Involvement of the 4-coumarate:coenzyme A ligase 4CL4 in rice phosphorus acquisition and rhizosphere microbe recruitment via root growth enlargement.

MAIN CONCLUSION: The 4-coumarate:coenzyme A ligase 4CL4 is involved in enhancing rice P acquisition and use in acid soil by enlarging root growth and boosting functional rhizosphere microbe recruitment. Rice (Oryza sativa L.) cannot easily acquire phosphorus (P) from acid soil, where root growth is inhibited and soil P is fixed. The combination of roots and rhizosphere microbiota is critical for plant P acquisition and soil P mobilization, but the associated molecular mechanism in rice is unclear. 4CL4/RAL1 encodes a 4-coumarate:coenzyme A ligase related to lignin biosynthesis in rice, and its dysfunction results in a small rice root system. In this study, soil culture and hydroponic experiments were conducted to examine the role of RAL1 in regulating rice P acquisition, fertilizer P use, and rhizosphere microbes in acid soil. Disruption of RAL1 markedly decreased root growth. Mutant rice plants exhibited decreased shoot growth, shoot P accumulation, and fertilizer P use efficiency when grown in soil-but not under hydroponic conditions, where all P is soluble and available for plants. Mutant ral1 and wild-type rice rhizospheres had distinct bacterial and fungal community structures, and wild-type rice recruited some genotype-specific microbial taxa associated with P solubilization. Our results highlight the function of 4CL4/RAL1 in enhancing rice P acquisition and use in acid soil, namely by enlarging root growth and boosting functional rhizosphere microbe recruitment. These findings can inform breeding strategies to improve P use efficiency through host genetic manipulation of root growth and rhizosphere microbiota.

Role of Silicon in Mediating Phosphorus Imbalance in Plants.

The soil bioavailability of phosphorus (P) is often low because of its poor solubility, strong sorption and slow diffusion in most soils; however, stress due to excess soil P can occur in greenhouse production systems subjected to high levels of P fertilizer. Silicon (Si) is a beneficial element that can alleviate multiple biotic and abiotic stresses. Although numerous studies have investigated the effects of Si on P nutrition, a comprehensive review has not been published. Accordingly, here we review: (1) the Si uptake, transport and accumulation in various plant species; (2) the roles of phosphate transporters in P acquisition, mobilization, re-utilization and homeostasis; (3) the beneficial role of Si in improving P nutrition under P deficiency; and (4) the regulatory function of Si in decreasing P uptake under excess P. The results of the reviewed studies suggest the important role of Si in mediating P imbalance in plants. We also present a schematic model to explain underlying mechanisms responsible for the beneficial impact of Si on plant adaption to P-imbalance stress. Finally, we highlight the importance of future investigations aimed at revealing the role of Si in regulating P imbalance in plants, both at deeper molecular and broader field levels.

Ammonium Alleviates Manganese Toxicity and Accumulation in Rice by Down-Regulating the Transporter Gene OsNramp5 Through Rhizosphere Acidification.

