Seed germination is the first phase in plant growth and is highly sensitive to stress conditions (Wang and Tanveer, 2023) including MP stress ( Table 2 ). MP stress reduces seed germination by clogging the pores in the seed capsule (Bosker et al., 2019), thus reducing water uptake and the imbibition process. Reduction in the imbibition process results in a reduced germination rate (Zhang et al., 2021). The seed germination rates and germination potential of three herbaceous ornamental plants, Trifolium repens, Orychophragmus violaceus, and Impatiens balsamina, were reduced in response to PS MP stress (Guo et al., 2022). Co-exposure of polymethyl methacrylate (PMMA) MPs and As (V) reduced the germination index, root length, and sprout length of rapeseed (Brassica campestris L.) (Dong et al., 2022a). Nonetheless, De Silva et al. (2022) argued that examining the conventional parameters of seed germination, including germination rate and germination viability, could not be reliable to examine the direct effects of MP stress on seed germination. As such measurements require destructive post-harvesting measurements, the use of techniques such as biospeckle optical coherence tomography (BOCT) can provide an explicit picture of the effects of MP stress on seed germination. In this context, De Silva et al. (2022) used BOCT and observed that PE MPinduced reduction in seed germination was associated with the reduction in the internal biological activity, most likely due to physical blockage of pores in seeds (Zhang et al., 2021), MP-induced cytotoxicity during SG (Bao et al., 2022), or due to the presence of plasticizers in MPs (Balestri et al., 2019). However, such alterations could be MPs dose-, plant species- and exposure time-dependent (Sahasa et al., 2023).

The degree of MP stress-induced reduction in seed germination also depends on the particle size of MPs. For instance, PS MPs with a particle size of 100 nm reduced tomato (Lycopersicon esculentum L.) seed germination in a relatively higher percentage when compared with PS MPs with a particle size of 5 μm (Liao et al., 2019), thereby indicating that MPs with large particle sizes reduce seed germination more significantly as compared with MPs with smaller particle sizes (Bosker et al., 2019; Wang et al., 2023a). Moreover, leachates produced during plastic degradation negatively affect seed germination. For instance, leachates of oxo-degradable PP reduced the germination of sorghum (Sorghum bicolor L.) (Schiavo et al., 2020). Likewise, leachates of PC showed negative effects on the germination of garden cress (Pflugmacher et al., 2020). However, MP-induced stimulatory effects on seed germination have also been reported (Lian et al., 2019). High concentrations of MPs in soil promoted wheat (Triticum aestivum L.) seed germination, while low and medium concentrations showed inhibitory effects on seed germination (Lian et al., 2019). This could be because of different particle sizes, agglomeration, and charge on MP particles, which may influence the ability of the wheat seed to germinate and grow under MP stress (Weber et al., 2018; Ziajahromi et al., 2018; Ge et al., 2021). Moreover, the seed germination of chickpea (Cicer arietinum L.) was increased after the exposure to PET MPs (Mondal et al., 2022), potentially due to the priming effects of MPs on seed germination or due to the ability of MPs to break seed coat and enhance the imbibition process due to better water uptake via microscopic pores.

Roots are ranked second amongst other plant parts that face direct and mechanical damage under MP stress. MPs can accumulate in roots and reduce root elongation, root activity, and root fresh and dry biomass production (Boots et al., 2019; Maity et al., 2022; Wang et al., 2023a). MP stress-induced root growth reduction has been reported in several crops ( Table 2 ), including wheat by LDPE MPs (Qi et al., 2018), broad bean (Vicia faba L.) and onion (Allium cepa L.) by PS MPs (Jiang et al., 2019; Lian et al., 2022), and broad bean by Bio-MP stress (Meng et al., 2022). Moreover, PS MPs can adhere to the root surface and cause physical blockage to root pores (Gao et al., 2021). Blockage of root pores by sharp-edged MPs can reduce root growth by altering water and nutrient uptake from soil. Moreover, exposure of roots to MPs resulted in physical damage to the root and induced oxidative stress (Dong et al., 2020), reduced root-branch ratio and root biomass (Qi et al., 2018), reduced nitrogen contents and transpiration rate (Urbina et al., 2020), and reduced respiration (Liu et al., 2019; Spano et al., 2022). A previous study found that PS MPs can accumulate in rice (Oryza sativa L.) root via the endocytosis mechanism. During the fragmentation of MPs, particle size gets smaller but gains more specific surface area, thus having more potential to be adsorbed on the root surface. However, MPs can also accumulate in the root via the endocytosis mechanism (Wu et al., 2021).

