On the specific difference in the local necrotic response to infection by tobacco mosaic and cabbage black ringspot viruses in tobaccos

Dědi?ně faktory podmiňující v rostlině schopnost reagovat na infekci virem mozaiky tabáku (Nicotiana virus 1) a virem ?erné krou?kovitosti zeli (Brassica virus 1) lokálními nekrotickými lézemi, resp. systémovoù infekcí, jsou u těchto dvou vir? Jen zdánlivě v inversní závislosti. Nicotiana tabacum var. Xanthi-nc, který získal nekrotický faktor z N. glutinosa, má je?tě nekrotický faktor pro infekci virem ?erné krou?kovitosti zelí.     

PR-Set7-dependent lysine methylation ensures genome replication and stability through S phase

PR-Set7/SET8 is a histone H4–lysine 20 methyltransferase required for normal cell proliferation. However, the exact functions of this enzyme remain to be determined. In this study, we show that human PR-Set7 functions during S phase to regulate cellular proliferation. PR-Set7 associates with replication foci and maintains the bulk of H4-K20 mono- and trimethylation. Consistent with a function in chromosome dynamics during S phase, inhibition of PR-Set7 methyltransferase activity by small hairpin RNA causes a replicative stress characterized by alterations in replication fork velocity and origin firing. This stress is accompanied by massive induction of DNA strand breaks followed by a robust DNA damage response. The DNA damage response includes the activation of ataxia telangiectasia mutated and ataxia telangiectasia related kinase–mediated pathways, which, in turn, leads to p53-mediated growth arrest to avoid aberrant chromosome behavior after improper DNA replication. Collectively, these data indicate that PR-Set7–dependent lysine methylation during S phase is an essential posttranslational mechanism that ensures genome replication and stability.     

Nitrate supply affects ammonium transport in canola roots

Plants may suffer from ammonium (NH4+) toxicity when NH4+ is the sole nitrogen source. Nitrate (NO3-) is known to alleviate NH4+ toxicity, but the mechanisms are unknown. This study has evaluated possible mechanisms of NO3- alleviation of NH4+ toxicity in canola (Brassica napus L.). Dynamics of net fluxes of NH4+, H+, K+ and Ca2+ were assessed, using a non-invasive microelectrode (MIFE) technique, in plants having different NO3- supplies, after single or several subsequent increases in external NH4Cl concentration. After an increase in external NH4Cl without NO3-, NH4+ net fluxes demonstrated three distinct stages: release (tau1), return to uptake (tau2), and a decrease in uptake rate (tau3). The presence of NO3- in the bathing medium prevented the tau1 release and also resulted in slower activation of the tau3 stage. Net fluxes of Ca2+ were in the opposite direction to NH4+ net fluxes, regardless of NO3- supply. In contrast, H+ and K+ net fluxes and change in external pH were not correlated with NH4+ net fluxes. It is concluded that (i) NO3- primarily affects the NH4+ low-affinity influx system; and (ii) NH4+ transport is inversely linked to Ca2+ net flux.     

Ammonium and nitrate uptake by the floating plant Landoltia punctata

BACKGROUND AND AIMS: Plants from the family Lemnaceae are widely used in ecological engineering projects to purify wastewater and eutrophic water bodies. However, the biology of nutrient uptake mechanisms in plants of this family is still poorly understood. There is controversy over whether Lemnaceae roots are involved in nutrient uptake. No information is available on nitrogen (N) preferences and capacity of Landoltia punctata (dotted duckweed), one of the best prospective species in Lemnaceae for phytomelioration and biomass production. The aim of this study was to assess L. punctata plants for their ability to take up NH4+ and NO3- by both roots and fronds. METHODS: NO3- and NH4+ fluxes were estimated by a non-invasive ion-selective microelectrode technique. This technique allows direct measurements of ion fluxes across the root or frond surface of an intact plant. KEY RESULTS: Landoltia punctata plants took up NH4+ and NO3- by both fronds and roots. Spatial distribution of NH4+ and NO3- fluxes demonstrated that, although ion fluxes at the most distal parts of the root were uneven, the mature part of the root was involved in N uptake. Despite the absolute flux values for NH4+ and NO3- being lower in roots than at the frond surface, the overall capacity of roots to take up ions was similar to that of fronds because the surface area of roots was larger. L. punctata plants preferred to take up NH4+ over NO3- when both N sources were available. CONCLUSIONS: Landoltia punctata plants take up nitrogen by both roots and fronds. When both sources of N are available, plants prefer to take up NH4+, but will take up NO3- when it is the only N source.     

