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.

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.

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.