Extreme Hydrolyzable Tannin Accumulation in Aphid-Induced Galls and Laccase-Mediated Tolerance in Schlechtendalia chinensis.

Plants redirect their secondary metabolism when herbivores feed on them, and the galls that the aphid Schlechtendalia chinensis induces on Rhus chinensis carry this response to an extreme, with hydrolyzable tannins (HTs) reaching 74.5% of gall wall dry weight, about 32 times the level in ungalled leaves. To find out how the plant builds this much tannin and how the aphid survives it, we followed HT accumulation and candidate gene expression through gall development, expressed the genes in Arabidopsis thaliana and characterised the aphid enzymes acting on tannin. Expression of phosphoglucomutase, UDP-glucosyltransferase BX9 and gallate 1-beta-d-glucosyltransferase (GDG) rose and fell with HT content, and GDG, whose product β-glucogallin is the first committed intermediate, tracked it most closely. In Arabidopsis, the galloyl compound pool rose up to about four-fold, so the three genes enlarge the galloyl precursor supply in this host. In the aphid, tannase activity stayed close to the detection limit of the assay throughout gall development, whereas laccase activity ran several orders of magnitude higher and was concentrated in the abdomen. Crude aphid extract removed 41.6%-54.2% of the HT from gall extracts, and heating it or adding the copper-oxidase inhibitor NaN3 prevented this. Recombinant Sc-Lac5, one of the three aphid laccases, oxidised 1,2,3,4,6-penta-O-galloyl-β-d-glucose, tannic acid and gall HT, consuming O2 equivalent to 3.77 electrons per PGG molecule and about two per galloyl group of gall tannin (Km = 187 µM, kcat = 2.83 s-1). Silencing Sc-Lac5 made 2% dietary tannic acid lethal to aphids that otherwise tolerate it and allowed HT to accumulate in their bodies, and feeding the purified enzyme reversed both. Laccase activity, and Sc-Lac5 in particular, underpins the ability of S. chinensis to live in the tannin its feeding creates.