSAGE: The effects of selenium in rice grain composition depend on the soil nitrogen supply. Selenium and nitrogen have the potential to modify rice grain composition; however, it is unclear how the combined effect of Se and nitrogen affects the grain nutritional quality of rice. In our study, grain Se concentration was positively associated with the increased availability of nitrogen in soil. The accumulation of Se in grain of rice plants treated with Se combined with nitrogen was accompanied by an increase in expression of NRT1.1B, a rice nitrate transporter and sensor, in root. Moreover, Se potentiates the response of nitrogen supply in expression of sulfate transporter OsSULTR1.2, phosphate transporter OsPT2 and silicon transporter OsNIP2.1 in root, thereby increasing root Se uptake capacity. The combination of Se with high nitrogen increased the concentrations of protein, carbohydrates, Se, Mo and Mg, but decreased concentrations of Fe, Mn, Cu and Zn in grain. Overall, our results revealed that many of the effects of Se in rice grain composition are due to a shift in the nitrogen status of the plant.

Selenomethionine induces oxidative stress and modifies growth in rice (Oryza sativa L.) seedlings through effects on hormone biosynthesis and primary metabolism.

Although the chemical characteristics of selenomethionine (SeMet) are similar to those of methionine (Met), the physiological activity of SeMet apparently differs in its ability to stimulate ethylene production in plant tissues. Since selenium alters root architecture of rice seedlings by modifying ethylene production, the investigation of the effect of SeMet and Met on rice growth would be a step forward towards unraveling factors that underlie selenium toxicity. Here, we report that SeMet increased concentrations of reactive oxygen species (ROS), inhibiting auxin and increasing ethylene production in rice seedlings. The effect of SeMet on seedlings was mediated by the inhibition of the abundance of transcripts encoding auxin transport and cell expansion proteins. Moreover, SeMet led to increased seedling respiration, which was positively correlated with organic acids consumption, but negatively with sugars consumption, thereby decreasing seedling growth. In contrast with SeMet treatment, Met did not affect ROS production, hormone biosynthesis and seedling growth, indicating an exclusive selenium effect. The singlet oxygen scavenger, 1,4-diazabicyclooctane, overrode the repressive effect of SeMet in seedling growth. Our results demonstrate a phytotoxic effect of SeMet for rice seedlings and reveal a relationship between reactive oxygen species, hormone homeostasis and carbon availability, which regulates growth responses.

Elevated CO2 induces age-dependent restoration of growth and metabolism in gibberellin-deficient plants.

MAIN CONCLUSION: The effect of elevated [CO2] on the growth of tomato plants with reduced gibberellin content is influenced by developmental stage. The impact of increased atmospheric carbon dioxide (CO2) on plants has aroused interest in the last decades. Signaling molecules known as plant hormones are fundamental controllers of plant growth and development. Elevated CO2 concentration ([CO2]) increases plant growth; however, whether plant hormones act as mediators of this effect is still an open question. Here, we show the response to elevated [CO2] in tomato does not require a functional gibberellin (GA) biosynthesis pathway. We compared growth and primary metabolism between wild-type (WT) and GA-deficient mutant (gib-1) plants transferred from ambient (400 ppm) to elevated (750 ppm) [CO2] at two different growth stages (either 21 or 35 days after germination, DAG). Growth, photosynthetic parameters and primary metabolism in the stunted gib-1 plants were restored when they were transferred to elevated [CO2] at 21 DAG. Elevated [CO2] also stimulated growth and photosynthetic parameters in WT plants at 21 DAG; however, only minor changes were observed in the level of primary metabolites. At 35 DAG, on the other hand, elevated [CO2] did not stimulate growth in WT plants and gib-1 mutants showed their characteristic stunted growth phenotype. Taken together, our results reveal that elevated [CO2] enhances growth only within a narrow developmental window, in which GA biosynthesis is dispensable. This finding could be relevant for breeding crops in the face of the expected increases in atmospheric CO2 over the next century.

Selenium downregulates auxin and ethylene biosynthesis in rice seedlings to modify primary metabolism and root architecture.

MAIN CONCLUSION: Selenium modulates the formation of primary and lateral roots through alterations in auxin and ethylene, leading to new patterns of root architecture in rice seedlings. Selenium (Se) at low concentrations can control root growth through interaction with hormone biosynthesis. Auxin and ethylene have been shown to control the root architecture, with most of the information obtained from the eudicots such Arabidopsis and Nicotiana tabacum. Here, we presented the effects of Se on auxin and ethylene pathways and examined their impact on primary metabolism and root system architecture in rice (Oryza sativa L.) seedlings. Se treatment increased elongation of primary root, but decreased the number and length of lateral roots. Se led to decreased expression of genes associated with the biosynthesis of auxin and ethylene, concomitantly with reduced production of these hormones by the roots. Moreover, Se decreased the abundance of transcripts encoding auxin transport proteins. Indole-3-acetic acid (IAA) treatment overrode the repressive effect of Se on lateral root growth. The ethylene synthesis inhibitor L-α-(2-aminoethoxyvinyl)-glycine (AVG) increased elongation of primary root, whereas the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) resulted in the opposite effect. Soluble sugars accumulate in roots of rice seedlings under Se treatment. Thus, Se modulates the formation of primary and lateral roots through alterations in auxin and ethylene, leading to new patterns of root architecture in rice seedlings.

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.

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

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.