1. Introduction
Role of gardens is expanding for biodiversity conservation as rapid urban expansion, and there are increasing needs for models of public garden with low-maintenance [
1,
2]. Native plants can be used as garden materials for effective maintenance, because they are good materials for gardens, restoration, and erosion control [
3,
4]. Recently, environmental stress to plant due to climate change is growing, and to cope with its brunt, plant breeding is valuable [
5,
6]. Therefore, the demand for developing new plant varieties based on native plants as garden materials against climate change has increased fairly.
Many plants in the family Poaceae are applied as food crops, fodder grasses, ornamental plants, or medical plants [
7]. Souza et al. [
8] described the capability of native Poaceae plants for usage as garden materials. Moreover, Dunster [
9] insisted that Poaceae must be used for not solely for ornamental but multi-functions in the age of climate change. However, some plant species in the family Poaceae are polyploid or have large and complicated genomes which make difficulties for breeding researches [
10]. To overcome this problem, model plants, which have a lot of advantages such as short lifecycle and small genome size [
11,
12], can be utilized in breeding researches of Poaceae plant species.
Model plants are extensively researched in plant science or agriculture [
11,
13].
Arabidopsis thaliana has been widely applied as a model plant from 1980s [
14]. However,
Arabidopsis is a dicotyledon in the family Brassicaceae, which is not advisable in some areas as a model plant of principal plants in the family Poaceae [
15,
16].
Brachypodium distachyon, which is distributed in the Mediterranean region, has been broadly investigated from the late 2000s by researchers and breeders on cereal crops notably wheat and barley which are valuable crops in the tribe Triticeae [
17]. However,
B. distachyon is not native in Korea so that it is not suitable to utilize as garden materials in Korea.
B. sylvaticum and
Setaria viridis have been lately proposed as model plants in the family Poaceae [
18].
B. sylvaticum can be utilized as a model plant for perennial grasses [
19].
S. viridis has a potentiality to be applied as a model plant for C
4 photosynthesis exploration [
20]. Both
B. sylvaticum and
S. viridis are native in Korea so that they are suitable to utilize as not only model plants but also garden materials in Korea.
Zoysia japonica, which is perennial C
4 grass, is the most popular warm-season turfgrass in Korea [
21,
22]. The reference genome of
Z. japonica and
Z. matrella was assembled and available [
23]. Also, transgenic
Z. japonica accessions were obtained by genetic transformation method [
24]. Therefore,
Z. japonica has not referred as a model plant yet, but it seems that
Z. japonica can also be used as a model plant for perennial C
4 plants as well as a garden material.
In this study, to explore the possibility of utilizing Korean native Poaceae plants as model plants and for breeding new cultivars, the candidate species were selected from the Korean Plant Names Index (KPNI). The modern research status of B. sylvaticum, S. viridis and Z. japonica has been summarized, and the genome size, life cycle, and other characteristics of these model plants have been compared.
2. Materials and Methods
The list of the Poaceae plant species was download from the KPNI (
http://www.nature.go.kr/kpni/). The scientific names of all plant were modified to delete information about authority, subspecies, or variety with leaving only the genus name and the species name. In the ‘Classification’ column, only ‘Species’ was selected while ‘Variety’, ‘Subspecies’, ‘Horticultural cultivar’, and ‘Cultivar’ were unchecked so that the list was filtered (
Supplementary Table S1). From the Published Plant Genomes (
https://www.plabipd.de/), the plants in which the reference genome was assembled were investigated. Based on the cladogram of the flowering plant menu, the list of the Poaceae plants in which the reference genome was assembled was created, and their genome size were investigated (
Supplementary Table S2). Finally, the Poaceae plants in which the reference genome was assembled were selected from the list of the Poaceae plants registered in the KPNI, and the characteristics including life cycle, and photosynthetic type were investigated (
Table 1). The life cycles of the selected plants were investigated from the Korean Biodiversity Information System (
http://www.nature.go.kr/) and the USDA PLANTS Database (
https://plants.usda.gov/). The photosynthetic types of the selected plants were investigated from the previous researches.
