Wildland Resources Department, Utah State University, Logan, Utah, United States of America.
Ecology Center, Utah State University, Logan, Utah, United States of America.
PLoS One. 2021 Sep 23;16(9):e0250395. doi: 10.1371/journal.pone.0250395. eCollection 2021.
A key component in understanding plant-insect interactions is the nature of host defenses. Research on defense traits among Pinus species has focused on specialized metabolites and axial resin ducts, but the role of lignin in defense within diverse systems is unclear. We investigated lignin levels in the outer bark and phloem of P. longaeva, P. balfouriana, and P. flexilis; tree species growing at high elevations in the western United States known to differ in susceptibility to mountain pine beetle (Dendroctonus ponderosae; MPB). Pinus longaeva and P. balfouriana are attacked by MPB less frequently than P. flexilis, and MPB brood production in P. longaeva is limited. Because greater lignification of feeding tissues has been shown to provide defense against bark beetles in related genera, such as Picea, we hypothesized that P. longaeva and P. balfouriana would have greater lignin concentrations than P. flexilis. Contrary to expectations, we found that the more MPB-susceptible P. flexilis had greater phloem lignin levels than the less susceptible P. longaeva and P. balfouriana. No differences in outer bark lignin levels among the species were found. We conclude that lignification in Pinus phloem and outer bark is likely not adaptive as a physical defense against MPB.
了解植物-昆虫相互作用的一个关键因素是宿主防御的性质。关于松树物种防御特征的研究集中在特殊代谢物和轴向树脂管上，但木质素在不同系统中的防御作用尚不清楚。我们调查了美国西部高海拔地区生长的长叶松（Pinus longaeva）、巴尔福氏松（P. balfouriana）和柔枝松（P. flexilis）的外树皮和韧皮部中的木质素水平；这些树种易感性不同，已知易受山松甲虫（Dendroctonus ponderosae；MPB）的侵袭。长叶松和巴尔福氏松比柔枝松遭受山松甲虫的攻击频率更低，并且长叶松中的山松甲虫幼虫产量受到限制。因为在相关属（如白皮松）中，增加取食组织的木质化已被证明可以提供对树皮甲虫的防御，因此我们假设长叶松和巴尔福氏松的木质素浓度会高于柔枝松。与预期相反，我们发现更易受山松甲虫侵袭的柔枝松的韧皮部木质素水平高于较不易受侵袭的长叶松和巴尔福氏松。在这些物种中外树皮木质素水平没有差异。我们的结论是，松树韧皮部和外树皮的木质化不太可能作为针对山松甲虫的物理防御具有适应性。
