Plants without arbuscular mycorrhizae. In: White PJ, Hammond JP, editors. The Ecophysiology of Plant-Phosphorus Interactions. Dordrecht: Springer Netherlands; 2008. pp. 117-42. [DOI: 10.1007/978-1-4020-8435-5_6]

Nitrate inhibits the remobilization of cell wall phosphorus under phosphorus-starvation conditions in rice (Oryza sativa)

NO₃^- not only inhibited the reutilization of cell wall P via decreasing root cell wall pectin content and PME activity, but also hampered the P translocation from root to shoot. The rice cultivars 'Kasalath' (Kas) and 'Nipponbare' (Nip) were used to demonstrate that the nitrogen source NO₃^- inhibits internal phosphorus (P) reutilization in rice under P-absence conditions. Analysis using Kas showed that the expression of - P-induced marker genes OsIPS1/2 and OsSPX1/2/3/5 are significantly higher under 1 mM NO ₃^- - P (1N - P) treatment than 0 mM NO ₃^- - P (0N - P) treatment. The absence of NO₃^- from the nutrient solution significantly increased cell wall P release by increasing pectin synthesis and increasing the activity of pectin methylesterase (PME), and also significantly improved the translocation of soluble P from the root to the shoot by increasing xylem sap P content under P-absence conditions. The rice seedlings grown in 0 mM NO₃^- accumulated significantly higher nitric oxide (NO) in the roots than those grown in 1 mM NO₃^-. Exogenously applying the NO donor sodium nitroprusside (SNP) revealed that NO is a major contributor to differential cell wall P remobilization in rice by mediating pectin synthesis and demethylation under different NO₃^- concentrations (0 and 1 mM) under P-deprived conditions.

Abscisic acid is involved in root cell wall phosphorus remobilization independent of nitric oxide and ethylene in rice (Oryza sativa)

BACKGROUND

Abscisic acid (ABA) is a well-studied phytohormone demonstrated to be involved in sub-sets of stress responses in plants, such as iron (Fe) deficiency and phosphorus (P) deficiency in Arabidopsis. However, whether ABA is involved in P deficiency in rice has not been frequently studied. The present study was undertaken to investigate the mechanism underlying ABA-aggravated P deficiency in rice (Oryza sativa).

RESULTS

P deficiency decreased ABA accumulation rapidly (within 1 h) in the roots. Exogenous ABA negatively regulated root and shoot soluble P contents by decreasing pectin content, inhibiting P deficiency-induced increases in pectin methylesterase activity and expression of the phosphate transporter gene-OsPT6, thereby decreasing the re-utilization of P from the cell wall and its translocation to the shoot. Moreover, neither the nitric oxide (NO) donor sodium nitroprusside nor ethylene precursor 1-aminocyclopropane-1-carboxylic acid had any effect on ABA accumulation, and application of ABA or the ABA inhibitor fluridone also had no effect on NO production and ethylene emission.

CONCLUSIONS

Under P deficiency, NO levels increase as quickly as ABA levels decrease, to inhibit both the ABA-induced reduction of pectin contents for the re-utilization of cell wall P and the ABA-induced down-regulation of OsPT6 for the translocation of P from roots to shoots. Overall, our results provide novel information indicating that the reduction of ABA under P deficiency is a very important pathway in the re-utilization of cell wall P in rice under P-deficient conditions, which should be a very effective mechanism for plant survival under P deficiency stress for common agronomic practice.

Nitric oxide acts upstream of ethylene in cell wall phosphorus reutilization in phosphorus-deficient rice

Nitric oxide (NO) and ethylene are both involved in cell wall phosphorus (P) reutilization in P-deficient rice; however, the crosstalk between them remains unclear. In the present study using P-deficient 'Nipponbare' (Nip), root NO accumulation significantly increased after 1 h and reached a maximum at 3 h, while ethylene production significantly increased after 3 h and reached a maximum at 6 h, indicating NO responded more quickly than ethylene. Irrespective of P status, addition of the NO donor sodium nitroprusside (SNP) significantly increased while the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) significantly decreased the production of ethylene, while neither the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) nor the ethylene inhibitor aminoethoxyvinylglycine (AVG) had any influence on NO accumulation, suggesting NO acted upstream of ethylene. Under P-deficient conditions, SNP and ACC alone significantly increased root soluble P content through increasing pectin content, and c-PTIO addition to the ACC treatment still showed the same tendency; however, AVG+SNP treatment had no effect, further indicating that ethylene was the downstream signal affecting pectin content. The expression of the phosphate transporter gene OsPT2 showed the same tendency as the NO-ethylene-pectin pathway. Taken together, we conclude that ethylene functions downstream of NO in cell wall P reutilization in P-deficient rice.