Small genome size is one of criteria for model plants [
11]. The genome sizes of
Brachypodium distachyon, which is the model plant for monocots but not native in Korea, and rice (
Oryza sativa), which is the representatively cultivated crop but not native in Korea, are 270 Mbps and 430 Mbps, respectively. In this point, the Korean native plant species with genome size smaller than that of rice (
O. sativa) were selected for candidate model plants. The current research states of the candidate model species were investigated. The candidate model plants were compared with the representative model plants,
Arabidopsis thaliana and
Brachypodium distachyon, and the properties of these plant were analyzed (
Table 2). Also, based on the Phytozome 13 (
https://phytozome-next.jgi.doe.gov/), the genomes of the 2 representative model plants and the 2 newly suggested model plants were summarized (
Table 3). For each plant species, two versions of genomes were selected and compared. Because it had no genome information in the Phytozome 13, it was hard to analyze
Zoysia japonica directly with the other 4 plants. Therefore, based on other researches [
23,
25], the genome of
Zoysia japonica was analyzed separately with those of other species in the genus
Zoysia such as
Z. matrella and
Z. pacifica which are cultivated plants in Korea (
Table 4).
3. Results
Of the 494 Poaceae plants registered in the KPNI, the 352 plants were registered as species (
Supplementary Table S1), and finally the 38 plants of them were analyzed in this study (
Table 1). The number of the Korean native plants were 14, and the number of both the cultivated plants and the exotic plants were 12, respectively. The number of the plants with the genome sizes less than 1 Gbps was 16. The number of the annual plants was 20 whereas the number of the perennial plants was 18. The number of the C
3 plants was 16 whereas the number of the C
4 plants was 22. The plants, whose genome sizes were less than the genome size of rice (
O. sativa), were selected, and a total of 5 plants were selected. Of them, the 3 plants (
Brachypodium sylvaticum,
Setaria viridis, and
Zoysia japonica) were native plants in Korea, whereas the 2 plants (
Z. matrella and
Z. pacifica) were cultivated plants in Korea.
The 3 Korean native plants were selected as the candidate model plants, and the properties of them and the 2 representative model plants (
Arabidopsis thaliana and
Brachypodium distachyon) were analyzed (
Table 2).
A. thaliana was eudicots in the family Brassicaceae whereas the others were monocots in the family Poaceae.
A. thaliana,
B. distachyon, and
S. viridis were annual but
B. sylvaticum and
Z. japonica were perennial.
A. thaliana,
B. distachyon, and
B. sylvaticum were C
3 plants whereas
S. viridis and
Z. japonica were C
4 plants. Both
A. thaliana and
B. distachyon were diploids with 10 chromosomes, but both
B. sylvaticum and
S. viridis were diploids with 18 chromosomes. Also,
Z. japonica was a tetraploid with 40 chromosomes. Except for
B. distachyon, the others were native plants in Korea.
The Information of the genomes of the 4 plants (
A. thaliana,
B. distachyon,
B. sylvaticum, and
S. viridis) was obtained from the Phytozome 13 and their reference publications (
Table 3). Within the same species, assembled genome sizes sometimes varied depending on the genome version but were approximately same. The genome size of
A. thaliana was the smallest, followed by
B. distachyon,
B. sylvaticum, and
S. viridis. Compared to
A. thaliana and
S. viridis,
B. distachyon and
B. sylvaticum showed relatively high differences in the number of contigs between the genome versions. No constant trend was found in the protein-coding transcripts and the protein-coding genes.
The genome of
Z. japonica was analyzed based on other researches (
Table 4). There were large differences of the genomes of
Z. japonica between Yang et al. [
25] and Tanaka et al. [
23]. Yang et al. [
25] used the PacBio long-read sequencing so that their average length and maximum length were longer than those of Tanaka et al. [
23]. Also, Tanaka et al. [
23] estimated the genome sizes of
Z. japonica,
Z. matrella, and
Z. pacifica by flow cytometry as 390 Mbps, 380 Mbps, and 370 Mbps, respectively. The obtained genome sizes of
Z. matrella and
Z. pacifica were larger than the estimated genome sizes, but the obtained genome size of
Z. japonica was smaller than the estimated genome size.
4. Discussion
Plants in the family Poaceae can be utilized for various uses [
7]. However, most plant breeders focus on cereal crops such as rice, wheat, and maize, and only few researchers perform a breeding program for ornamental purposes [
26]. Ornamental grasses in the family Poaceae are utilized in garden formation for landscaping, and gardens are economically important in climate change acclimatization and extenuation [
27,
28]. Also, native plants have some economic and ecological benefits, and the utilization of native plants is positive in a garden [
29,
30].
