Members only: induced systemic resistance to herbivory in a clonal plant network

The stoloniferous herb Trifolium repens was used to study the expression of induced systemic resistance (ISR) to the generalist caterpillar Spodoptera exigua in interconnected ramets of clonal fragments. The ISR was assessed as caterpillar preference in dual choice tests between control and systemically induced plants. The ISR was detected in young ramets, after inducing older sibling ramets on the same stolon by a controlled herbivore attack. However, older ramets did not receive a defense induction signal from younger ramets unless the predominant phloem flow was reversed by means of basal shading. This provides evidence for the notion that in T. repens the clone-internal expression of ISR is coupled to phloem transport and follows source–sink gradients. The inducibility of the genotypes was not linked to their constitutive ability to produce cyanide, implying the absence of a trade-off between these two defense traits. To our knowledge, this is the first study that explores ISR to herbivory in the context of physiological integration in potentially extensive clonal plant networks.     

Metabolic profiles and genetic diversity of denitrifying communities in activated sludge after addition of methanol or ethanol

External carbon sources can enhance denitrification rates and thus improve nitrogen removal in wastewater treatment plants. The effects of adding methanol and ethanol on the genetic and metabolic diversity of denitrifying communities in activated sludge were compared using a pilot-scale plant with two parallel lines. A full-scale plant receiving the same municipal wastewater, but without external carbon source addition, was the reference. Metabolic profiles obtained from potential denitrification rates with 10 electron donors showed that the denitrifying communities altered their preferences for certain compounds after supplementation with methanol or ethanol and that methanol had the greater impact. Clone libraries of nirK and nirS genes, encoding the two different nitrite reductases in denitrifiers, revealed that methanol also increased the diversity of denitrifiers of the nirS type, which indicates that denitrifiers favored by methanol were on the rise in the community. This suggests that there might be a niche differentiation between nirS and nirK genotypes during activated sludge processes. The composition of nirS genotypes also varied greatly among all samples, whereas the nirK communities were more stable. The latter was confirmed by denaturing gradient gel electrophoresis of nirK communities on all sampling occasions. Our results support earlier hypotheses that the compositions of denitrifier communities change during predenitrification processes when external carbon sources are added, although no severe effect could be observed from an operational point of view.     

Survival of yogurt bacteria in the human gut

Whether Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus can be recovered after passage through the human gut was tested by feeding 20 healthy volunteers commercial yogurt. Yogurt bacteria were found in human feces, suggesting that they can survive transit in the gastrointestinal tract.     

Conjugation to Nedd8 instigates ubiquitylation and down-regulation of activated receptor tyrosine kinases

When appended to the epidermal growth factor receptor (EGFR), ubiquitin serves as a sorting signal for lysosomal degradation. Here we demonstrate that the ubiquitin ligase of EGFR, namely c-Cbl, also mediates receptor modification with the ubiquitin-like molecule Nedd8. EGF stimulates receptor neddylation, which enhances subsequent ubiquitylation, as well as sorting of EGFR for degradation. Multiple lysine residues, located within the tyrosine kinase domain of EGFR, serve as attachment sites for Nedd8. A set of clathrin coat-associated binders of ubiquitin also bind Nedd8, but they undergo ubiquitylation, not neddylation. We discuss the emerging versatility of the concerted action of ubiquitylation and neddylation in the process that desensitizes growth factor-activated receptor tyrosine kinases.     

A requirement for dimerization of HP1Hsalpha in suppression of breast cancer invasion

The development and progression of cancer is controlled by gene expression, often regulated through chromatin packaging. Heterochromatin protein 1(Hsalpha) (HP1(Hsalpha)), one of three human HP1 family members, participates in heterochromatin formation and gene regulation. HP1(Hsalpha) possesses an amino-terminal chromodomain, which binds methylated lysine 9 of histone H3 (meK9 H3), and a carboxyl-terminal chromoshadow domain (CSD) that is required for dimerization and interaction with partner proteins. HP1(Hsalpha) is down-regulated in invasive metastatic breast cancer cells compared with poorly invasive nonmetastatic breast cancer cells. Expression of EGFP-HP1(Hsalpha) in highly invasive MDA-MB-231 cells causes a reduction in in vitro invasion, without affecting cell growth. Conversely, knock-down of HP1(Hsalpha) levels in the poorly invasive breast cancer cell line MCF-7 increased invasion, without affecting cell growth. To determine whether functions of the CSD were required for the regulation of invasion, mutant forms of HP1(Hsalpha) were expressed in MDA-MB-231 cells. A W174A mutation that disrupts interactions between HP1(Hsalpha) and PXVXL-containing partner proteins reduced invasion similar to that of the wild type protein. In contrast, an I165E mutation that disrupts dimerization of HP1(Hsalpha) did not decrease invasion. No gross changes in localization and abundance of HP1(Hsbeta), HP1(Hsgamma), and meK9 H3 were observed upon expression of wild type and mutant forms of HP1(Hsalpha) in MDA-MB-231 cells. Taken together, these data demonstrate that modulation of HP1(Hsalpha) alters the invasive potential of breast cancer cells through mechanisms requiring HP1 dimerization, but not interactions with PXVXL-containing proteins.