Abstract
Interactions between plant and pathogen often occur in the extracellular space and especially nucleotides like ATP and NAD have been identified as key players in this scenario. Arabidopsis mutants accumulating nucleosides in the extracellular space were generated and studied with respect to susceptibility against Botrytis cinerea infection and general plant fitness determined as photosynthetic performance. The mutants used are deficient in the main nucleoside uptake system ENT3 and the extracellular nucleoside hydrolase NSH3. When grown on soil but not in hydroponic culture, these plants markedly accumulate adenosine and uridine in leaves. This nucleoside accumulation was accompanied by reduced photosystem II efficiency and altered expression of photosynthesis related genes. Moreover, a higher susceptibility toward Botrytis cinerea infection and a reduced induction of pathogen related genes PR1 and WRKY33 was observed. All these effects did not occur in hydroponically grown plants substantiating a contribution of extracellular nucleosides to these effects. Whether reduced general plant fitness, altered pathogen response capability or more direct interactions with the pathogen are responsible for these observations is discussed.
Highlights
Nucleotide metabolism is an essential process in all living organisms as nucleotides function as energy providers, building blocks for nucleic acids, as a signaling component and as precursor for the biosynthesis of the phytohormone cytokinin (Buchanan et al, 2002)
We describe the generation of a mutant lacking the main nucleoside import activity as well as extracellular nucleoside hydrolase activity. This ent3:nsh3 double mutant combines characteristics of corresponding single mutants as increased resistance against the toxic purine and pyrimidine nucleoside analogs 2-chloro-adenosine and 5-fluoro-uridine, a diminished capacity to degrade extracellular purine nucleosides and a reduced ability to grow on inosine as sole nitrogen source
Inosine is a preferred substrate for ENT3 and NSH3
Summary
Nucleotide metabolism is an essential process in all living organisms as nucleotides function as energy providers, building blocks for nucleic acids, as a signaling component and as precursor for the biosynthesis of the phytohormone cytokinin (Buchanan et al, 2002). Nucleotide metabolism can be roughly divided into three parts: (i) de novo synthesis, (ii) salvage of nucleosides and nucleobases, and (iii) catabolism of purines and pyrimidines (Zrenner et al, 2006). Cytosolic NSH1 was attributed a key regulator at the branch point between salvage processes fueling the nucleotide pool and pyrimidine catabolism (Jung et al, 2009). One member of the Arabidopsis NSH protein family, NSH3, belongs to the purine specific IAG-NH family (inosine–adenosine–guanosine nucleoside hydrolases) and was shown to accept adenosine and inosine as substrates (Jung et al, 2011). The results of two Nucleoside Accumulation Affects Pathogenicity independent proteome analyses (Borderies et al, 2003; Kwon et al, 2005) as well as the analysis of apoplastic sap, identified NSH3 as an extracellular protein, assumed to function in extracellular nucleoside turnover (Jung et al, 2011). Incubation of seedlings with jasmonic acid leads to an increased NSH3 expression, indicating potential participation of NSH3 in plant wound or pathogen response (Taki et al, 2005; Jung et al, 2011)
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