A friendly-environmental strategy: application of arbuscular mycorrhizal fungi to ornamental plants for plant growth and garden landscape

The demand for ornamental plants is increasing due to urban greening and rural construction, while the growing environment of plants, especially the soil environment, is deteriorating. Hence, sustainable methods of ornamental plant cultivation need to be developed quickly. The application of arbuscular mycorrhizal fungi (AMF) to ornamental plants can be one of the eco-friendly ways to achieve the objective. Soil AMF establish mycorrhizal symbiosis with roots of ornamental plants, which can develop a marvelous mycorrhizal mycelium network in the rhizosphere to stimulate nutrient and water acquisition of host plants. Numerous researches have proven that AMF improved the quality of ornamental plants, like fruit yield, height, biomass, seed quality, the size and number of flowers, leaf, and root. In addition, mycorrhizal fungi also improve nutrient uptake and endogenous hormone balance of host plants. Another important function of AMF is to regulate the physiological, biochemical, and molecular responses of host plants to adversity, including drought stress, temperature stress, heavy-metal stress, and insect and disease stress. From the perspective of the ecological garden landscape, AMF richness would maintain plant abundance, nutrient and energy balance, and higher productivity in normal and soil environment stress, thus, establishing a friendly-environmental ecosystem. This review also provides the basis to exploit and improve the commercial application of AMF in ornamental plants in the future.

. Colonization of arbuscular mycorrhizal fungi in roots of ornamental plants. a: Idesia polycarpa; b: Trifolium repens AMF effects on ornamental quality of ornamental plants There are many paths to improve flowering, growth, and field of ornamental plants, such as temperature, light, and plant growth regulator. However, these paths would be too costly in terms of capital and energy consumption, and long-term use of plant growth regulators harms the soil environment. AMF can be a friendly environmental approach to promote the quality of ornamental plants. Studies indicated that AMF significantly affected flowering, such as the size, the number of flowers, and the phenological stage of flowering (Shamshiri et al., 2012). Comparing with non-AMF plants, AMF-inoculated plants had relatively higher flower number in ornamental plants, such as marigold, geranium, and harlequin (Engel et al., 2016;Varga and Kytöviita, 2010;Scagel, 2004), and also increased size and color in carnation and hyacinth (Navarro et al., 2012;Xie and Wu, 2015;Xie et al., 2018). Similarly, the flowering time of Medicago truncatula was earlier by inoculation with AMF (Liu et al., 2018). The total flowering stage was significantly prolonged in tomato after mycorrhization (Banla et al., 2015), but was shortened in Chrysanthemum morifolium by AMF colonization (Sohn et al., 2003). Garmendia and Manga (2012) reported that inoculation with Glomus mosseae in Rosa hybrida was not significantly affected on early flowering and the number of flowers. The species of AMF have different effects on the flower of host plants and depend on how AMF regulate the nutrient element content in host plants and a certain amount of carbohydrates accumulated in plants (Liu et al., 2018). Similarly, AMF also can directly or indirectly affect the balance of endogenous hormones in plants, thus affecting the growth and flowering of mycorhizal plants. Endogenous hormones, such as indoleacetic acid (IAA), gibberellin (GA), and cytokinin (CTK), are growth-promoting hormones, and are affected by AMF (Perner et al., 2007). And Song et al. (2012) has also proved that inoculation with G. intraradices and G. mosseae can significantly enhance the level of IAA, GA and zeatin (ZR) in Amorpha fruticosa. During seed germination, leaf growth, stem elongation, pollen tube elongation, flower and fruit development, and flower transformation, endogenous hormone levels are regulated by AMF for better plant growth responses (Swain and Singh, 2005;Razem et al., 2006). In addition, the hyphae of AMF can produce CTK and GA to effect growth and flowering of ornamental plants ( Barea and Azconaguilar, 1982).
AMF do not just change morphological of flower, but also affect inclusion of flower. polyphenol, as an important component of flowering ornament plants, has been affected by AMF in Calendula officinalis, Melissa officinalis, and Origanum majorana (Engel et al., 2016). Not only that, AMF also can influence the quality of ornamental plant progeny by indirectly affecting ornamental plant pollination, AMF alter the seed quality of some Rosaceae by indirectly affecting plant pollination (Barber and Gorden, 2006). AMF increased nectar yield and the male or hermaphrodite flower number and size, to attract pollinators more easily to ensure the formation and quality of progeny seed (Asikainen and Mutikainen, 2005;Kiers et al., 2010;Varga and Kytöviita, 2010). At the same time, AMF cause transgenerational effects on the offspring of plants through increasing the proportion of methylated DNA in seed (Varga and Soulsbury, 2017).