Some MPs, such as PS MPs, are hydrophobic and can easily be absorbed on the root surface, thus reducing root growth (Nel et al., 2009). Moreover, MP stress reduces nutrient uptake due to the hetero aggregation of opposite charges and pore blockage in the cell wall (Xu et al., 2022). MP stress resulted in the upregulation of organic metabolic pathways with concomitant alteration of MP mobility and absorption through the electrostatic and hydrophobic interactions between the root exudations and MPs (Xu et al., 2022). Nonetheless, the application of both negatively and positively charged PS MPs reduced Arabidopsis growth by altering gene expression (Sun et al., 2020).

The surface charge groups of MPs also determine the negative effects of MPs on root growth, for instance, functional groups such as -NH₂ and -SO₃H reduced root and shoot biomass and root volume (Xu et al., 2022). The -NH₂ functional group contains a positive charge, which can easily occupy the binding sites and be easily adsorbed by the cell wall, thereby hindering the adsorption of cationic elements such as K⁺ Ca²⁺ or H⁺, Pb²⁺ around roots (Li et al., 2020; Song et al., 2023). Contrarily, -SO₃H is of strong hydrophilic nature and contains a negative charge, and can closely combine with the hydrophobic functional group of the phospholipids in the cell membrane, thus it can easily enter the cells to cause cytotoxic effects in roots (Feng et al., 2019). Besides surface charge or functional group types, the zeta potential of MP particles also influences MP adsorption and translocation of MPs from soil to root (Hu et al., 2020). MPs also caused cell atrophy, increased conduct numbers, and lignification in roots (Xu et al., 2022). Lignification results in cell wall thickening and hardening, thereby reducing root activity (Rui et al., 2016; Singh et al., 2019). MPs with large surface areas adsorb essential elements around roots, thus reducing root growth (Abbasi et al., 2020). As mentioned above, MPs with different surface charge groups affect the kinetics of essential elements uptake and adsorption by roots. For example, functional groups such as -OH, -COOH, -SO₃H, or -NH₂ group might reduce elemental adsorption by blocking the extracellular adsorption pathway (Li et al., 2020a), by directly repelling elements with the same surface charges (Fu et al., 2022), or by the attraction and hetero-aggregation of opposite charge to each other, thereby reducing the element adsorption by roots (Li et al., 2021a; Xia et al., 2021; Song et al., 2023).

In addition to the aforementioned factors, different MPs cause different effects on root growth. For instance, PE MPs caused more negative effects on root weight, while PS MPs had more negative effects on root length (Shi et al., 2022). Moreover, Lian et al. (2022) showed that elliptical-shaped PE MPs have fewer negative effects on plant growth and bacterial communities in the soil as compared with sharp-edged PLA MPs. Such differential response of roots to different MPs might be due to (i) different shapes of MPs along with particle size, (ii) plant species-specificity to different MPs, and (iii) the toxicity level of different MPs after fragmentation and degradation.

MPs enter the shoot via xylem vessels as an action of transpiration pull and accumulate in leaves (Xu et al., 2022). After the accumulation of MPs in roots, they can enter the vasculature of the stems and leaves via the apoplastic pathway (Li et al., 2019); however, factors such as chemical composition and geometry of plastic debris, root surface area and volume, cell membrane potential, and xylem properties can influence the translocation of MPs from root to leaves via shoot. MPs after fragmentation can enter roots via cracks on newly developed roots ( Figure 2A ) and enter xylem or phloem vessels (Li et al., 2020). Thus, the likeliness of the entry of MPs through cracks in roots provides instant access to conducting tissues. MP stress negatively affects the growth of shoots and leaves ( Table 2 ) by reducing cell elongation during the developmental phase, affecting the supply of nutrients, causing physical damage to xylem vessels, and influencing shoot and leaf biomass production. Furthermore, PS MP stress reduced leaves’ fresh weight, number of leaves per plant, leaf surface area, SPAD value, and overall plant height in a dose-dependent manner (Wang et al., 2023c). Liu et al. (2021) showed that PE MPs reduced shoot weight and shoot height significantly at high concentrations but stimulated root elongation, thus suggesting tissue-specific responses to MP stress.