Growth,photosynthesis and H+-ATPase activity in two Jerusalem artichoke varieties under NaCl-induced stress

In order to evaluate differential growth, photosynthesis and H⁺-ATPase activity responses to salt-induced stress, two Jerusalem artichoke (Helianthus tuberosus L.) genotypes (Nanyu No. 1 and Qingyu No. 2) were used in sand-culture experiment with different concentrations of NaCl (0, 30, 60, 90, 120 and 150 mM). After 20 days of growth, the NaCl stress resulted in a decrease of biomass accumulation, relative leaf expansion rate and photosynthetic rate, but an increase of proline content in both genotypes. Compared with Qingyu No. 2, Nanyu No. 1 had lower biomass, photosynthetic rate, gas exchange and transpiration rate, but higher proline content, activities of plasma membrane H⁺-ATPase (PM H⁺-ATPase) and vacuolar membrane H⁺-ATPase (VM H⁺-ATPase). Hence, the NaCl adaptation strategy in Nanyu No. 1 was by lowering photosynthetic rate, stomatal conductance and transpiration rate while maintaining high H⁺-ATPase activities, whereas the adjustment of Qingyu No. 2 was by keeping much higher rate of proline accumulation and concentration of chlorophyll. The differences in salt tolerance showed that different adaptation mechanisms existed between cultivars of Jerusalem artichoke. The findings offered the possibility of selecting salt-tolerant genotypes of Jerusalem artichoke.     

Rhizosphere Properties of Poaceae Genotypes Under P-limiting Conditions

Plant genotypes differ in P efficiency, i.e. their capacity to grow in soil with low P availability. Plant properties such as root and root hair length, release of P mineralising and mobilising compounds by the roots and P requirement for optimal growth are known to influence P efficiency. In order to improve the understanding of the role of rhizosphere properties in plant P uptake, we grew three Poaceae genotypes [two wheat (Triticum aestivum L.) genotypes (the P-efficient Goldmark and the P-inefficient Janz), and the Australian native grass Austrostipa densiflora L.] to maturity in an acidic loamy sand with low P availability. Addition of 120 mg P as FePO₄ kg⁻¹ (P120) improved the growth of all three genotypes. In both P0 and P120, growth and P uptake were smaller in Janz than in Goldmark. During the vegetative phase, growth and P uptake of Austrostipa were smaller than in Goldmark in P0 but greater in P120. These differences can be explained by plant properties such as root growth, specific P uptake, mobilisation of inorganic and organic P by root exudates and P utilisation efficiency. In P120, P availability in the rhizosphere was least in Janz and greatest in Austrostipa. Microbial biomass P in the rhizosphere was least in Janz. Acid phosphatase activity was greatest in the rhizosphere of Austrostipa and least in Janz. Plant growth and P uptake were positively correlated with microbial P, acid phosphatase activity and resin P in the rhizosphere, suggesting that microorganisms contribute to uptake of P by plants in this soil. Microbial community composition in the rhizosphere [analysed by fatty acid methylester (FAME) analysis and denaturing gradient gel electrophoresis (DGGE)] differed among genotypes, changed during plant development and was affected by P addition to the soil. Genotype-specific microbial community composition in the rhizosphere may have contributed to the observed differential capacity of plants to grow at low P availability.     

Nitrogen Utilization Efficiency in Canola Cultivars at Grain Harvest

Canola (Brassica napus L.) cultivars with improved nitrogen utilization efficiency (NUE) at grain harvest are of interest to growers to reduce fertilizer inputs. Our objective was to determine whether cultivar-specific responses in NUE (seed yield per N accumulated in the whole plant) could be related to the differences in dry matter and N partitioning among various plant parts. Four spring canola cultivars were grown in a glasshouse under the conditions of low and high N supply. When compared to high-N treatment, deficient N conditions resulted in a similar decrease in dry weight for all cultivars, averaging 46% for shoot, 47% for root, and 45% for dropped leaves. The reductions in N concentrations at low-N compared to high-N treatment were much smaller and averaged 15% for shoot, 16% for root and 10% for dropped leaves. Although significant variations occurred for dry weight, N concentration and N uptake in various plant sections, all cultivars accumulated a similar amount of N in total plant biomass at harvest. However, significant differences in plant biomass, seed yield and consequently, NUE existed because more N-efficient cultivars Eyre and Charlton produced larger seed yields than less N-efficient cultivars Pinnacle and Rainbow. No consistent variations in N concentration in various plant parts could be established among tested cultivars. Thus, cultivar-specific responses in NUE were mainly attributed to the differences in the root-to-shoot ratio and harvest index. N-efficient Eyre produced seed yield similar to the highest yielding Charlton, though it had the smallest plant dry weight of all cultivars. In contrast, N-inefficient Rainbow had the largest plant biomass, but produced the smallest seed yield because of its lowest harvest index and the highest root-to-shoot ratio. The absence of cultivar×N treatment interactions indicated that cultivars performed similarly for plant biomass, N uptake and seed yield across two contrasting N supplies. Canola cultivars significantly differed in NUE despite a similar amount of absorbed N in plant biomass; more N-efficient cultivars outyielded less N-efficient ones primarily because of cultivar-specific variations in the root-to-shoot ratio and harvest index.