Hydrogen sulfide (H2S) plays a vital role in Al3+ stress resistance in plants, but the underlying mechanism is unclear. In the present study, pretreatment with 2 μM of the H2S donor NaHS significantly alleviated the inhibition of root elongation caused by Al toxicity in rice roots, which was accompanied by a decrease in Al contents in root tips under 50 μM Al3+ treatment. NaHS pretreatment decreased the negative charge in cell walls by reducing the activity of pectin methylesterase and decreasing the pectin and hemicellulose contents in rice roots. This treatment also masked Al-binding sites in the cell wall by upregulating the expression of OsSATR1 and OsSTAR2 in roots and reduced Al binding in the cell wall by stimulating the expression of the citrate acid exudation gene OsFRDL4 and increasing the secretion of citrate acid. In addition, NaHS pretreatment decreased the symplasmic Al content by downregulating the expression of OsNRAT1, and increasing the translocation of cytoplasmic Al to the vacuole via upregulating the expression of OsALS1. The increment of antioxidant enzyme [superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), and peroxidase (POD)] activity with NaHS pretreatment significantly decreased the MDA and H2O2 content in rice roots, thereby reducing the damage of Al3+ toxicity on membrane integrity in rice. H2S exhibits crosstalk with nitric oxide (NO) in response to Al toxicity, and through reducing NO content in root tips to alleviate Al toxicity. Together, this study establishes that H2S alleviates Al toxicity by decreasing the Al content in the apoplast and symplast of rice roots.

Ethylene functions downstream of nitric oxide in cell wall phosphorus reutilization in P-deficient rice through controlling pectin biosynthesis and expression of phosphate transporter gene OsPT2.

Background and aims Plants are able to grow under phosphorus (P)-deficient conditions by coordinating Pi acquisition, translocation from roots to shoots and remobilization within the plant. Previous reports have demonstrated that cell-wall pectin contributes greatly to rice cell-wall Pi re-utilization under P-deficient conditions, but whether other factors such as ethylene also affect the pectin-remobilizing capacity remains unclear.Methods Two rice cultivars, 'Nipponbare' (Nip) and 'Kasalath' (Kas) were cultured in the þP (complete nutrient solution), ÀP (withdrawing P from the complete nutrient solution), þPþACC (1-amino-cyclopropane-1-carboxylic acid, an ethylene precursor, adding 1 lM ACC to the complete nutrient solution) and ÀPþACC (adding 1 lM ACC to ÀP nutrient solution) nutrient solutions for 7 d.Key Results After 7 d ÀP treatment, there was clearly more soluble P in Nip root and shoot, accompanied by additional production of ethylene in Nip root compared with Kas. Under P-deficient conditions, addition of ACC significantly increased the cell-wall pectin content and decreased cell-wall retained P, and thus more soluble P was released to the root and translocated to the shoot, which was mediated by the expression of the P deficiency-responsive gene OsPT2, which also strongly induced by ACC treatment under both P-sufficient and P-deficient conditions.Conclusions Ethylene positively regulates pectin content and expression of OsPT2, which ultimately makes more P available by facilitating the solubilization of P fixed in the cell wall and its translocation to the shoot.Key words: Rice, phosphorus, ethylene, cell-wall polysaccharides, pectin, transport, remobilization, gene expression. INTRODUCTIONPhosphorous (P) is a macro-element that is essential for plant growth and development. It not only provides the backbone for the biosynthesis of nucleic acids, phospholipids and the energycarrying molecule ATP, but also has a regulatory role in metabolism and signal transduction through phosphoryl group transfer and protein activation (Marschner, 1995). However, its high chemical fixation rate, slow diffusion rate and substantial fraction of organically bound P render Pi (the form of P available to plants) one of the least available nutrients for crops (Vance et al., 2003;Wu et al., 2013). For this reason, crops are often supplied with inorganic P fertilizers (Hammond et al., 2004). However, the non-renewable nature of inorganic P fertilizers means that cheap sources of P, such as phosphate rocks, will be exhausted within the next 60-90 years (Runge-Metzger, 1995). In addition, excessive P added to crops can pollute local watercourses, contributing to the process of eutrophication (Withers et al., 2001). Therefore, there is a need to develop more P-efficient crops.To cope with P deficiency, many plant species have developed two strategies (Zhu et al., 2014). One is based on maximizing P uptake from the soil. Production of more lateral roots, root hairs and root biomass has been defined as one of the most valuable P ...

+ is a major source of inorganic nitrogen for rice (Oryza sativa), and NH 4 + is known to stimulate the uptake of phosphorus (P). However, it is unclear whether NH 4 + can also stimulate P remobilization when rice is grown under P-deficient conditions. In this study, we use the two rice cultivars 'Nipponbare' and 'Kasalath' that differ in their cell wall P reutilization, to demonstrate that NH 4 + positively regulates the pectin content and activity of pectin methylesterase in root cell walls under 2P conditions, thereby remobilizing more P from the cell wall and increasing soluble P in roots and shoots. Interestingly, our results show that more NO (nitric oxide) was produced in the rice root when NH 4 + was applied as the sole nitrogen source compared with the NO 3 2. The effect of NO on the reutilization of P from the cell walls was further demonstrated through the application of the NO donor SNP (sodium nitroprusside) and c-PTIO (NO scavenger 2-(4-carboxyphenyl)-4, 4, 5, 5-tetramethylimidazoline-1-oxyl-3-oxide). What's more, the P-transporter gene OsPT2 is up-regulated under NH 4 + supplementation and is therefore involved in the stimulated P remobilization. In conclusion, our data provide novel (to our knowledge) insight into the regulatory mechanism by which NH 4 + stimulates Pi reutilization in cell walls of rice.