Plant yield refers to a plant’s end-of-life journey, and producing seeds is considered very important for maintaining a plant’s niche from the ecological point of view and for producing grains as a source of food for humans. MP stress significantly reduces plant yield by altering plant growth and development (Khalid et al., 2020). Nonetheless, intra-species responses among two rice cultivars (XS123 and Y900) were also observed in terms of rice yield under PS MP stress; such variation in rice yield was associated with the upregulation of the transcripts levels and accumulation of metabolites relating to energy expenditure pathways and metabolic accumulation pathways in XS123, thereby indicating the reprogramming of metabolites accumulation as a component of PS MP tolerance in XS123 (Wu et al., 2022). Likewise, Yi et al. (2023) showed that PE MP stress did not alter rice grain yield in a common cultivar, whereas hybrid rice showed a significant reduction in the number of grains per panicle (13%) and rice grain yield (23%). Such variation in rice yield was due to higher total amino acid contents in hybrid rice grain, whereas conventional rice grain did not show any difference in total amino acid content under PE MP stress and control (Yi et al., 2021). Moreover, the reduction in rice yield was ascribed to the MP stress-induced reduction in the hemoglobin content by directly destroying its tertiary structure and inhibiting root activity (Dong et al., 2022b). Likewise, the individual and combined application of PS MPs and Pb significantly reduced 100 seed weight and seed yield per plant, which was associated with the reduction in root weight, surface area, total amino acids, Rubisco activity, and impaired hormonal regulation (Chen et al., 2023a). Contrarily, exposure of rice to three types of MPs, including PET, PAN, and PE, did not cause any negative effects on rice yield, but rather increased rice yield and N contents in rice grains (Chen et al., 2022). In conclusion, MP stress-induced effects on yield are highly plant species-, cultivar-, and MP type-specific.

MP-induced physical growth reduction is accompanied by the alteration in several key physiological mechanisms including photosynthesis, redox regulation, ionic homeostasis, and hormonal regulation. These mechanisms are sensitive to stressful conditions and any alteration in these mechanisms leads to crop growth reduction. In this section, we discuss the impacts of MPs on shoot and leaf physiology and root physiology.

Effects on shoot and leaf physiology: Given the MP stress-induced reduction in shoot and leaf growth and biomass production, it is important to explore what physiological processes have been or could have been impacted by MP stress. Photosynthesis is an important mechanism in plants, which yields oxygen and energy in the form of sugars and depends on many factors including biosynthesis of photosynthetic pigments, chlorophyll fluorescence and leaf gas exchange, ionic homeostasis, and redox regulation. MP stress negatively regulates these factors, thus reducing photosynthesis in plants (Yang and Gao, 2022; Figure 2B ). For instance, Lian et al. (2021) showed that the application of PS MPs reduced carotenoids and chlorophyll a and b by 12.5%, 9.1%, and 8.7% respectively, indicating one of the methods by which PS MP stress reduced shoot dry weight, shoot height, and leaf area in lettuce (Lactuca sativa L.). Likewise, in several other plant species, such as cucumber (Cucumis sativus L.), tomato, cabbage (Brassica oleracea L.), lettuce, and pakchoi (Brassica rapa L.), the application of PAN MPs showed a negative correlation with the chlorophyll a and b contents in leaves, thus reducing their growth (Ahammed et al., 2012). Liu et al. (2021) showed that the combined application of PAN with PE MPs significantly reduced photosynthesis by reducing the contents of chlorophyll a and b and carotenoids in a highly MP dose-dependent manner. In pumpkins, the application of PVC and PE MPs resulted in a dose-dependent reduction of chlorophyll contents and photosynthesis rate (Colzi et al., 2022). Gao et al. (2019) showed in lettuce that PE MP-induced reduction in photosynthesis was associated with a reduction in chlorophyll contents, leaf gas exchange, and Rubisco activity. Nonetheless, Mondal et al. (2022) observed a higher chlorophyll (a/b) ratio, probably because of the inhibition of chlorophyll ‘b’ synthesis under MP stress, suggesting the reduction in photosynthesis under MP stress could be due to less production of chlorophyll b (Pflugmacher et al., 2021). Conversely, Chai et al. (2023) showed that the PVC, PP, and PE MP-induced reduction in the photosynthesis of mangrove (Kandelia obovate L.) was due to a reduction in the concentration of carotenoids and chlorophyll a, and efficiency of ETC, while no significant effects of MP stress were observed on leaf gas exchange and chlorophyll b contents. These contrasting results can be explained by plant species-specificity and MP dose dependency. The plant species-specific response to MP stress was ascribed to the heterogenous responses of amino acids and hormones (Bouaicha et al., 2022).

