Abstract
The larch adelgid Adelges laricis laricis Vallot is a specialist insect parasite of Picea koraiensis (Korean spruce) and forms fish scale-like galls that damage the growth of the host plants. Our investigation reveals that both these galls and the fruits (cones) of P. koraiensis display lower concentrations of phytosynthetic pigments and accumulate anthocyanin cyanidin-3-O-glucoside and soluble sugars in the mature stages. Interestingly, high concentrations of 6-benzylaminopurine (BAP) both in the cauline gall tissues and in the larch adelgids themselves (4064.61 ± 167.83 and 3655.42 ± 210.29 ng/g FW, respectively), suggested that this vital phytohormone may be synthesized by the insects to control the development of gall tissues. These results indicate that the galls and cones are sink organs, and the development of gall tissues is possibly regulated by phytohormones in a way similar to that of the growth of cones. The concentrations of phytohormones related to growth [indole-3-acetic acid (IAA), cytokinins (CTK), and gibberellins (GAs)] and defense [salicylic acid (SA)], as well as SA-related phenolics [benzoic acid (BA) and p-hydroxybenzoic acid (pHBA)] in gall tissues were positively correlated with those in cones during the development stage. The levels of 1-aminocyclopropane-1-carboxylic acid (ACC) in the developmental stage of the cones correlates negatively with their concentrations in the gall tissues (R = −0.92, p < 0.001), suggesting that downregulation of ACC might be the reason why galls are not abscised after a year. Our results provide a new perspective on the potential mechanism of the development of cauline galls on P. koraiensis, which are regulated by phytohormones.
Highlights
Herbivorous insects and plants are hugely important components of terrestrial communities and have coevolved over millions of years (Stahl et al, 2018)
Galls caused by larch adelgids on P. koraiensis are about 15–20 mm both in length and width and have a fish-scale like surface with strawberry shaped
From morphology, the larch adelgid galls resemble the cones of P. koraiensis (Figure 1)
Summary
Herbivorous insects and plants are hugely important components of terrestrial communities and have coevolved over millions of years (Stahl et al, 2018). Insects are mobile and have evolved sophisticated and effective strategies to enable them to live on their host plants. An example of a strategy evolved by an insect to counter plant host defenses is that the salivary secretions of certain herbivorous insect caterpillars contain the enzyme glucose oxidase, which counteract the production of defensive metabolites induced by the caterpillar feeding on the plant (Musser et al, 2002). The digestive proteases in the guts of phytophagous insects are able to counter plant defense proteins (Zhu-Salzman and Zeng, 2015). The complex chemical networks involved in the interactions between plants and insects resulting from thousands of years of evolution remain a hot topic in research
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