The phosphate transporters LjPT4 and MtPT4 mediate early root responses to phosphate status in non mycorrhizal roots

Arbuscular mycorrhizal (AM) symbiosis improves host plant phosphorous (P) status and elicits the expression of AM-inducible phosphate transporters (PTs) in arbuscule-containing cells, where they control arbuscule morphogenesis and P release. We confirmed such functions for LjPT4 in mycorrhizal Lotus japonicus. Promoter-GUS experiments showed LjPT4 transcription not only in arbusculated cells but also in root tips, in the absence of the fungus: here LjPT4 transcription profile depended on the phosphate level. In addition, quantitative RT-PCR confirmed the expression of Lotus and Medicago truncatula PT4 in the tips of non-mycorrhizal roots. Starting from these observations, we hypothesized that AM-inducible PTs may have a regulatory role in plant development, irrespective of the fungal presence. Firstly, we focused on root development responses to different phosphate treatments in both plants demonstrating that phosphate starvation induced a higher number of lateral roots. By contrast, Lotus PT4i plants and Medicago mtpt4 mutants did not show any differential response to phosphate levels, suggesting that PT4 genes affect early root branching. Phosphate starvation-induced genes and a key auxin receptor, MtTIR1, showed an impaired expression in mtpt4 plants. We suggest PT4 genes as novel components of the P-sensing machinery at the root tip level, independently of AM fungi

Ammonium sorption and ammonia inhibition of nitrite-oxidizing bacteria explain contrasting soil N₂O production

Better understanding of process controls over nitrous oxide (N₂O) production in urine-impacted 'hot spots' and fertilizer bands is needed to improve mitigation strategies and emission models. Following amendment with bovine (Bos taurus) urine (Bu) or urea (Ur), we measured inorganic N, pH, N₂O, and genes associated with nitrification in two soils ('L' and 'W') having similar texture, pH, C, and C/N ratio. Solution-phase ammonia (slNH₃) was also calculated accounting for non-linear ammonium (NH₄⁺) sorption capacities (ASC). Soil W displayed greater nitrification rates and nitrate (NO₃⁻) levels than soil L, but was more resistant to nitrite (NO₂⁻) accumulation and produced two to ten times less N₂O than soil L. Genes associated with NO₂⁻oxidation (nxrA) increased substantially in soil W but remained static in soil L. Soil NO₂⁻was strongly correlated with N₂O production, and cumulative (c-) slNH₃ explained 87% of the variance in c-NO₂⁻. Differences between soils were explained by greater slNH₃ in soil L which inhibited NO₂⁻oxidization leading to greater NO₂⁻ levels and N₂O production. This is the first study to correlate the dynamics of soil slNH₃, NO₂⁻, N₂O and nitrifier genes, and the first to show how ASC can regulate NO₂⁻ levels and N₂O production. © 2015 Macmillan Publishers Limited

Nitrification gene ratio and free ammonia explain nitrite and nitrous oxide production in urea-amended soils

The atmospheric concentration of nitrous oxide (N₂O), a potent greenhouse gas and ozone-depleting chemical, continues to increase, due largely to the application of nitrogen (N) fertilizers. While nitrite (NO₂⁻) is a central regulator of N₂O production in soil, NO₂⁻ and N₂O responses to fertilizer addition rates cannot be readily predicted. Our objective was to determine if quantification of multiple chemical variables and structural genes associated with ammonia (NH₃)- (AOB, encoded by amoA) and NO₂⁻ -oxidizing bacteria (NOB, encoded by nxrA and nxrB) could explain the contrasting responses of eight agricultural soils to five rates of urea addition in aerobic microcosms. Significant differences in NO₂⁻ accumulation and N₂O production by soil type could not be explained by initial soil properties. Biologically-coherent statistical models, however, accounted for 70–89% of the total variance in NO₂⁻ and N₂O. Free NH₃ concentration accounted for 50–85% of the variance in NO₂⁻ which, in turn, explained 62–82% of the variance in N₂O. By itself, the time-integrated nxrA:amoA gene ratio explained 78 and 79% of the variance in cumulative NO₂⁻ and N₂O, respectively. In all soils, nxrA abundances declined above critical urea addition rates, indicating a consistent pattern of suppression of Nitrobacter-associated NOB due to NH₃ toxicity. In contrast, Nitrospira-associated nxrB abundances exhibited a broader range of responses, and showed that long-term management practices (e.g., tillage) can induce a shift in dominant NOB populations which subsequently impacts NO₂⁻ accumulation and N₂O production. These results highlight the challenges of predicting NO₂⁻ and N₂O responses based solely on static soil properties, and suggest that models that account for dynamic processes following N addition are ultimately needed. The relationships found here provide a basis for incorporating the relevant biological and chemical processes into N cycling and N₂O emissions models

Phylogenetic analysis of P_i-responsive transporters of <i>Petunia hybrida</i> compared to <i>Arabidopsis</i> transporters for nitrate, nitrite, and peptides.