Pollutants including MPs can disrupt the molecular structure of thylakoid membranes and can reduce the efficiency of ETC by inhibiting the activity of electron transfer in chloroplast, thus reducing photosynthesis. Though how MP stress induces these alterations remains elusive, this can be explained at least by the different surface functional groups of different MPs. MPs with -NH₂ and -SO₃H functional groups shut down the reaction center with a concomitant decrease in the number of election receptors in PSII (Kalhor et al., 2018), which results in reduced transfer of elections to QA-receptors in PSII (Wang et al., 2017), thereby reducing the overall efficiency of PSII under MP stress. Nonetheless, the kinetic curve of chlorophyll fluorescence was OJIP type, indicating the structure and function of the PSII photosystem were intact and unaltered after MP stress (Zha et al., 2021; Xu et al., 2022). MP stress reduced chlorophyll contents by converting the chlorophyll contents into phytol (Xu et al., 2022) and by inducing a high rate of the exfoliation of oxygen evolution complex (Wang et al., 2017; Xu et al., 2022), thus blocking the transfer of electrons to PSII reaction centers, which subsequently affects ETC reaction in the PSII photosystem. Reduction in photosynthesis was not only associated with a reduction in chlorophyll contents in the reaction center (Peng et al., 2021) but could also be associated with oxidation and reduction reactions and transfer of electrons in the reaction center of the PSII (Lu et al., 2020). Moreover, anthocyanins are very important pigments in plants, assisting in protecting leaves from photoinhibition by absorbing excess protons, thus also acting as non-enzymatic antioxidants. However, a reduction in anthocyanin biosynthesis also compromises photosynthesis under MP stress (Wang et al., 2023b).

Besides influencing the stomatal and non-stomatal limitations of photosynthesis, MP stress also reduces photosynthesis by affecting shoot and leaf ionome and causing the production of ROS ( Figure 2B ). Any alteration in uptake or accumulation of essential macro- and micro-nutrient in photosynthetically active tissues in shoots or leaves results in significant growth reduction. For instance, PP and PET MPs increased K concentration in shoots and leaves, while PVC MPs decreased K concentration thereby reducing photosynthetic efficiency (Colzi et al., 2022). Moreover, PE and PVC MPs increased nickel (Ni) concentration in shoots and leaves, which decreased after PP and PET MPs (Colzi et al., 2022). Reduction in photosynthesis, stomatal conductance, and SPAD values under PS and PVC MP stress were in line with the reduction in the concentration of Ca, K, N, and P in the shoot and leaves (Ma et al., 2022), nonetheless, these effects were highly MP type-specific and dose-dependent. Likewise, MP stress-induced reduction in N uptake may cause a low carbon fixation rate, and the disruption of chlorophyll contents or the decline in iron (Fe) contents in the shoot and leaves after exposure to PP- and PVC- MPs (Colzi et al., 2022) can partially inhibit the electron transport chain and lead to the loss of photosynthetic efficiency, and to the remodeling of the photosynthetic Fe-dependent protein and apparatus (Saito et al., 2014; Hantzis et al., 2018; Akmakjian et al., 2021). On the other hand, MPs such as PVC and PE increased Ni concentration in shoots and leaves, while PET and PP decreased Ni concentration in shoots and leaves (Colzi et al., 2022). Such imbalance in Ni concentration, either excessive uptake or deficiency, can reduce photosynthesis either by causing ROS production, in the case of excessive Ni concentration, or reduced N metabolism and reduced Fe uptake, in the case of Ni deficiency (Shahzad et al., 2018b).

The chloroplast is among the primary sites of ROS production during photosynthesis ( Figure 2C ), as the chloroplast converts light energy into energy required for chemical bonding. Under normal and optimal growth conditions, chlorophyll absorbs light and triggers several redox reactions in thylakoid membranes including oxidation of H₂O, development of H⁺ gradient across thylakoid membranes, and reduction of NADP⁺ to NADPH (Khorobrykh et al., 2020). However, these reactions are highly sensitive to any abnormality experienced during any change in growth conditions, such as abiotic or biotic stress conditions (Krieger-Liszkay and Shimakawa, 2022). Exposure to different MPs resulted in a significantly high production of hydrogen peroxide (H₂O₂) (Pignattelli et al., 2021), one of the most important ROS, thereby reducing photosynthesis. However, Ren et al. (2018) found no difference in H₂O₂ production in MP-treated plants compared to control plants and this could be due to the pleiotropic effects of H₂O₂, or the activation of the antioxidant defense system and/or sensitivity of plant species to MP stress. Nonetheless, PS MPinduced higher ROS production may cause oxidative damage to the thylakoid membrane and chloroplast structure, thereby inhibiting photosynthesis (Lian et al., 2021). Moreover, MP stress reduced photochemical efficiency and caused higher production of H₂O₂ and thiobarbituric acid reactive substances, thereby contributing to a higher leaf loss rate under MP stress (Menicagli et al., 2022). It has also been suggested that MP stress might reduce the photosynthesis of C3 plants by inhibiting light use efficiency, while in C4 plants, reduction in photosynthesis could be mediated by the inhibition of carbon fixation (Liu et al., 2020; Zhang et al., 2023). Under such conditions, the stroma in the chloroplast may gather more electrons, which concomitantly may induce the production of ROS.