Nowadays, breeders can use genomic resources such as reference genomes for molecular breeding of crop improvement [
31]. A lot of species persists uncharted even though thousands of genomes have been explored [
32]. Due to the recent technological development, various sequencing methods have developed and their cost have been cheaper than before [
33]. However, assembly of the reference genome is still a costly, energy demanding, and protracted task [
34]. Furthermore, due to insufficient information, there are difficulties in utilization of wild plants for breeding [
35]. Information obtained from model plants can be hypothesized to the target species of breeding, making the researcher easy to conduct research on studies of those plant species [
36]. Therefore, building a foundation through researches using model plants may play an important role in the breeding of wild native plants which have not yet been explored.
In this study, to explore the possibility of utilizing Korean native plants in the family Poaceae for breeding new cultivars and as model plants and garden materials, the candidate species were explored from the KPNI. A total of 3 Korean native plants in the family Poaceae including Brachypodium sylvaticum, Setaria viridis, and Zoysia japonica were selected, and their properties and their genome information were compared with the representative model plants, Arabidopsis thaliana and Brachypodium distachyon.
Brachypodium distachyon was first suggested as a model plant for cereals and forage grasses at 2001 [
37].
B. distachyon is an annual C
3 grass and distributed in the Mediterranean region (
Figure 1A). In Japan, a country geographically close to Korea,
B. distachyon was first discovered from the Shimizu Port in 1953, and it is classified as a naturalized plant [
38,
39]. In Korea,
B. distachyon has been used in researches since the late 2000s [
40,
41]. However, the discovery of
B. distachyon in the wilds of Korea has not been reported for over a decade. According to the Köppen-Geiger classification system,
B. distachyon distributes on Bsh, Csa, Csb/Bsk, and Cfa/Cfb regions [
42]. Also, most parts of Japan belong to Cfa [
43], so that
B. distachyon can survive in Japan. However, most of the Korean Peninsula shows Dwa climate, and Cfa is mainly observed in some southern regions including Wando and Jeju [
44,
45]. Actually, in some island regions of the southern part of the Korean Peninsula mainly Jeju Island, there are some plants not distributed in the Korean Peninsula but distributed in China, Japan, and Taiwan [
46]. Therefore,
B. distachyon would be able to adapt naturally and survive only to some southern regions of Korea, and it is inevitable to artificially cultivate with cost and effort to utilize
B. distachyon as a garden material in most regions of Korea.
Unlike
B. distachyon,
B. sylvaticum is a perennial C
3 grass and native in Korea (
Figure 1B). Both
B. sylvaticum and
B. distachyon are plants in the genus
Brachypodium of the subfamily Pooideae so that they are genetically close to each other [
47]. Genetically close species can be utilized for breeding with a hybridization and an introgression [
48]. The first version of the reference genome of
B. distachyon was announced at 2010 [
49], by comparison, the reference genome of
B. sylvaticum was recently reported [
50]. Steinwand et al. [
19] suggested
B. sylvaticum as a model plant for perennial grasses. Also, according to Kim [
51],
B. sylvaticum was one of potential candidates for ornamental grasses and it was applied from abroad but not in Korea. Therefore,
B. sylvaticum can be utilized as not only a model plant for perennial C
3 grasses but also a garden material in Korea.