Inoculation with G. intraradices or Gigaspora albida significantly increased collar diameter and root length in Eucalyptus hybrid (Sastry et al., 2000). In the urban garden, inoculation with AMF had positive effects on total shoot length, trunk diameter, shoot and root biomass, and shoot to root ratio in Acacia smallii and Fraxinus uhdei (Stabler et al., 2001). In addition, root structure was greatly improved by AMF to delay the senescence of Cryptomeria japonica (Hishi et al., 2016). Similarly, the most significant effect of AMF on ornamental grasses is to increase the biomass of shoot and root and the root activity for promoting density and coverage of lawn (Watts-Williams et al., 2019). However, there was no effect on plant height in mycorrhizal tall fescue, while plant height of both eastern gamagrass and big bluestem was positively increased in mycorrhizal plants ( Thorne et al., 2013), suggesting that the AMF effect is dependent on host and AMF species. Therefore, when AMF are applied to any ornamental plant, efficient screening of mycorrhizal fungi is required.
Improvement of mineral nutrition in mycorrhizal ornamental plants Ornamental plants need adequate industrial chemical fertilizer, which would pollute the soil environment. AMF, a natural biological fertilizer, could not result in environmental pollution. Studies have proven that AMF significantly improved the absorption of nutrient elements, like N, P, and K in mycorrhizal Chrysanthemum morifolium, Petunia hybrida, Tegetes erecta, Callistephus chinensis, Papaver rhoeas, and Dianthus caryophyllus, as well Fe, Mn, Cu, and Zn (Table 1) (Sohn et al., 2003;Gaur and Adholeya, 2005).
The nutritional improvement under mycorrhizal is closely related to hyphae of AMF that increases the absorption of nutrient elements. The mycorrhizal hyphae expand the area of absorption of nutrient element, but also increase the ability to absorb nutrient elements (Mathur et al., 2018). The content of N, P, and K was significantly increased in Hyacinthus orientalis L. by inoculation with Funneliformis mosseae, but not Diversispora spurca and D. versiformis (Xie and Wu, 2015). In pelargonium, inoculation with AMF significantly increased P and K content, not N concentration (Perner et al., 2007). Huang et al. (2020) reported that in walnut (Juglans regia), inoculation with AMF (Acaulospora scrobiculata, D. spurca, G. etunicatum, G. mosseae and G. versiforme), to some extent, increased root nutrient contents, dependent on AMF species and mineral types. The above results fully indicate that AMF inoculation has a positive effect on nutrient uptake of ornamental plants, which depends on mineral elements, host plants, and AMF species.
It was proved that extraradical mycelium of AMF contribute P requirements for up to 80% plants (Marschner and Dell, 1994) because the network of hyphae expands the volume of P absorption, but also secretes phosphatases to catalyze organic compounds into phosphate (Hayashi et al., 2018). The gene of P transporters is up-regulated by AMF to associate P uptake in many plants (Fellbaum et al., 2014). The expression of MtPT4 was induced by AMF in the root of Medocago truncatula. AMF promote organic and inorganic N absorption and transportation for host plants by extratradical mycelium, and NH4 + is the major form to be absorbed (Leigh et al., 2009). Wang et al. (2020) proved that about 42% of total N was obtained via mycorrhizal rice roots under NO3supply condition, and putative nitrate transporter gene OsNPF4.5 had been strongly induced in rice roots. Especially, the increase of N nutrient was more obviously in leguminous by AMF, contributed to the nodule number and N metabolism-related enzyme activity . A higher K + /Na + ratio in mycorrhizal Zelkova serrata seedling subjected to salt stress protected protein synthesis and cellular enzymatic processes (Wang et al., 2019a). Paying attention to other nutrients in ornamental plants should be involved. Tall fescue was not affected by AMF, while plant growth of big bluestem and eastern gamagrass was enhanced Thorne et al., 2013 Enhancement of stress resistance in ornamental plants by AMF

Drought stress
Researches have confirmed that AMF significantly enhanced drought resistance in ornamental plants (Wu et al., 2013Zhang et al., 2020). AMF usually improved the root structure of ornamental plants, such as root number, length, surface area, and diameter (Wu et al., 2013). The great root system obtains more water in wider and deeper soil, but also allow more abundant extraradical hyphae of AMF, which are much thinner than fine roots, to expand beyond depletion zone and penetrate smaller pores to absorb water and nutrient (Allen, 2011;Smith and Smith, 2011;Zhang et al., 2018a;Zou et al., 2020). Meanwhile, Khalvait and Ruth (2005) indicated that 4% of water was transported to the roots of host plants by hyphae. And the hyphae increase the total water absorption rate by 20% (Ruth et al., 2011). Therefore, mycorrhizal ornamental plants improve drought tolerance, which attributes to the increase of hydraulic conductivity (Robert et al., 2008).
