Host cues mediate growth and establishment of oak mistletoe (Phoradendron leucarpum, Viscaceae), an aerial parasitic plant.

The oak mistletoe (Phoradendron leucarpum, Viscaceae) is well-documented to exhibit preference for a few potential host species in a given locality, even when many potential host species are present. In trying to explain this distribution, we examined the mechanisms by which mistletoe seedlings recognize potentially suitable hosts in the Piney Woods ecoregion of east Texas. An initial survey of patterns of infection on the campus of Sam Houston State University revealed that water oak (Quercus nigra) was host to nearly half of the mistletoes observed, despite comprising less than 15% of trees surveyed. Field experiments demonstrated that light, host physiochemistry, and volatiles released from potential host trees serve as cues affecting the viability and establishment of mistletoe seedlings. These results provoked further study in controlled laboratory settings, in which it was demonstrated that chemical compounds in the bark of local host trees (compared to trees that serve as hosts elsewhere, but not in our survey) induce significantly although slightly greater seedling viability. Establishment of haustoria depended only on the presence of these chemicals, regardless of host species. Importantly, we demonstrated that three common monoterpenes, limonene, β-myrcene, and β-phellandrene induce a positive growth response of mistletoe radicles. These results taken together suggest a model to explain local host preference in P. leucarpum, in which covariation between mistletoe fruit maturity and monoterpene production by hosts determines the distribution of successful haustorial establishment.

已有充分文献证实，橡树槲寄生（Phoradendron leucarpum，槲寄生科Viscaceae）在特定生境中会偏好少数潜在寄主物种，即便该区域存在大量潜在寄主树种。为阐释这一分布格局，我们以得克萨斯州东部的松林林木生态区（Piney Woods ecoregion）为研究区域，探究了槲寄生幼苗识别适宜寄主的潜在机制。我们先在山姆休斯顿州立大学（Sam Houston State University）校园内开展了侵染格局调查，结果显示：尽管调查样地内水栎（Quercus nigra）的占比不足15%，但近半数观测到的槲寄生均寄生于该树种上。田间试验表明，光照、寄主生理化学特性以及潜在寄主释放的挥发性物质，均可作为影响槲寄生幼苗活力与定植的信号因子。上述研究结果推动了后续受控实验室研究，结果证实：相较于其他区域的寄主树种（非本调查中的寄主物种），本地寄主树皮中的化学成分可显著（虽增幅较小）提升槲寄生幼苗的活力。而吸器（haustoria）的形成仅取决于这类化学成分的存在，与寄主物种无关。尤为重要的是，我们证实三种常见单萜类物质（monoterpenes）——柠檬烯（limonene）、β-月桂烯（β-myrcene）与β-水芹烯（β-phellandrene）——可诱导槲寄生胚根（radicles）产生正向生长响应。综合上述结果，我们提出了一套可解释P. leucarpum本地寄主偏好性的模型：槲寄生果实成熟度与寄主单萜类物质合成量之间的协同变化，决定了成功定植吸器的分布格局。