Ammonium ( N H 4 + ) alleviates manganese (Mn) toxicity in various plant species, but the underlying mechanisms are still unclear. In this study, we compared the effects of N H 4 + and nitrate ( N O 3 - ) on rice (Oryza sativa L.) growth, accumulation and distribution of Mn, accumulation of iron (Fe), zinc (Zn) and copper (Cu), root cell wall components, and expression of Mn and Fe transporter genes. After rice seedlings were grown in non-pH-buffered nutrient solution for 2 days, the pH of growth medium changed from an initial value of 4.5 to 3.5 and to 5.5 in the presence of N H 4 + and in the presence of N O 3 - , respectively. Compared with N O 3 - , ammonium decreased nutrient-solution pH and alleviated Mn toxicity and accumulation in rice under non-pH-buffered conditions. This alleviation disappeared when 5 mM Homo-PIPES pH buffer was added. Regardless of N form, roots, shoots, root cell sap, and xylem sap accumulated much lower Mn at pH 3.5 than at pH 5.5, whereas Mn distribution in different leaves and Mn accumulation in root cell walls was affected by neither N form nor pH. Ammonium decreased the expression of the Mn influx transporter gene OsNramp5 in roots under non-pH-buffered conditions, but not under pH-buffered ones. OsNramp5 expression was down-regulated at pH 3.5 compared with pH 5.5. Another efflux Mn transporter gene, OsMTP9, was not regulated by either N form or pH. High pH (5.5) enhanced the expression of the Fe transporter gene OsIRT1 and increased the accumulation of Zn but not Fe or Cu in shoots compared with pH 3.5. Taken together, our results indicate that N H 4 + alleviates Mn toxicity and accumulation in rice through the down-regulatory effects of rhizosphere acidification on the Mn influx transporter gene OsNramp5. In addition, the up-regulation of OsIRT1 expression may contribute to the increased Zn uptake by rice at high pH of nutrient solution.

Putrescine alleviates aluminum toxicity in rice (Oryza sativa) by reducing cell wall Al contents in an ethylene-dependent manner.

Aluminum (Al3+ ) toxicity in acidic soils limits crop productivity worldwide. In this study, we found that putrescine (PUT) significantly alleviates Al toxicity in rice roots. The addition of 0.1 mM PUT promoted root elongation and reduced the Al content in the root apices of Nipponbare (Nip) and Kasalath (Kas) rice under Al toxicity conditions. Exogenous treatment with PUT reduced the cell wall Al content by reducing polysaccharide (pectin and hemicellulose) levels and pectin methylesterase (PME) activity in roots and decreased the translocation of Al from the external environment to the cytoplasm by downregulating the expression of OsNRAT1, which responsible to encode an Al transporter protein Nrat1 (Nramp aluminum transporter 1). The addition of PUT under Al toxicity conditions significantly inhibited ethylene emissions and suppressed the expression of genes involved in ethylene biosynthesis. Treatment with the ethylene precursor 1-aminocylopropane-1-carboxylic acid (ACC) significantly improved ethylene emission, inhibited root elongation, increased the Al accumulation in root tips and the root cell wall, and increased cell wall pectin and hemicellulose contents in both rice cultivars under Al toxicity conditions. The ethylene biosynthesis antagonist aminoethoxyvinylglycine (AVG, inhibitor of the ACC synthase) had the opposite effect and reduced PME activity. Together, our results show that PUT decreases the cell wall Al contents by suppressing ethylene emissions and decreases the symplastic Al levels by downregulating OsNRAT1 in rice.

Boron reduces cell wall aluminum content in rice (Oryza sativa) roots by decreasing H2O2 accumulation.

When boron (B) deficiency and aluminum (Al) toxicity co-exist in acidic soils, crop productivity is limited. In the current study, we found that 3 µM of B pretreatment significantly enhances rice root elongation under Al toxicity conditions. Pretreatment with B significantly decreases the deposition of Al in rice apoplasts, suppresses the synthesis of cell wall pectin, inhibits cell wall pectin methylesterase (PME) activity and its gene expression, and increases the expression of OsSTAR1 and OsSTAR2, which are responsible for reducing the Al content in the cell walls. In addition, B pretreatment significantly increases OsALS1 expression, thereby facilitating the transfer of Al from the cytoplasm to the vacuoles. However, B pretreatment had no effect on Al uptake and citric acid secretion. Pretreatment with B significantly increases the activity of ascorbate peroxidase (APX), peroxidase (POD), and catalase (CAT), thus increasing the elimination rate of H2O2 in rice roots. Co-treatment using B and H2O2 does not increase root growth under Al toxicity conditions; it also improves pectin synthesis, enhances PME activity, and increases Al deposition in root cell walls. However, the co-treatment of B and H2O2 scavenger 4-hydroxy-TEMPO has an opposite effect. The above results indicate that applying B fertilizers in acidic soil can help decrease the side effects of Al toxicity on rice growth.