Mycorrhizal ornamental plants can maintain higher water-use efficiency and relative water content than non-AMF plants (Yang et al., 2014b), providing favorable condition for accumulation of carbohydrates and gas exchange (Zhu et al., 2012). Under soil water deficit, mycorrhizal plants remarkably increase the accumulation of carbon compounds to respond to oxidative burst (Barros et al., 2018). In addition, AMF induce antioxidant defense systems to mitigate the accumulation of reactive oxygen species in mycorrhizal plants . Li et al. (2019) showed that AMF reduced malondialdehyde content, increased catalase and superoxide dismutase activity in Leymus chinensis under water stress. Furthermore, AMF can AMF regulate the expression of drought-related genes in plants (Cheng et al., 2020a). Two functional aquaporin genes from G. intraradices, GintAQPF1 and GintAQPF2, were expressed strongly in cortical cells with rich intraradical mycelia and extraradical mycelia of roots under drought stress (Li et al., 2013). However, host AQPs were down-regulated or unchanged by AMF inoculation under drought stress . Hence, Cheng et al. (2020a) proposed the synergistic effect of host and fungal AQPs on water status of hosts. AMF are involved in the induced expression of P5CS genes encoding a rate-limiting enzyme in proline synthesis (Porcel et al., 2004) and NCED genes encoding a key enzyme in ABA synthesis during drought stress and recovery (Aroca et al., 2008). Hence, mycorrhizal ornamental plants have a greater capacity to tolerate drought stress by a series of physiological and molecular mechanisms.

Temperature stress
In recent years, extreme weather is happening with increasing frequency by global climate change. Ornamental plants are always subjected to low-or high-temperature stress. AMF help plant responses to stress by altering plant physiological activities (Duhamel et al., 2013), Thus, inoculation with AMF maybe be an efficient strategy to cope with low and high temperature through promoting nutrient absorption, changing cell membrane structure, activating the antioxidant system, and regulating temperature-related gene expression (Tu et al., 2019). Bunn et al. (2009) had proven that AMF showed better heat tolerance than roots, with the increasing temperature. Under high-temperature soil, Dichanthelium lanuginosm plants inoculated with AMF significantly increased plant biomass, root length and diameter, and proportion of flowers and promoted early flowering (Bunn et al., 2009). Five AMF species collectively promoted shoot and root biomass, and reduced indices of leaf and root browning in strawberry under high-temperature stress, and G. mosseae and G. aggregatum had most effective (Matsubara et al., 2004). Additionally, mycorrhizal plants hold higher photosynthetic capacity and avoid the damage to the photosynthetic apparatus under high temperature (Mathur et al., 2020). Under low temperature stress, Ornithopus compressus and Lolium rigidum inoculated with AMF significantly increased plant growth than non-AMF plants (Carvalho et al., 2015). That could be AMF inoculation stimulated cyclic electron flow process in chloroplasts to reduce damage of stress, but also affected electrons transmission and phosphoric acid production in mitochondria, thus, promoting carbon metabolism under temperature stress (Mathur et al., 2020). Simultaneously, AMF induced the cold-tolerant gene expression of the host, and twenty-four DEGs identified were associated with the metabolism of photosynthesis and respiratory (Li et al., 2020). AMF increased secondary metabolites content and antioxidant enzyme activities, and induced expression of stress-related genes under low temperature (Chen et al., 2013). Therefore, mycorrhizal fungi are an important protocol for ornamental plants to resist temperature stress, which should be paid more attention.

