To elucidate the mechanism of clubroot suppression under neutral soil pH, a highly reproducible germination assay system under soil culture conditions was designed based on the hypothesis that germinated spores of Plasmodiophora brassicae could be identified by the absence of a nucleus (i.e. having released a zoospore to infect a root hair of the host plant). Brassica rapa var. perviridis seedlings were inoculated with a spore suspension of P. brassicae at a rate of 2·0 × 10 6 spores gsoil and grown in a growth chamber for 7 days. The spores were recovered from rhizosphere and non-rhizosphere soils and stained with both Fluorescent Brightener 28 (cell-wall-specific) and SYTO 82 orange fluorescent nucleic-acid stain (nucleus-specific stain). Total numbers of spores were counted under UV-excitation, and spores with a nucleus that fluoresced orange under G-excitation were counted. The significant increase in the percentage of spores without a nucleus (germinated spores) in the rhizosphere after 7 days' cultivation and the correlation with root-hair infections validated the assay system. Applications of calcium-rich compost or calcium carbonate to neutralize the soil significantly reduced the percentage of germinated spores in the rhizosphere, as well as the number of root-hair infections. The present study provides direct evidence that the inhibition of spore germination is the primary cause of disease suppression under neutral soil pH.
Clubroot disease of cruciferous plants caused by the soil-borne pathogen Plasmodiophora brassicae is difficult to control because the pathogen survives for a long time in soil as resting spores. Disease-suppressive and conducive soils were found during the long-term experiment on the impact of organic matter application to arable fields and have been studied to clarify the biotic and abiotic factors involved in the disease suppression. The fact that a large amount of organic matter, 400 t ha−1 yr−1 farmyard manure (FYM) or 100 t ha−1 yr−1 food factory sludge compost (FSC), had been incorporated for more than 15 yr in the suppressive soils and these soils showed higher pH and Ca concentration than the disease conducive soil led us to hypothesize that an increase in soil pH due to the long-term incorporation of Ca-rich organic matter might be the primary cause of the disease suppression. We have designed a highly reproducible bioassay system to examine this hypothesis. The suppressive and conducive soils were mixed with the resting spores of P. brassicae at a rate of 106 spore g−1 soil, and Brassica campestris was grown in a growth chamber for 8 d. The number of root hair infections was assessed on a microscope. It was found that the incorporation of FYM and FSC at 2.5% (w/w) to the conducive soil suppressed the infection and that the finer particles (5 mm) of FSC inhibited the infection and increased soil pH more effectively. Neutralization of the conducive soil by Ca(OH)2, CaCO3 and KOH suppressed the infection, but the effectiveness of KOH was less than those of Ca(OH)2 and CaCO3. Acidification of the suppressive soils by H2SO4, promoted the infection. The involvement of soil biota in the disease suppression was investigated using the sterilized (γ-ray irradiation) suppressive soils with respect to soil pH. The γ-ray irradiation promoted the infection at pH 5.5, but no infection was observed at pH 7.4 irrespective of the sterilization status. All these observations suggest that soil pH is a major factor in disease suppression by organic matter application and that Ca and soil biota play certain roles in the suppression under the influence of soil pH
Eukaryotes possess eight highly conserved Lsm (like Sm) proteins that assemble into circular, heteroheptameric complexes, bind RNA, and direct a diverse range of biological processes. Among the many essential functions of Lsm proteins, the cytoplasmic Lsm1-7 complex initiates mRNA decay, while the nuclear Lsm2-8 complex acts as a chaperone for U6 spliceosomal RNA. It has been unclear how these complexes perform their distinct functions while differing by only one out of seven subunits. Here, we elucidate the molecular basis for Lsm-RNA recognition and present four high-resolution structures of Lsm complexes bound to RNAs. The structures of Lsm2-8 bound to RNA identify the unique 2′,3′ cyclic phosphate end of U6 as a prime determinant of specificity. In contrast, the Lsm1-7 complex strongly discriminates against cyclic phosphates and tightly binds to oligouridylate tracts with terminal purines. Lsm5 uniquely recognizes purine bases, explaining its divergent sequence relative to other Lsm subunits. Lsm1-7 loads onto RNA from the 3′ end and removal of the Lsm1 C-terminal region allows Lsm1-7 to scan along RNA, suggesting a gated mechanism for accessing internal binding sites. These data reveal the molecular basis for RNA binding by Lsm proteins, a fundamental step in the formation of molecular assemblies that are central to eukaryotic mRNA metabolism.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.