Arbuscular mycorrhizal fungi (AMF), which maintain symbiotic relationships with most terrestrial plants, appeared on land more than 460 million years ago, and they helped plants adapt to the terrestrial environment . AMF evolved specialized organs such as arbuscules, vesicles, etc., to interact with roots of plants and they provide resistance to environmental stresses (e.g., draught and salinity), help defend against disease, and absorb minerals, all of which benefit the host plants. The number of species of vascular plants worldwide is estimated at approximately 270,000 , while that of AMF is around 240 ; this asymmetric symbiotic relationship has generated tremendous academic interest. It is generally accepted that AMF exhibit relatively high genetic and functional diversity to overcome their limited species diversity .
In Korea, the history of AMF-related study is relatively short (ca. 30 years). Vascular plants distributed in the Korean peninsula have been reported to contain 3,000 to 5,000 taxa [5,6]; however, precise estimates of AMF species in Korea are currently lacking. Here, we review the state of AMF research in Korea in an attempt to determine the diversity of AMF species and their host plants in order to provide a basis for future AMF studies. Published papers were examined to ascertain the results of AMF-related research carried out in Korea, and a taxonomical treatment was conducted according to Schüßler and Walker .
AMF species diversity
A total of 89 species of AMF have been reported in Korea; however, new species have not been discovered (Table 1). Among them, species in the genera Acaulospora and Glomus have the most abundant (19 species), followed by Scutellospora (15 species), Pascispora and ParaGlomus (two species each), and Diversispora and Redeckera (one species each). Gigaspora margarita was the most frequently appeared species in the literatures. Also, Acaulospora scrobiculata, Funneliformis mosseae and Sclerocystis rubiformis were also frequently appeared in the literatures.
In most of studies, AMF were identified using morphological characteristics of spores extracted from fieldcollected soils. Of the 89 AMF species, approximately 25% was identified by using both morphological and molecular data, but the remainder by using only morphological characteristics (Table 1). AMF, compared with vascular plants and animals, have few key morphological characteristics and the morphology of spores extracted from field soil has been affected by various environmental factors, leading to misidentification; thus, sound molecular data are needed for accurate AMF identification.
Host plant diversity
Host plants for AMF are divided into three categories, herbaceous, woody, and crop species, and, in total, AMF were found from about 100 host plants (Table 2). Among them, about 25% are crop species, 24% woody plants, and 51% herbaceous plants. Most crop species are also herbaceous plants; thus, it would be reasonable to focus on woody plants for investigating AMF biodiversity. The literature shows that most host plants are common species; thus, rare species such as alpine or endangered species that are sensitive to climatic change or human development also need to be investigated.
The distribution of sampling sites for AMF in Korea to date is illustrated in Fig. 1. The region of greatest focus is the center of South Korea (e.g., Chungbuk province and the regions in its vicinity). Therefore, these would be appropriate reference sites for selecting new sampling sites for future studies of AMF. The northern and the southern part of South Korea have not been evaluated extensively in AMF studies. The vegetation in the southern Korea is different from that in the rest of Korea, and similar to that in other countries such as southern China and Kyushu in Japan. Moreover, these countries share some plant species; thus, comparative analysis of AMF between plants or between regions of these countries would be possible. In the case of the north western Korea, there are many alpine regions; hence, its diversity with respect to alpine plants and geological features would likely affect AMF species diversity and distribution.
From the perspective of this review, AMF-related research in Korea is relatively narrow in scope compared with that in other countries with respect to AMF diversity. In particular, the number of investigated host plants is merely 1/300 to 1/500 of the entire Korean plant taxa, and regionally and geologically, only Chungbuk province has been investigated relatively intensively. Thus, investigations that are more comprehensive are needed to establish a more complete understanding of AMF diversity in Korea. With respect to biodiversity, basing future AMF study of undiscovered areas and host plants on the results of this review will be a good strategy for improving Korean biodiversity, including that of AMF.