Heavy-metal stress
In cities, a large amount of ornamental plants is planted in soil polluted by heavy metals, which severely limits plant growth and survival. Fortunately, AMF have been considered as a cost-effective and environmentally friendly protocol in phytoremediation and ecological restoration (Yang et al., 2015b). Elsholtzia splendens applied to a Cu-contaminated soil that significantly delayed the first-flowering dates and full-bloom stage and shorted flowering duration without AMF, whereas AMF recovered it and even promoted the flowering period (Jin et al., 2015). Generally, mycorrhizal hyphae and spores of AMF combine with heavy metal ions to reduce their mobility in the soil (Janoušková and Pavlíková, 2010). The hyphae also decrease the distribution of heavy metal from root to leaf, protect leaf tissues from injury (Yang et al., 2015a;Kushwaha et al., 2016). Similarly, Zhou et al. (2017) adopted a spectrum to analyze Cu content at the cross section of root tip of Tagetes patula, and confirmed that intraradical hyphae could selectively sequestrate a great deal of free Cu through sorption and barrier mechanisms. In general, soil nutrients are always low under excessive heavy metals condition, while mycorrhizal ornamental plants usually maintain higher nutrient levels. For example, mycorrhizal Medicago sativa had higher biomass and N, P, K and Ca contents than non-mycorrhizal plants under Cd stress . AMF also secrete a special glycoprotein, glomalin, to contribute soil nutrient pools and form protein-metal compounds for reducing the level of heavy metal in soil (Chern et al., 2007;Gonzalez-Chavez et al., 2009;He et al., 2020;Meng et al., 2020). Inoculation with Funneliformis mosseae enhanced the levels of ATP binging cassette (ABC) and metallothioneins (MET), transcripts in tall fescue roots under Ni stress (Shabani et al., 2016). Thereinto, GintABC1 isolated from extraradial mycelium of G. intraradices, participated in reducing toxicity of Cu and Cd (González-Guerrero et al., 2010). Furthermore, RintZnT1, a Zn transporter, participated in isolation of Zn in vacuolar (González-Guerrero et al., 2005). Therefore, an important research hotspot of mycorrhizas in ornamental plants is the mechanism of heavy metal pollution soil adaptive to mycorrhizal fungi in ornamental plants.
Diseases and insect AMF affect the population of pathogenic microbes and harmful rhizospheric pests (Cheng et al., 2020b). Slezack et al. (1999) discovered that inoculation with Glomus mosseae significantly reduced the red rot of pea roots by Aphanomyces euteiches. The date palm (Phoenix dactylifera L.) against inoculated with AMF reduced the index of pathogen occurrence by 8-77% (Jaiti et al., 2007). The mechanisms of AMF on enhancing the tolerance of diseases and insects is due to the competition between AMF and pathogens and insect for rhizospheric microbes, colonization sites, and nutrient substances (Al-Aska et al., 2010). Perhaps AMF compensate the damage of diseases and insect through improvement of plant health, root structure, and nutrient acquisition (Majewska et al., 2017). However, there was no significant difference between G. intraradices-colonized plants and non-AMF-colonized plants infected with white rot (Prados-Ligeo., 2002). AMF also inhibited the damage of nematode in menthol mint (Ratti et al., 2000), and reduced the damage of Pratylenchus coffea (Elsen et al., 2003). Additionally, many genes (ChtA3, gluB, CEVI16, OSM-8e and PR-1) have been predicted to participate in defense responses to enhance the disease resistance of mycorrhizal plants via transcript profiles (Liu et al., 2007;Ismail and Hijri, 2012).
As a whole, when ornamental plants grow in poor soil, inoculation with AMF not only increases the adaptability of ornamental plants to adversity but also improves the soil environment, which will be beneficial to the further application of ornamental plants in urban greening.
AMF and ecological garden landscape Ornamental plants are frequently used in the ecological garden that is distributed in urban green space, rural area, and a damaged environment land. A higher richness of community productivity is found in the ecological system with various ornamental plants than in a conventional system (Lacombe et al., 2009). It is well-known that the interaction between plant and microbial communities affects the balance of biodiversity and ecosystem function . AMF are the vital component of the underground biome and widely distributed in various eco-systems with kinds of plants (Lee et al., 2013). In addition, AMF directly affect the performance of host plants and indirectly affect plant communities via the increase of nutrient absorption and availability of soil nutrients by underground mycorrhizal networks (Werner et al., 2015). In contrast, the diversity and richness of host plants also affect AMF communities, which could give priority to providing carbon to beneficial symbionts (Vogelsang et al., 2006). AMF have effect on a variety of ecosystem functions by various pathways. Perhaps AMF are ubiquitous in terrestrial ecosystems and have a greater effect in the ecosystem than other soil microbes, like phosphate-solubilizing bacteria and nitrifiers (Powell et al., 2018). Additionally, AMF have positive effects on soil microorganism population and soil physical and chemical properties, which benefit the ecological restoration (Yang et al., 2016b). Hereinto, AMF reduce water loss of soil and soil hydrophobicity by AMF-released glomalin, resulting in low soil erosion in mycorrhizal soils (Rillig, 2004;Rillig et al., 2010).
