Abstract
Many plants have evolved adaptations in order to survive in low nitrogen environments. One of the best-known adaptations is that of plant symbiosis with nitrogen-fixing bacteria; this is the major route by which nitrogen is incorporated into plant biomass. A portion of this plant-associated nitrogen is then lost to insects through herbivory, and insects represent a nitrogen reservoir that is generally overlooked in nitrogen cycles. In this review we show three specialized plant adaptations that allow for the recovery of insect nitrogen; that is, plants gaining nitrogen from insects. First, we show specialized adaptations by carnivorous plants in low nitrogen habitats. Insect carnivorous plants such as pitcher plants and sundews (Nepenthaceae/Sarraceniaceae and Drosera respectively) are able to obtain substantial amounts of nitrogen from the insects that they capture. Secondly, numerous plants form associations with mycorrhizal fungi that can provide soluble nitrogen from the soil, some of which may be insect-derived nitrogen, obtained from decaying insects or insect frass. Finally, a specialized group of endophytic, insect-pathogenic fungi (EIPF) provide host plants with insect-derived nitrogen. These soil-inhabiting fungi form a remarkable symbiosis with certain plant species. They can infect a wide range of insect hosts and also form endophytic associations in which they transfer insect-derived nitrogen to the plant. Root colonizing fungi are found in disparate fungal phylogenetic lineages, indicating possible convergent evolutionary strategies between taxa, evolution potentially driven by access to carbon-containing root exudates.
Highlights
Nitrogen exists in the soil in numerous forms, and these forms can be rapidly converted from inorganic to organic and vice versa [1]
Carnivorous plants are able to thrive in areas of extremely low nitrogen, a fact that speaks to the enormous potential of insects as a source of nitrogen
EIPF have a broad insect host range, which would be advantageous to the plant as well as to the fungus, i.e., a larger insect nitrogen reservoir to use in trading for carbon
Summary
Nitrogen exists in the soil in numerous forms, and these forms can be rapidly converted from inorganic to organic and vice versa [1]. Soil bacteria and fungal symbionts, such as Rhizobia and Glomus, respectively, are able to provide plants with substantial amounts of inorganic nitrogen per year in natural settings, as well as in commercial agricultural fields [2,6,7]. Organic nitrogen is present in soil ecosystems bound within leaf litter and other plant detritus, as well as in insect cadavers, insect frass, and living insects [8]. This organic nitrogen is made available to plants through microbial decomposition. In this review we discuss how the nitrogen reservoir present within insects is an integral part of the soil nitrogen cycle, and the three major adaptations by which insects can be exploited by plants for their nitrogen
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