The Oligopeptide Permease (Opp) of the Plant Pathogen <Emphasis Type="Italic">Xanthomonas axonopodis</Emphasis> pv. <Emphasis Type="Italic">citri</Emphasis>

The oligopeptide permease (Opp), a protein-dependent ABC transporter, has been found in the genome of Xanthomonas axonopodis pv. citri (Xac), but not in Xanthomonas campestris pv. campestris (Xcc). Sequence analysis indicated that 4 opp genes (oppA, oppB, oppC, oppD/F), located in a 33.8-kbp DNA fragment present only in the Xac genome, are arranged in an operon-like structure and share highest sequence similarities with Streptomyces roseofulvus orthologs. Nonetheless, analyses of the GC content, codon usage, and transposon positioning suggested that the Xac opp operon does not have an exogenous origin. The presence of a stop codon at one of the ATP-binding domains of OppD/F would render the uptake system nonfunctional, but detection of a single polycistronic mRNA and periplasmic OppA in actively growing bacteria suggests that the Opp permease is active and could contribute to the distinct nutritional requirements and host specificities of the two Xanthomonas species.     

Towards soil management with Zn and Mn: estimates of fertilisation efficacy of Citrus trees

Nutrient management recommendations for fruit crops lack the understanding of the efficiency of soil fertilisation with manganese (Mn) and zinc (Zn), which could substitute, in part, the traditional foliar applications. Fruit yield of trees in response to Zn and Mn supply via soil may be limited either by sorption reactions with soil colloids or low solubility of fertilisers. We investigated the effects of fertiliser sources and rates of Mn and Zn applied to soils with different sorption capacities on nutrient uptake, biochemical responses and biomass of Citrus. Two experiments were carried out with 2‐year‐old sweet orange trees that received applications of Mn or Zn. The first experiment evaluated the application of Mn fertilisers (MnCO₃ and MnSO₄) at three levels of the nutrient (0, 0.7 and 3.5 g plant^?1 of Mn) in two types of soil (18.1% and 64.4% of clay, referred to as sandy loam and clay soils, respectively). The second experiment, likewise, evaluated Zn fertilisers (ZnO and ZnSO₄) and nutrient levels (0, 1.0 and 5.0 g plant^?1 of Zn). Application of Mn and Zn increased nutrient availability in the soils as well as leaf nutrient concentrations in the trees. The lowest rates, 0.7 g plant^?1 of Mn and 1.0 g plant^?1 of Zn, both as sulphate, were sufficient to supply these micronutrients to sufficient levels in leaves, flowers and fruits. Metal toxicity to plants occurred with higher doses of both nutrients and to a large extent in the sandy soil. In this case, protein bands lower than 25 kDa were observed as well a decrease on leaf chlorophyll content. In the clay soil, despite increased micronutrient concentrations in the plant, responses were less pronounced because of higher adsorption of metals in the soil. Superoxide dismutase (SOD, EC 1.15.1.1) isoenzyme activity was determined by non‐denaturing polyacrylamide gel electrophoresis (PAGE). The Cu/Zn‐SOD isoenzymes increased with increased Zn rates, but in contrast, when Mn was applied at the highest rate, the activity of Cu/Zn‐SODs decreased. The SOD activity pattern observed indicated increased production of superoxide and consequently an oxidative stress condition at the highest rates of Zn and Mn applied. The results demonstrated that the soil application of Mn and Zn can supply nutrient demands of orange trees, however the low solubility of fertilisers and the high sorption capacity of soils limit fertilisation efficiency. On the contrary, application of sulphate source in sandy soils may cause excess uptake of Mn and Zn and oxidative stress, which impairs the photosynthetic apparatus and consequently tree growth.     

The possible role of extra magnesium and nitrogen supply to alleviate stress caused by high irradiation and temperature in lemon trees

Plant and Soil - The aims of this study were to (i) assess how the P concentration in leaves of common Australian native plant species varies across a broad geographic region that includes...     

Far-red light blocks greening of Arabidopsis seedlings via a phytochrome A-mediated change in plastid development.

We have characterized a far-red-light response that induces a novel pathway for plastid development in Arabidopsis seedlings. This response results in the inability of cotyledons to green upon subsequent white light illumination, and the response is suppressed by exogenous sucrose. Studies with mutants showed that this far-red block of greening is phytochrome A dependent and requires an intact downstream signaling pathway in which FHY1 and FHY3 may be components but in which HY5 is not. This highlights a previously undefined branchpoint in the phytochrome signaling pathway. Ultrastructural analysis showed that the far-red block correlates with both the failure of plastids to accumulate prolamellar bodies and the formation of vesicles in the stroma. We present evidence that the far-red block of greening is the result of severe repression of protochlorophyllide reductase (POR) genes by far-red light coupled with irreversible plastid damage. This results in the temporal separation of phytochrome-mediated POR; repression from light-dependent protochlorophyllide reduction, two processes that normally occur in coordination in white light.     