Urban ecological landscapes are usually built on developed construction sites, including urban garden, residential landscape, and road afforesting, with a very small number of microbial species. Ornamental plants could be colonized by AMF in urban environments, but the propagule abundance or infectivity is relatively lower than the natural environment (Wisemanand Wells, 2005). Plenty of ornamental plants are applied to the urban ecological garden landscape by tree transplanting, while the plant survival rate is dramatically low under such poor ecological conditions. Dag et al. (2009) indicated that AMF promoted olive transplanting with higher height, shoot and root biomass, and leaf and root nutrient levels without fertilization to increase tolerance of transplant shock. The AMF species richness and communities are dependent on ornamental plant species (Wang et al., 2019b). Therefore, the combination with ornamental plants and appropriate AMF community built wonderful garden ecosystems that have less interspecific competition, better water and nutrient levels, and greater tolerance of abiotic and biotic stress (Teste et al., 2017).
Ornamental plants often apply to build beautiful rural, which need to be in line with the local natural conditions. Unlike urban environment, the rural environment has higher species diversity, organisms' number, and total biomass (Bainard et al., 2011). Therefore, exotic ornamental plants are difficult to blend into the local rural ecology without causing a threat by changing native plant species, community composition, and ecological functions (Aerts et al., 2017). The richness and composition of AMF indirectly influence plant survival and establish in an unfamiliar environment (Yang et al., 2014a;Lin et al., 2015). When exotic ornamental plants are introduced into a new environment, the native AMF from rhizosphere of the exotic plants must be introduced to increase the plant survival rate and growth (Egidi et al., 2018;Policelli et al., 2019). Hence, it is necessary to know AMF diversity in the native and new habitat. In some cases, exotic plants and AMF can overcome the lack of AMF group, and the native AMF are crucial for the transplanting of ancient trees (Sulzbacher et al., 2018). However, indigenous AMF may cause ecological risks in the new environment, which needs to be noted (Davison et al., 2015).
Possibly, ornamental plants in ecological landscapes cannot be used to remediate the terrible soil environment, such as the soil with heavy metal, pesticide residue, and organic pollutant, but they beautify the polluted environment. AMF as an effective protocol can be considered to apply soil remediation (Joner et al., 2001). Yang et al. (2016a) showed that AMF strengthened nutrient complementarities between plants through an underground mycorrhizal hyphal network. Similarly, under hydrocarbon-contaminants soil, AMF colonization promoted the absorption of polycyclic aromatic hydrocarbon, and AMF also degraded organic pollutants in the soil (Singer et al., 2003;Rajtor and Piotrowskaseget, 2016). AMF species and number in the ecosystem having multiple plants are relatively higher than those in the ecosystem with a single plant, and thus an ecosystem of diverse plant species with abundant AMF diversity has more glomalin released from AMF to chelate heavy metals in the soil (Bedini et al., 2010).

Conclusions
At present, the beneficial effects of AMF are confirmed in ornamental plants with better plant growth performance, more nutrient acquisition, and higher resistance to stress (Figure 2). AMF play a vital role in the establishment and maintenance of the ecological landscape in various soil environments. In the future, more applied studies should be carried out as soon as possible to make AMF commercially available. However, there are still many problems to be solved before the AMF can be used: i) There is no qualitative breakthrough in AMF propagation in vitro. Large-area application of AMF on ornamental plants requires a lot of mycorrhizal inoculums. Therefore, the development of economic, reliable, and efficient protocol for AMF propagation still needs to be paid attention; ii) AMF promote flowering and improve the ornamental quality of ornamental plants, while the underlying mechanism at the molecular level is still unclear and needs to be researched. iii) When exogenous AMF are applied to the rhizosphere of ornamental plants, the biological risk must be concerned, so as not to destroy the community of indigenous AMF. iv) The effect of AMF on ornamental plants depends on their compatibility. Therefore, an effective evaluation system of AMF for ornamental plants should be established to screen effective AMF to early apply it in the nursery. Figure 2. A schematic diagram regarding the effects of arbuscular mycorrhizal fungi (AMF) on ornamental plants and their rhizosphere for establishing and maintaining landscape ecosystem. AMF colonize the root system of an ornamental plant, establish a developed extraradical mycelium network, and further colonize the neighboring plants. Therefore, the developed mycelium network establishes the plants in an ecosystem as an organic whole. These mycorrhizal networks and extracellular mycelium help ornamental plants that grow better and have better nutrients and resistance of abiotic and biotic stress. Therefore, AMF can be used as an environmentally friendly medium to construct garden landscape.