<p>For the EST sequences listed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090841#pone-0090841-t001" target="_blank">Table 1</a> the full-length predicted cDNA sequences were derived from the petunia genome sequence. Predicted petunia protein sequences were compared with <i>Arabidopsis thaliana</i> transporters for nitrate and nitrite (NRT and Nitr1, respectively), and for peptide transporters (PTR). Note the clear separation of the nitrate transporter subfamilies NRT1 and NRT2. The NRT1 family also comprises the nitrite transporter AtNitr1 and several peptide transporters, of which only two are represented (AtPTR2 and AtPTR5). Petunia has two very closely homologous representatives of the high affinity nitrate transporter family NRT2 (cn8666 and cn7864). In addition, there is a putative nitrite transporter (corresponding to the EST sequences CL1918 and cn5943), and two additional members of the low affinity NRT1 family. Genes boxed in green were analyzed by qPCR (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090841#pone-0090841-g008" target="_blank">Figure 8</a>). Cn8665, which is almost identical to cn8666, and CL5245, which is predicted to encode a nitrogen limitation adaptation gene (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090841#pone-0090841-t001" target="_blank">Table 1</a>), were omitted from phylogenetic analysis.</p

<i>R. irregularis</i> increases nutrient content of plants supplied with water or with nutrient solution.

<p>Nutrient levels in leaves were determined 36 days after inoculation in the plants shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090841#pone-0090841-g003" target="_blank">Figure 3</a>. Values are the mean of three biological replicates. Error bars represent standard deviations. Asterisks indicate significant differences between mycorrhizal roots (black columns) and non-mycorrhizal controls (white columns). (<b>a</b>) Plants were fertilized with basic nutrient solution. Values are expressed relative to the non-mycorrhizal fertilized controls that were set to 100% for each nutrient. (<b>b</b>) As in (a), but without nutrient solution. Values are expressed relative to the non-colonized water-treated controls that were set to 100% for each nutrient.</p

Shoot weight, shoot/root ratio and N/P ratio as indicators of nutritional status.

<p>Treatments were as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090841#pone-0090841-g003" target="_blank">Figure 3</a>, shown are the values of the final time point (48 days after inoculation). Columns represent the average of three biological replicates, error bars represent standard deviations. Asterisks indicate significant differences between mycorrhizal and non-mycorrhizal plants (white vs. black columns), crosses indicate significant differences between the non-mycorrhizal nutrient treatments vs. the non-mycorrhizal water treatment (i.e. between the different white columns). (a) Shoot weight of plants grown with <i>R. irregularis</i> (black column) or without (white columns) under different nutritional conditions. (b) Shoot/root ratio of plants inoculated with <i>R. irregularis</i> (black columns) or without (white columns) under various nutritional conditions. A ratio of 3.5–4 indicates that plants are well supplied with mineral nutrients, whereas a ratio around 2 indicates that plants are starved and allocate relatively large amounts of resources to the root system to compensate nutritional deficits. (c) N/P ratio of the same plants as in (a),(b). In the absence of exogenous P_i supply, mycorrhizal plants (black columns) exhibited lower N/P ratios than non-mycorrhizal controls, reflecting increased mycorrhizal P_i supply. Administration of 5 mM KH₂PO₄ reduced N/P ratio even stronger than AM, in particular if only P_i was supplied.</p

Exogenous phosphate and nitrate inhibit root colonization by <i>Rhizophagus irregularis</i>.

<p>Plants inoculated with <i>Rhizophagus irregularis</i> were watered with the basic nutrient solution, additionally supplemented with the indicated nutrient concentrations. The first column to the left in each graph corresponds to the concentration in the basic nutrient solution, except for KH₂PO₄ (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0090841#pone.0090841.s004" target="_blank">Table S2</a>); the other columns represent elevated nutrient levels as indicated. Columns represent the average of four replicate plants with standard deviations.</p

Inhibition of AM colonization by exogenous phosphate depends on the supply with other nutrients.

<p>KH₂PO₄ was applied to inoculated plants at the indicated concentrations either with basic nutrient solution (black columns) or alone (white columns). Additional treatments involved the application of KH₂PO₄ with only micronutrients (light grey) or only macronutrients (dark grey), respectively. Columns represent the average of four replicate plants with standard deviations. Asterisks indicate significant differences between phosphate alone (white bars) and phosphate with micronutrients (light grey bars), respectively, vs. the treatment with a combination of P_i and basic nutrient solution (black bars).</p

Correction: Phosphorus and Nitrogen Regulate Arbuscular Mycorrhizal Symbiosis in <i>Petunia hybrida</i>

<p>Correction: Phosphorus and Nitrogen Regulate Arbuscular Mycorrhizal Symbiosis in <i>Petunia hybrida</i></p