In the genus
Brachypodium of the subfamily Pooideae, there is no species which is native or cultivated in Korea apart from
B. sylvaticum. In the subfamily Pooideae, there are many significant C
3 perennial grasses such as bentgrasses (
Agrostis spp.), bluegrasses (
Poa spp.), fescues (
Festuca spp.), and ryegrasses (
Lolium spp.), applied as turf in temperate zones [
52]. Except for annual cereal crops such as wheat, barley, and oat, and their relatives, only few plants for a perennial turf in the subfamily Pooideae such as
Poa pratensis and
Lolium perenne have been studied for the reference genome assembly [
53,
54]. Therefore,
B. sylvaticum can be utilized as a model plant for the perennial cool season grasses whose reference genome have not been reported such as bentgrasses (
Agrostis spp.) and fescues (
Festuca spp.) in Korea.
S. viridis is an annual C
4 grass in the subfamily Panicoideae, which included many economically valuable C
4 species such as maize, sorghum, and sugarcane [
55]. Brutnell et al. [
20] suggested
S. viridis as a model plant for C
4 photosynthesis. The reference genomes of
S. viridis were first reported at 2020 [
56,
57]. Therefore, compared to
B. distachyon,
S. viridis were received attention relatively later as a model plant. However,
S. viridis can be transformed with the floral-dip method which has not been reported in
B. distachyon yet [
58]. As a result,
S. viridis is used for genome editing research such as CRISPR/Cas9 [
59]. Additionally, various researches on the C
4 photosynthesis using
S. viridis as a model plant were conducted [
60,
61]. Therefore,
S. viridis is highly valuable to conduct breeding research on the family Poaceae apart from
B. distachyon.
In the genus
Setaria, some species were applied as garden materials.
S. italica, which is cultivated for food or forage in Korea, was planted and analyzed for composition and utilization in garden [
62]. Also, according to Frey and Moretti [
63], 4 species in the genus
Setaria, (
S. italica,
S. pumila,
S. verticillata, and
S. viridis) could be discovered in urban gardens. Additionally, in the subfamily Panicoideae, the genus
Paspalum and the genus
Axonopus are applied for lawns [
64]. Apart from
S. viridis, the reference genomes of
S. italica and
Paspalum notatum have been reported [
65,
66], but those of
S. pumila,
S. verticillata, and carpet grasses (
Axonopus spp.) have not been reported yet. Also,
S. viridis has a smaller genome than
S. italica [
57,
67]. Therefore,
S. viridis can be utilized as a model plant for the annual C
4 grass for garden materials.
Z. japonica is a widely used turfgrass and distributed in East Asia including Korea, Japan, and China [
68,
69]. The genomes of
Z. japonica were reported by Tanaka et al. [
23] and Yang et al. [
25]. However, there were large differences between the two genomes, therefore, further researches should be required to improve an accuracy (
Table 4). Also, considering the errors in
Z. japonica, the estimated genome sizes of the other species,
Z. matrella and
Z. pacifica, could be uncertain as well. Therefore, genome assemblies of both
Z. matrella and
Z. pacifica using other accessions would be required to estimate more accurate genome sizes of both species. Also, it would be inevitable to artificially cultivate with cost and effort to utilize
Z. matrella and
Z. pacifica, which are not native but cultivated in Korea, as garden materials in Korea.
B. distachyon,
B. sylvaticum, and
S. viridis were reported as model plants for annual C
3 grasses, perennial C
3 grasses, and annual C
4 grasses, respectively, whereas a model plant for perennial C
4 grasses has not been reported. The genus
Zoysia, which is consisted of 11 species, is a perennial C
4 grass in the subfamily Chloridoideae and native in the western Pacific Rim and Indian Ocean [
69,
70].
Z. japonica,
Z. matrella, and
Z. pacifica have been utilized as turf and ornate grasses [
68]. Also, their genome sizes were relatively small [
23], so that one species in the genus
Zoysia, which are perennial C
4 grasses, can be utilized as a model plant for perennial C
4 grasses. However, compared to
B. sylvaticum and
S. viridis, plants in the genus
Zoysia were less studied, probably because they are not native in Europe or America. Additionally, plants in the genus
Zoysia were allotetraploids but Flavell [
71] presented diploid genetics as one of characteristics of model plants. Therefore,
Zoysia species are suitable as garden materials but can be unsuitable as model plants. For an appearance of a model plant for perennial C
4 grasses, a discovery of a diploid perennial C
4 species with a small genome size would be necessary.