Plant pathogens use effector proteins to promote host colonisation. The mode of action of effectors from root-invading pathogens, such as Fusarium oxysporum (Fo), is poorly understood. Here, we investigated whether Fo effectors suppress pattern-triggered immunity (PTI), and whether they enter host cells during infection. Eight candidate effectors of an Arabidopsis-infecting Fo strain were expressed with and without signal peptide for secretion in Nicotiana benthamiana and their effect on flg22-triggered and chitin-triggered reactive oxidative species (ROS) burst was monitored. To detect uptake, effector biotinylation by an intracellular Arabidopsis-produced biotin ligase was examined following root infection. Four effectors suppressed PTI signalling; two acted intracellularly and two apoplastically. Heterologous expression of a PTI-suppressing effector in Arabidopsis enhanced bacterial susceptibility. Consistent with an intracellular activity, host cell uptake of five effectors, but not of the apoplastically acting ones, was detected in Fo-infected Arabidopsis roots. Multiple Fo effectors targeted PTI signalling, uncovering a surprising overlap in infection strategies between foliar and root pathogens. Extracellular targeting of flg22 signalling by a microbial effector provides a new mechanism on how plant pathogens manipulate their host. Effector translocation appears independent of protein size, charge, presence of conserved motifs or the promoter driving its expression.
Flavonoids are plant pigments that provide health benefits for human and animal consumers. Understanding why domesticated crops have altered pigmentation patterns and unraveling the molecular/genetic mechanisms that underlie this will facilitate the breeding of new (healthier) varieties. We present an overview of changes in flavonoid pigmentation patterns that have occurred during crop domestication and, where possible, link them to the molecular changes that brought about the new phenotypes. We consider species that lost flavonoid pigmentation in the edible part of the plant at some point during domestication (like cereals). We also consider the converse situation, for example eggplant (aubergine), which instead gained strong anthocyanin accumulation in the skin of the fruit during domestication, and some varieties of citrus and apple that acquired anthocyanins in the fruit flesh. Interestingly, the genes responsible for such changes are sometimes closely linked to, or have pleiotropic effects on, important domestication genes, suggesting accidental and perhaps inevitable changes of anthocyanin patterning during domestication. In other cases, flavonoid pigmentation patterns in domesticated crops are the result of cultural preferences, with examples being found in varieties of citrus, barley, wheat, and maize. Finally, and more recently, in some species, anthocyanins seem to have been the direct target of selection in a second wave of domestication that followed the introduction of industrial food processing.
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