Photosynthesis is also regulated by several genes involved in chlorophyll biosynthesis, carbohydrate metabolism, and ATP production, and MP stress has also been shown to impact the regulation of such genes, thus reducing photosynthesis in plants. For instance, Liu et al. (2022) showed that PBAT-MP stress significantly affected photosynthesis in Arabidopsis (Arabidopsis thaliana L.) by downregulating the gene expression of genes that encode light-harvesting chlorophyll a/b binding (LHCB) proteins. Moreover, the downregulation of LHCB proteins affects plant stress adaptive responses by regulating redox homeostasis, and sensitivity of stomata to abscisic acid (Xu et al., 2012; Chen et al., 2023b), thereby suggesting PBAT MP stress-induced alteration in photosynthesis might relate to the ABA-mediated stomatal response (Liu et al., 2022). In tobacco (Nicotiana tabacum L.), the application of PE MPs resulted in the downregulation of more than 80% of differentially expressed genes relating to ETC, PSI, and PSII in chloroplast and light harvesting (Tang et al., 2022). Given the above discussion, it can be suggested that MP stress-induced reduction in photosynthesis ( Figure 2B ) results from six complementary mechanisms: (i) higher ROS production, leading to oxidative stress, (ii) structural damage and modification of membrane proteins in PSII, (iii) conversion of chlorophyll to phytol and plausibly higher activity of chlorophyllase enzyme due to higher ROS production, (iv) peroxidation of the chloroplast and thylakoid membrane, (v) alteration in leaf ionome, and (vi) alteration in gene expression relating to the photosynthesis system.

Effects on root physiology: MP stress disrupts the balance between ROS production and ROS scavenging by increasing the ROS production beyond the capacity of plants to activate the antioxidant defense system, which subsequently disrupts the integrity of membranes, decreases mitotic cell division, and even causes cytogenetic anomalies and micronuclei (Giorgetti et al., 2020) in roots. MP stress significantly increases ROS (O₂ ^- and H₂O₂) production in roots (Gao et al., 2019) and causes genotoxic effects (Jiang et al., 2019), thus reducing the number of rootlets (Li et al. (2021b). Nonetheless, Biba et al. (2023) observed no drastic changes in O₂ ^- production in onion roots under different PS MP stress levels; however, when compared with the effects of PMMA MPs, PS MP treatment showed relevantly higher O₂ ^- production, thereby indicating MP-type response of ROS production in the root. Several factors can support such results such as the time point of ROS measurement, upregulation of the antioxidant defense system, and innate ability of roots to encounter ROS toxicity. Contrarily, the application of PS MPs resulted in a significant reduction in root length due to higher ROS production and cyto-genotoxicity (Giorgetti et al., 2020; Maity et al., 2020). Such contrasting results can also be explained by the particle size of MPs, as MPs with a small particle size can easily be internalized into root tissues and cause oxidative stress when compared to MPs with a large particle size (Roy et al., 2023), therefore plant species, plant age, and the particle size of MPs should be considered when studying MP induced oxidative damage to plants.