Ammonium mitigates Cd toxicity in rice (Oryza sativa) via putrescine-dependent alterations of cell wall composition.

In plants, different forms of nitrogen (NO3- or NH4+) affect nutrient uptake and environmental stress responses. In the present study, we tested whether NO3- and NH4+ affect the ability of rice (Oryza sativa) to tolerate the toxic heavy metal cadmium (Cd). Compared with NO3-, NH4+ treatment significantly increased chlorophyll contents and reduced Cd2+ levels in rice cultivars Nipponbare (japonica) and Kasalath (indica) grown in 0.2 mM Cd2+. NH4+ significantly reduced the pectin and hemicellulose contents and inhibited the pectin methylesterase (PME) activity in rice roots, thereby reducing the negative charges in the cell wall and decreasing the accumulation of Cd2+ in roots. In addition, NH4+ reduced the absorption and root-to-shoot translocation of Cd2+ by decreasing the expression of OsHMA2 and OsNramp5 in the root. Levels of the signaling molecule putrescine were significantly higher in the roots of both rice cultivars provided with NH4+ compared with NO3-. The addition of putrescine reduced Cd2+ contents in both rice cultivars and increased the chlorophyll content in shoots by reducing root cell wall pectin and hemicellulose contents, inhibiting PME activity and suppressing the expression of OsHMA2 and OsNramp5 in the root. Taken together, these results indicate that NH4+ treatment alleviated Cd toxicity, enabling rice to withstand the noxious effects of Cd by modifying the cell wall Cd-binding capacity due to alterations of pectin and hemicellulose contents and Cd transport, processes induced by increasing putrescine levels. Our findings suggest methods to decrease Cd accumulation in rice by applying NH4+ fertilizers.

[Effects of different rotation systems on greenhouse gas (CH4 and N2O) emissions in the Taihu Lake region, China].

We conducted a greenhouse gas emissions study of different rice-based cropping systems in the Taihu Lake region. The results indicated that the seasonal CH4 emission initially increased, but declined over time. CH4emission mainly occurred during the early stages of rice growth and decreased after the paddy soil dried. N2O emission mainly occurred during the fertilizer application and paddy field drying stages. Compared with N20 emission, CH4 emission contributed significantly more to the global warming potential (GWP) during the rice season. The proportion of CH4 emission to the total greenhouse gas emissions, which this study aimed to reduce, ranged from 94.7%-99.6%. CH4emissions and their GWP during the rice season varied significantly under different rotation systems, with the order of wheat-rice rotation>Chinese milk vetch-rice rotation>fallow-rice rotation, while the N2O emissions and their GWP exhibited no significant differences. Compared with no nitrogen fertilization, applying N fertilizer significantly reduced CH4 emission and GWP of the Chinese milk vetch-rice rotation. However, CH4 emission and GWP did not vary with N application rates. The rice yield was largest when the N application rate was 240 kg · hm⁻². Taking economic and environmental benefits into account, we found that a N application rate of 240 kg · hm⁻² and a straw-return application of Chinese milk vetch not only reduced greenhouse gas emissions but also achieved the highest rice grain yield, which was recommended as a suitable cropping system for the Taihu Lake region.

Differential Effects of Nitrogen Forms on Cell Wall Phosphorus Remobilization Are Mediated by Nitric Oxide, Pectin Content, and Phosphate Transporter Expression.

NH4 (+) is a major source of inorganic nitrogen for rice (Oryza sativa), and NH4 (+) is known to stimulate the uptake of phosphorus (P). However, it is unclear whether NH4 (+) 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 NH4 (+) positively regulates the pectin content and activity of pectin methylesterase in root cell walls under -P 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 NH4 (+) was applied as the sole nitrogen source compared with the NO3 (-) 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 NH4 (+) 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 NH4 (+) stimulates Pi reutilization in cell walls of rice.