Abstract
Grapevine trunk diseases (GTDs) are progressively affecting vineyard longevity and productivity worldwide. To be able to understand and combat these diseases, we need a different concept of the signals exchanged between the grapevine and fungi than the well-studied pathogen-associated molecular pattern and effector concepts. We screened extracts from fungi associated with GTDs for their association with basal defence responses in suspension cells of grapevine. By activity-guided fractionation of the two selected extracts, O-methylmellein was identified as a candidate modulator of grapevine immunity. O-Methylmellein could not induce immune responses by itself (i.e. does not act as an elicitor), but could amplify some of the defence responses triggered by the bacterial elicitor flg22, such as the induction level of defence genes and actin remodelling. These findings show that Eutypa lata, exemplarily selected as an endophytic fungus linked with GTDs, can secrete compounds that act as amplifiers of basal immunity. Thus, in addition to elicitors that can trigger basal immunity, and effectors that down-modulate antibacterial basal immunity, once it had been activated, E. lata seems to secrete a third type of chemical signal that amplifies basal immunity and may play a role in the context of consortia of mutually competing microorganisms.
Highlights
In contrast to most animals, plants are sessile organisms, which makes immunity even more crucial in warding off intruders
These findings show that Eutypa lata, exemplarily selected as an endophytic fungus linked with grapevine trunk disease (GTD), can secrete compounds that act as amplifiers of basal immunity
As a rapid readout for defence, we monitored changes of extracellular pH, since activation of the defence-related calcium influx through proton co-import will lead to apoplastic alkalinization (Jabs et al, 1997; Felix et al, 1999; Nürnberger and Scheel, 2001). This pH shift has been shown for our experimental system to be blocked by Gd3+, an inhibitor of calcium channels (Qiao et al, 2010), corroborating that it can be used as a readout.To find candidate ‘pools’ of defence modulators, we tested the effects of fungal filtrates on the V. rupestris green fluorescent protein (GFP)–AtTUB6 cell line by measuring alkalinization of the culture medium.The cells were incubated with 15 culture extracts from fungus-associated grapevine diseases, with a focus on GTD, E. lata (Supplementary Table S3)
Summary
In contrast to most animals, plants are sessile organisms, which makes immunity even more crucial in warding off intruders. Plants lack a somatic adaptive immune system. Instead, they have evolved innate immunity of two layers: pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), a broad-spectrum basal immunity triggered by molecular patterns generic for a class of microbes (microbe-associated molecular patterns and PAMPs); and effector-triggered immunity (ETI), which is often specific for one pathogen and usually accompanied by programmed cell death (for a recent review see Boutrot and Zipfel, 2017). The most comprehensive understanding of plant responses to PAMPs has been achieved for the PTI triggered by bacterial flagellin (Gómez-Gómez and Boller, 2002).This building. Hunting modulators of grapevine basal immunity | 3711 Pathogen effectors can support infection success by indirectly modulating targets outside of innate immunity (Pelgrom and Van den Ackerveken, 2001)
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