Maintaining ionic homeostasis is very important for the activation of several adaptive responses including osmotic adjustment, water and nutrient uptake, and overall plant growth. Any disruption in ionic homeostasis especially under salt stress (Khan et al., 2020) or heavy metal stress disrupts ion transport (Zhao et al., 2019) via increasing membrane depolarization, ROS-activated cation efflux channels, and desensitization of ion channels (Huang et al., 2022). MP stress physically damages the roots, which affects the ionic homeostasis in plants. For instance, the exposure of lettuce to PS MP stress resulted in the downregulation of genes involved in ionic homeostasis, such as transition metal ions, cellular metal ions, and cellular manganese (Mn) ions in a dose-dependent manner (Wang et al., 2023c), thereby indicating that MP mediated disruption in ionic homeostasis resulted in hypertonic injury in lettuce roots. The foliar application of PS MPs resulted in a significant reduction in Fe, Mn, and Cu contents in roots as compared with the control, and given the essentiality of these elements in plant physiology (Lian et al., 2021), it is not surprising that the reduction in the accumulation of mineral nutrients is correlated with the overall root growth under MP stress. Moreover, reduction in elements such as Ni, Zn, or Fe also affects the antioxidant activity, and the biosynthesis of amino acids or specific proteins (Shahzad et al., 2018b; Colzi et al., 2022). Studies comparing the elemental profile of roots under MP stress showed that regardless of MP types and exposure time, MPs impaired root growth by reducing N, P (Shorobi et al., 2023), Fe (Briat et al., 2015), and K contents (Ma et al., 2022). However, Lian et al. (2019) showed that PVC MPs decreased Fe contents in the root, while PET MPs reduced Mg, Zn, and K, thereby suggesting the effects of MPs on ionic homeostasis might be MP type-specific. However, it is still unclear how MP stress affects ionic homeostasis in plants. Thus, we presented a model ( Figure 3 ) and according to this model, we speculated that upon the entry of MP particles in roots, MPs induce membrane depolarization and increase ROS production in the cytosol. Both membrane depolarization and excessive ROS production in the cytosol cause the activation of depolarization activated- and ROS activated- cation (e.g., K, Ca, Fe, and Zn) efflux channels at the plasma membrane, thereby decreasing the concentration of cations in the cytosol. This model needs to be validated in future studies. Furthermore, Liu et al. (2023b) recently showed that PP MPs and rubber crumbs significantly increased ROS production and damaged the plasma membrane of root cells, as revealed by the decreased number of xylem vessels and reduced N uptake by roots. Thus, these results support our model.

MP stress also induces alterations in gene expression (Xu et al., 2020) in roots. As mentioned previously, MP stress induces cytotoxic and genotoxic effects on root growth, which might be due to an alteration in gene expression. For instance, Li et al. (2021b) showed that PAN MP treatment increased root aging or even death by affecting the expression of CDC2 (a gene regulating the cell division cycle) and CDK (the coding gene of cyclin-dependent kinase). As both genes play a very crucial role in regulating the cell cycle, any alteration in the expression of these genes results in altered root growth and development. Application of PS MPs altered the overall root and plant growth by downregulating the genes involved in nitrogen metabolism and linolenic acid metabolism (Zhang et al., 2021), by altering the transcriptome and expression of genes encoding proteins involved in the tricarboxylic acid cycle (Wu et al., 2022) and accumulation of soluble protein (Yu et al., 2020). A recent study reported that PS MP stress-induced negative effects on root and shoot growth were associated with the downregulation of genes involved in ROS scavenging and ionic homeostasis (Wang et al., 2023c). Thus, MP stress affects root physiology by altering gene expression. However, a variation may exist among plant species, MP particle size, MP type, and other growth conditions, which needs to be further explored in future studies.

The activation of plant adaptive response to stress conditions is closely linked with the regulation of phytohormones (Tanveer et al., 2019; Huang et al., 2022), as hormonal regulation is very important in regulating several key physiological processes including ionic homeostasis, ROS scavenging, osmotic adjustment, and overall growth in plants. For instance, in wheat root, the application of PAN MP stress resulted in the upregulation of MiR164, which enhanced the silencing of NAC1 and weakened the association between auxin signaling and adventitious root initiation (Li et al., 2021b). Auxin and cytokinin govern the root system by regulating the formation of root hairs, crown roots, and lateral roots (Neogy et al., 2021). Li et al. (2021c) showed that PS MP stress reduces the number of rootlets and concentrations of three phytohormones including IBA, OPA, and cis-zeatin riboside in roots, thereby indicating PS MP stress affects the development of roots by changing cytokinin and auxin concentration. In rice, PS MP stress inhibited jasmonate and lignin contents by altering the expression of genes involved in their biosynthesis (Zhou et al., 2021). In conclusion, plant physical root growth is altered by five complementary mechanisms under MP stress, including ROS metabolism, altered gene expression, impaired ionic homeostasis, and hormonal regulation. Nonetheless, these mechanisms are highly plant species-specific, MP type-specific, particle size-specific, and dose-dependent.