Uptake and Distribution of Soil Applied Zinc by Citrus Trees—Addressing Fertilizer Use Efficiency with 68Zn Labeling

The zinc (Zn) supply increases the fruit yield of Citrus trees that are grown, especially in the highly weathered soils of the tropics due to the inherently low nutrient availability in the soil solution. Leaf sprays containing micronutrients are commonly applied to orchards, even though the nutrient supply via soil could be of practical value. This study aimed to evaluate the effect of Zn fertilizers that are applied to the soil surface on absorption and partitioning of the nutrient by citrus trees. A greenhouse experiment was conducted with one-year-old sweet orange trees. The plants were grown in soils with different textures (18.1 or 64.4% clay) that received 1.8 g Zn per plant, in the form of either ZnO or ZnSO₄ enriched with the stable isotope ⁶⁸Zn. Zinc fertilization increased the availability of the nutrient in the soil and the content in the orange trees. Greater responses were obtained when ZnSO₄ was applied to the sandy loam soil due to its lower specific metal adsorption compared to that of the clay soil. The trunk and branches accumulated the most fertilizer-derived Zn (Zn_dff) and thus represent the major reserve organ for this nutrient in the plant. The trees recovered up to 4% of the applied Zn_dff. Despite this relative low recovery, the Zn requirement of the trees was met with the selected treatment based on the total leaf nutrient content and increased Cu/Zn-SOD activity in the leaves. We conclude that the efficiency of Zn fertilizers depends on the fertilizer source and the soil texture, which must be taken into account by guidelines for fruit crop fertilization via soil, in substitution or complementation of traditional foliar sprays.     

Absorption of 15NH3 volatilized from urea by Citrus trees

Background and aims

Gaseous losses of ammonia (NH₃) have been observed in citrus orchards when urea is surface-applied to the soils, and this loss might significantly limit the effectiveness of the nitrogen (N) fertilizer. However, a portion of the volatilized NH₃ might be absorbed by the plants through the leaves. To quantify the contribution of the leaf absorption of ¹⁵NH₃, a study with sweet oranges was conducted in two field areas where trees were grown at standard (480 trees ha^?1) and high densities (617 trees ha^?1).

Methods

Plastic trays were filled with soil, covered with mown grass to simulate field management conditions, fertilized with ¹⁵N labeled urea (12 atom % excess) and placed under each of three trees in the orchards. This experimental procedure prevented the uptake of N from the labeled urea by the roots. Two weeks after ¹⁵N fertilization, the trays were removed from the field, and the soil was homogenized and sampled for chemical analyses. The citrus trees under which the trays were placed were destructively harvested, and the total N concentrations and ¹⁵N/¹⁴N ratios were determined.

Results

After urea application, the NH₃ losses peaked within three days and subsequently decreased to negligible amounts after 10 days. The total NH₃ losses accounted for 55–82 % of the applied N. Although the NH₃ absorption by the citrus leaves was proportional to the tree density in the field, only 3–7 % of the ¹⁵NH₃ volatilized from the soil was recovered by the citrus trees, and the NH₃ absorption was also influenced by the proximity of citrus trees to the site of urea application and the leaf areas of the trees.

Conclusions

The citrus trees can absorb the NH₃ volatilized from urea, even though, the amount recovered by the trees is small and does not represent a significant proportion of total gaseous N losses, what demonstrates the importance of enhanced N use efficiency practices in field to reduce losses of NH₃ when urea is applied to soil surfaces.     

fhy1 defines a branch point in phytochrome A signal transduction pathways for gene expression

Physiological analysis of the fhy1 mutant of Arabidopsis has led to the proposal that the mutant is deficient in a downstream component of the phytochrome A signal transduction pathway. To define this lesion at the molecular level, we have examined the expression of a range of phytochrome-regulated genes in fhy1. In far-red light, the regulation of genes such as CHS and CHI is blocked in fhy1, whereas the induction of CAB and NR genes is affected minimally. In contrast, the induction of all genes tested is blocked in a phytochrome A-deficient mutant, confirming that gene expression in far-red light is regulated solely by phytochrome A. Thus, fhy1 defines a branch point in phytochrome A signal transduction pathways for gene expression. Contrary to the general opinion that responses to continuous red light are mediated by phytochrome B and other photostable phytochromes, we have shown also that red light-induction of CHS is mediated almost entirely by phytochrome A. Furthermore, phytochrome A-mediated induction of CHS by red light is blocked in fhy1. The induction of CHS by blue light, however, is normal in fhy1, suggesting that although FHY1 is a component of the phytochrome A signaling pathway, it is not a component of the blue-light signaling pathway for CHS expression.     

Parallel measurement of Ca2+ binding and fluorescence emission upon Ca2+ titration of recombinant skeletal muscle troponin C. Measurement of sequential calcium binding to the regulatory sites

Calcium binding to chicken recombinant skeletal muscle TnC (TnC) and its mutants containing tryptophan (F29W), 5-hydroxytryptophan (F29HW), or 7-azatryptophan (F29ZW) at position 29 was measured by flow dialysis and by fluorescence. Comparative analysis of the results allowed us to determine the influence of each amino acid on the calcium binding properties of the N-terminal regulatory domain of the protein. Compared with TnC, the Ca(2+) affinity of N-terminal sites was: 1) increased 6-fold in F29W, 2) increased 3-fold in F29ZW, and 3) decreased slightly in F29HW. The Ca(2+) titration of F29ZW monitored by fluorescence displayed a bimodal curve related to sequential Ca(2+) binding to the two N-terminal Ca(2+) binding sites. Single and double mutants of TnC, F29W, F29HW, and F29ZW were constructed by replacing aspartate by alanine at position 30 (site I) or 66 (site II) or both. Ca(2+) binding data showed that the Asp --> Ala mutation at position 30 impairs calcium binding to site I only, whereas the Asp --> Ala mutation at position 66 impairs calcium binding to both sites I and II. Furthermore, the Asp --> Ala mutation at position 30 eliminates the differences in Ca(2+) affinity observed for replacement of Phe at position 29 by Trp, 5-hydroxytryptophan, or 7-azatryptophan. We conclude that position 29 influences the affinity of site I and that Ca(2+) binding to site I is dependent on the previous binding of metal to site II.