Resistance against Leucoptera sinuella (Lepidoptera: Lyonetiidae) among hybrid clones of Populus spp. in central Chile

Leucoptera sinuella (Reutti) (Lepidoptera: Lyonetiidae) is a leaf miner specialist on Salicaceae recently introduced to Chile and Argentina, where it is causing economic damage to poplar plantations. We report a field survey in a poplar nursery naturally infested showing that regardless of the poplar hybrid taxon, high variability in resistance was observed among clones within families for oviposition and leaf-mining damage. A group of susceptible and resistant hybrid poplar clones was then selected for a laboratory evaluation of oviposition (antixenosis) and leaf-mining damage (antibiosis) on potted, rooted shoot cuttings. The concentration of condensed tannins (CTs) and salicinoid phenolic glucosides (SPGs) of the leaves of the selected clones from the laboratory study was also measured. Total oviposited eggs were positively correlated with leaf area, with the lowest oviposition on TMxT 11372 clone. The lowest percentage of mined leaf area was obtained for clones TMxT 11372, TMxT 11463, and TDxD 17574, but surprisingly no correlation between the percentage of mined leaf area and concentration of CTs and SPGs was found. Resistant poplar hybrids of our study could be suitable for breeding programs aimed for L. sinuella integrated pest management.

A willow UDP-glycosyltransferase involved in salicinoid biosynthesis

The salicinoids are phenolic glycosides that are characteristic secondary metabolites of the Salicaceae, particularly willows and poplars. Despite the well-known pharmacology of salicin, that led to the development of aspirin >100 years ago, the biosynthetic pathways leading to salicinoids have yet to be defined. Here, we describe the identification, cloning, and biochemical characterization of SpUGT71L2 and SpUGT71L3-isozymic glycosyltransferases from Salix purpurea-that function in the glucosylation of ortho-substituted phenols. The best substrate in vitro was salicyl-7-benzoate. Its product, salicyl-7-benzoate glucoside, was shown to be endogenous in poplar and willow. Together they are inferred to be early intermediates in the biosynthesis of salicortin and related metabolites in planta. The role of this UDP-glycosyltransferase was confirmed via the metabolomic analysis of transgenic plants produced by RNAi knockdown of the poplar orthologue (UGT71L1) in the hybrid clone Populus tremula×P. alba, INRA 717-1B4.

An optimal defense strategy for phenolic glycoside production in Populus trichocarpa--isotope labeling demonstrates secondary metabolite production in growing leaves

Large amounts of carbon are required for plant growth, but young, growing tissues often also have high concentrations of defensive secondary metabolites. Plants' capacity to allocate resources to growth and defense is addressed by the growth-differentiation balance hypothesis and the optimal defense hypothesis, which make contrasting predictions. Isotope labeling can demonstrate whether defense compounds are synthesized from stored or newly fixed carbon, allowing a detailed examination of these hypotheses. Populus trichocarpa saplings were pulse-labeled with 13CO2 at the beginning and end of a growing season, and the 13C signatures of phenolic glycosides (salicinoids), sugars, bulk tissue, and respired CO2 were traced over time. Half of the saplings were also subjected to mechanical damage. Populus trichocarpa followed an optimal defense strategy, investing 13C in salicinoids in expanding leaves directly after labeling. Salicinoids turned over quickly, and their production continued throughout the season. Salicin was induced by early-season damage, further demonstrating optimal defense. Salicinoids appear to be of great value to P. trichocarpa, as they command new C both early and late in the growing season, but their fitness benefits require further study. Export of salicinoids between tissues and biochemical pathways enabling induction also needs research. Nonetheless, the investigation of defense production afforded by isotope labeling lends new insights into plants' ability to grow and defend simultaneously.