Research

I am broadly interested in the diversity of biological interactions and the natural history of intriguing organisms in the terrestrial ecosystem. I have tackled pollination biology, mycorrhizal symbiosis, host-parasite interaction, seed dispersal mutualism, spore dispersal mutualism, and so on... Specifically, I am focusing on the following theme:

Host Association of Mycoheterotrophic plants

Non-photosynthetic mycorrhizal plants (i.e. mycoheterotrophs) have long attracted the curiosity of botanists and mycologists. In fact, these puzzling plants have dominated the very beginnings of the field of mycorrhizal biology. Only recently, however, has the mycorrhizal biology of this diverse group of plants begun to be systematically unraveled.

Molecular methods have dramatically enhanced the identification of host mycorrhizal fungi and revolutionized the understanding of the ecology and evolution of mycoheterotrophic plants. Suetsugu et al. (2020) showed that radiocarbon (¹⁴C) is also useful for estimating the trophic strategies of mycoheterotrophic plants, as it can be used to estimate the mean age of C in plant biomass. This is because the ¹⁴C level in atmospheric CO₂ was elevated globally due to nuclear weapon testing during the early 1950s, but it has been steadily declining since the atmospheric nuclear test ban treaty of 1963. The ¹⁴C level in newly synthesized organic matter is representative of atmospheric CO₂ at the time; therefore, the age of C in organic matter can be estimated from the level of Δ¹⁴C. Consequently, mycoheterotrophic plants that associate with wood‐decaying fungi have extremely high Δ¹⁴C values owing to the indirect acquisition of ¹⁴C‐enriched C from deadwood, which was initially fixed decades before sampling. This indicates that some mycoheterotrophic plants do not obtain their carbon by tapping into existing mycorrhizal networks, but recruit saprotrophic fungi into novel mycorrhizal symbioses.

We have also shown that some green orchids can obtain C fixed in wood during the past decades via saprotrophic fungi. These studies illustrates a novel pathway for the evolution of full mycoheterotrophy and represents a major advancement in the current understanding of trophic interactions between orchids and fungi. Mixotrophy with wood‐decaying fungi may be more widespread than previously thought. We have continued to unveiling host associations of mycoheterotrophic plants including partial ones to reveal the enigmatic evolution!

Related articles

Suetsugu K, Matsubayashi J, Tayasu I (2020) Some mycoheterotrophic orchids depend on carbon from dead wood: Novel evidence from a radiocarbon approach. New Phytologist, 227: 1519–1529.

Suetsugu K, Yamato M, Miura C, Yamaguchi K, Takahashi K, Ida Y, Shigenobu S, Kaminaka H (2017) Comparison of green and albino individuals of the partially mycoheterotrophic orchid Epipactis helleborine on molecular identities of mycorrhizal fungi, nutritional modes, and gene expression in mycorrhizal roots. Molecular Ecology, 26: 1652–1669.

Taxonomy of Mycoheterotrophic Plants

A common feature of most mycoheterotrophic plants is their extreme scarcity and small size. In addition, most species are found in the dark understory of forests, only discoverable during the flowering and fruiting period when aboveground organs appear through the leaf litter (Leake 1994 New. Phytol. 127: 171–216.). For example, the majority of mycoheterotrophic Thismia species are known exclusively from the type collection, or appear to have been collected only once or a few times. Thus, we still have scant knowledge on the precise distribution on most mycoheterotrophic plants, even in Japan where we have been conducting intensive botanical fieldwork. Moreover, there seems to be many unrecorded mycoheterotrophic species and undescribed taxa even in Japan. We have described many new mycoheterotrophic species.

Related articles

Suetsugu K, Kinoshita A (2020) Sciaphila kozushimensis (Triuridaceae), a new mycoheterotrophic plant from Kozu Island, Izu Islands, Japan, based on morphological and molecular data. Phytotaxa, 436: 157–166.

Suetsugu K, Nakanishi O, Kobayashi T, Kurosaki N (2018) Thismia kobensis (Burmanniaceae), a new and presumably extinct species from Hyogo Prefecture, Japan. Phytotaxa, 369: 121–125.

Suetsugu K, Nishioka T (2017) Sciaphila sugimotoi (Triuridaceae), a new mycoheterotrophic plant from Ishigaki Island, Japan. Phytotaxa, 314: 279–284.

Pollination Biology of the Family Orchidaceae

The family Orchidaceae is one of the most species-rich and morphologically diverse plant families on earth. The floral diversity of orchids has long intrigued evolutionary biologists. Adaptation to specific pollinators is an important attribute of the impressive floral diversity of orchids (Cozzolino & Widmer, 2005 Trends Ecol. Evol. 20: 487–494). For example, of all the orchid species, an estimated 60% are pollinated by only one species of insect (Tremblay, 1992 Can. J. Bot. 70: 642–650), which is both a prezygotic and ethological mechanism of importance in determining the reproductive isolation of species (Cozzolino & Widmer, 2005 Trends Ecol. Evol. 20: 487–494).

In addition, most wild orchid species have recently experienced a drop in their population, primarily as a result of habitat destruction and overharvesting. An understanding of the reproductive biology of these species could help determine whether a failure in the recruitment cycle is limiting the success of their reproduction, leading to their constrained population growth (Gale 2007 Plant Syst. Evol. 268: 59–73). This information, when integrated with data regarding other factors of their life history, could provide an estimate of the persistence of a population in a given location (Gale 2007 Plant Syst. Evol. 268: 59–73). This approach is necessary when planning intervening measures to conserve endangered orchid species, given their taxonomic diversity and the intricacies and inefficiencies of their pollination systems (Tremblay et al. 2005 Biol. J. Linn. Soc. 84: 1–54).

However, most studies of the reproductive biology of orchids have been conducted in Europe, North America and the Southern Hemisphere. Little is known about the breeding systems and pollination biology of orchids in north temperate Asia. We have conducted pollination experiments in Japanese endangered orchids to characterize the breeding system, such as the identity of pollinator and the capacity for self-fertilization.

Related articles

Suetsugu K (2019) Rain-triggered self-pollination in Liparis kumokiri, an orchid that blooms during the rainy season. Ecology 100: e02683.

Suetsugu K, Tetsu S, Hiraiwa KM, Tsutsumi T (2019) Thrips as a supplementary pollinator in an orchid with granular pollinia: is this mutualism? Ecology 100: e02535.