Phosphate Uptake and Allocation - A Closer Look at Arabidopsis thaliana L. and Oryza sativa L

This year marks the 20th anniversary of the discovery and characterization of the two Arabidopsis PHT1 genes encoding the phosphate transporter in Arabidopsis thaliana. So far, multiple inorganic phosphate (Pi) transporters have been described, and the molecular basis of Pi acquisition by plants has been well-characterized. These genes are involved in Pi acquisition, allocation, and/or signal transduction. This review summarizes how Pi is taken up by the roots and further distributed within two plants: A. thaliana and Oryza sativa L. by plasma membrane phosphate transporters PHT1 and PHO1 as well as by intracellular transporters: PHO1, PHT2, PHT3, PHT4, PHT5 (VPT1), SPX-MFS and phosphate translocators family. We also describe the role of the PHT1 transporters in mycorrhizal roots of rice as an adaptive strategy to cope with limited phosphate availability in soil.

Responses of root architecture development to low phosphorus availability: a review

BACKGROUND

Phosphorus (P) is an essential element for plant growth and development but it is often a limiting nutrient in soils. Hence, P acquisition from soil by plant roots is a subject of considerable interest in agriculture, ecology and plant root biology. Root architecture, with its shape and structured development, can be considered as an evolutionary response to scarcity of resources.

SCOPE

This review discusses the significance of root architecture development in response to low P availability and its beneficial effects on alleviation of P stress. It also focuses on recent progress in unravelling cellular, physiological and molecular mechanisms in root developmental adaptation to P starvation. The progress in a more detailed understanding of these mechanisms might be used for developing strategies that build upon the observed explorative behaviour of plant roots.

CONCLUSIONS

The role of root architecture in alleviation of P stress is well documented. However, this paper describes how plants adjust their root architecture to low-P conditions through inhibition of primary root growth, promotion of lateral root growth, enhancement of root hair development and cluster root formation, which all promote P acquisition by plants. The mechanisms for activating alterations in root architecture in response to P deprivation depend on changes in the localized P concentration, and transport of or sensitivity to growth regulators such as sugars, auxins, ethylene, cytokinins, nitric oxide (NO), reactive oxygen species (ROS) and abscisic acid (ABA). In the process, many genes are activated, which in turn trigger changes in molecular, physiological and cellular processes. As a result, root architecture is modified, allowing plants to adapt effectively to the low-P environment. This review provides a framework for understanding how P deficiency alters root architecture, with a focus on integrated physiological and molecular signalling.

Mobilization and acquisition of sparingly soluble P-Sources by Brassica cultivars under P-starved environment II. Rhizospheric pH changes, redesigned root architecture and pi-uptake kinetics

Non-mycorrhizal Brassica does not produce specialized root structures such as cluster or dauciform roots but is an effective user of P compared with other crops. In addition to P-uptake, utilization and remobilization activity, acquisition of orthophosphate (Pi) from extracellular sparingly P-sources or unavailable bound P-forms can be enhanced by biochemical rescue mechanisms such copious H(+)-efflux and/or carboxylates exudation into rhizosphere by roots via plasmalemma H(+) ATPase and anion channels triggered by P-starvation. To visualize the dissolution of sparingly soluble Ca-phosphate (Ca-P), newly formed Ca-P was suspended in agar containing other essential nutrients. With NH(4)(+) applied as the N source, the precipitate dissolved in the root vicinity can be ascribed to rhizosphere acidification, whereas no dissolution occurred with nitrate nutrition. To observe in situ rhizospheric pH changes, images were recorded after embedding the roots in agar containing bromocresol purple as a pH indicator. P-tolerant cultivar showed a greater decrease in pH than the sensitive cultivar in the culture media (the appearance of typical patterns of various colors of pH indicator in the root vicinity), and at stress P-level this acidification was more prominent. In experiment 2, low P-tolerant class-I cultivars (Oscar and Con-II) showed a greater decrease in solution media pH than low P-sensitive class-II (Gold Rush and RL-18) cultivars, and P-contents of the cultivars was inversely related to decrease in culture media pH. To elucidate P-stress-induced remodeling and redesigning in a root architectural system, cultivars were grown in rhizoboxes in experiment 3. The elongation rates of primary roots increased as P-supply increased, but the elongation rates of the branched zones of primary roots decreased. The length of the lateral roots and topological index values increased when cultivars were exposed to a P-stress environment. To elucidate Pi-uptake kinetics, parameters related to P influx: maximal transport rate (V(max)), the Michaelis-Menten constant (K(m)), and the external concentration when net uptake is zero (C(min)) were tested in experiment 4. Lower K(m) and C(min) values were better indicative of the P-uptake ability of the class-I cultivars, evidencing their adaptability to P-starved environmental cues. In experiment 5, class-I cultivars exuded two- to threefold more carboxylates than class-II cultivars under the P-stress environment. The amount and types of carboxylates exuded from the roots of P-starved plants differed from those of plants grown under P-sufficient conditions. Nevertheless, the exudation rate of both class-I and class-II cultivars decreased with time, and the highest exudation rate was found after the first 4 h of carboxylates collection. Higher P uptake by class-I cultivars was significantly related to the drop in root medium pH, which can be ascribed to H(+)-efflux from the roots supplied with sparingly soluble rock-P and Ca(3)(PO(4))(2). These classical rescue strategies provided the basis of P-solubilization and acquisition from sparingly soluble P-sources by Brassica cultivars to thrive in a typically stressful environment.