Manganese (Mn) toxicity in plants a comprehensive overview: a review

Plants experience an array of abiotic stresses that seriously restrict their productivity and yield. In this context, the acidity of soils is a matter of serious concern which comprises about 40–50% of the total ice-free land, primarily in humid climates (Von Uexküll & Mutert, 1995). The low soil pH directly or indirectly affects the mineral uptake in plants, drastically limiting agriculture activities around the world. Moreover, acid soil results in the toxicity of plants on account of enhanced availability of metal ions (Fe, Mn, Al) and the deficiency of crucial elements required for plant growth (Ca, P, Mg, K, Mo, B) (Kochian et al ., 2004; Chetry & Sharma, 2023). Mineral nutrients which have specific and essential functions in plant metabolism are classified as macronutrients and micronutrients. Manganese (Mn) is one of the 17 essential micronutrients required for the normal growth and development of plants. This metalloenzyme (Mn) cluster serves as an activator for several enzymes necessary for chlorophyll synthesis and the proper functioning of photosynthesis (Terry & Ulrich, 1974). However, Mn becomes toxic in poorly drained soil with low pH (Marschner, 1995; Pittman, 2005). Unlike aluminium (Al), excess Mn generally affects shoots more than roots in low-pH soil (Foy et al ., 1978), as the accumulation of Mn has been predominantly reported in shoot rather than root systems (Page & Feller, 2005). The thresholds of Mn toxicity and tolerance vary significantly among plant species and among cultivars within a species (Foy et al ., 1988). For example, rice is considered a Mn-tolerant species among the cereal crops, especially flooded or paddy rice (Lindon, 2001). Moreover, rice species have been reported to accumulate Mn in their leaves at concentrations as high as 5000 µg g^-1 DW without showing any toxic symptoms, which is remarkably high when compared with the Mn concentration recorded in barley with toxicity of 150 µg g^-1 DW (Vlamis & Williams, 1964).

Mn (II) the ubiquitously available form for plants, which can be easily oxidised to Mn (III) and Mn (IV) in the presence of an acidic environment (Marschner, 2011). The bioavailability of Mn depends on a range of environmental factors, viz., soil acidity, redox potential, temperature and moisture, which gradually increase the concentration of Mn either individually or in a sequence (George et al., 2012). Mn toxicity results when the normal concentration of biologically available Mn is increased above the threshold level. Thus, the excess amount of Mn causes drastic physiological as well as biochemical changes in plants. Hence, an attempt was made to understand the phytotoxic effect of Mn in plants, which would provide insight into the physiological processes functioning during Mn toxicity as well as the detoxification mechanisms inbuilt in plants to sustain such abiotic stress.

Mn uptake in plants

The uptake of Mn depends on the soil pH as well as soil redox potential, as acid soil results in an increase in bioavailable Mn by promoting the reduction of soil-bound Mn (Mn3+ and Mn4+ to Mn2+) (Goulding, 2016). Moreover, the uptake of Mn has been reported to be an active process where H+-ATPases are used to create an electrochemical gradient across the plasma membrane of root cells (Rengel, 2000). The mechanism of Mn uptake has been depicted in two phases. (a) first phase consists of the absorption of Mn2+ via the negatively charged cell wall, which constitutes the apoplast of the root epidermal cells (b) second phase comprises Mn2+ transported to other parts via the symplastic pathway. It is interesting to note that the transporters associated with Mn uptake also compete with other divalent cations, such as Fe2+, Zn2+, Cu2+, Cd2+, Ca2+, Co2+ and Ni2+, because of their non-specificity and low requirement for Mn2+ for plant nutrition. Once Mn uptake from the soil takes place, diverse families of transport proteins are known to maintain Mn homeostasis, which have been classified as importers and exporters respectively. The importers mainly translocate Mn from the extracellular space into the cytosol, whereas the exporters are responsible for the exclusion of Mn from the cytosol into intracellular compartments. The natural resistance-associated macrophage protein (NRAMP) family, the zinc-regulated transporter/iron-regulated transporter (ZRT/IRT)-related protein (ZIP) family and the yellow stripe-like (YSL) family have members involved in the transport of Mn2+ into the cytosol. In contrast, the cation diffusion facilitator/metal transport protein (CDF/MTP)

family, the vacuolar iron transporter (VIT) family, the Ca²⁺/cationantiporter (CaCA) superfamily, the bivalent cation transporter (BICAT) family and the P 2A -type ATPase family have members involved in the transport of Mn²⁺ out of the cytosol (Alejandro et al ., 2020). The sequestration of surplus Mn in the vacuoles, endoplasmic reticulum, or Golgi bodies has key roles in Mn tolerance (Williams & Pittman, 2010). A variety of transporter proteins belonging to the family, viz., cation exchanger (CAX), cation diffusion facilitator (CDF) and P 2A -type ATPase, mediate these processes, particularly in Arabidopsis thaliana . (Hirschi et al ., 2000; Delhaize et al ., 2007; Wu et al ., 2002).

Phytotoxicity of Mn in plants

Mn toxicity limits plant productivity in acid soils after aluminium (Al), where it prevents the uptake and transport of various other essential plant nutrients because of their resemblance with the ionic radius and ligand binding ability (Clark, 1982). The common symptoms of Mn toxicity are marginal chlorosis and necrosis of leaves, which strongly vary depending on the plant species. For example, Mn restricts the number and size of nodules and causes bronze speckle in marigold or geranium, crinkle leaf necrosis in cotton, stem streak necrosis in potato, internal bark necrosis in apple, tip burn in carnation, and fruit cracking in muskmelon (Foy et al ., 1978; Foy, 1983). Although a low concentration of Mn is a basic requirement for plant growth, the excess Mn in the soil not only harms plant productivity but also influences their yield and quality. Additionally, the excess concentration of Mn represents an important factor in environmental contamination, which causes various phytotoxic effects (Figure 1) (Pitman, 2005). Higher (Mn) concentrations are responsible for oxidative bursts with the production of reactive oxygen species (ROS) like superoxide radicals (O 2 ^-), hydrogen peroxide (H 2 O 2 ), and hydroxyl radicals (OH·) (Demirevska-

Kepova et al ., 2004; Boojar & Goodarzi, 2008). To cope with these ROS, the combined action of enzymatic antioxidant systems like the production of superoxide dismutases, catalases, peroxidases and the synthesis of non-enzymatic antioxidants like ascorbate and glutathione is necessary (Sharma & Dietz, 2009). The threshold of Mn toxicity and the tolerance to excess Mn concentrations vary characteristically according to plant species and their cultivars (Foy et al ., 1988).

Mn toxicity typically causes the oxidation of excess Mn²⁺ to Mn³⁺ in the apoplast, which in turn results in strong oxidative damage of proteins and lipids (Fecht-Christoffers et al ., 2006). The visible symptoms of Mn toxicity, i.e., brown necrotic spots on the leaves, have been proposed to occur due to the accumulation of high levels of oxidised phenolics in the apoplast (Wissemeier & Horst, 1992). Mn toxicity in wheat results in reduced shoot fresh weight, leaf extension, and nodal root growth, which cause death of the seminal root system and early senescence of the lower leaves (Khabaz-Saberi et al ., 2006).

Effects of Mn toxicity on photosynthesis

carboxylation efficiency in various plant species. The decline in photosynthesis rate is considered one of the major mechanisms constituting the toxic effects of excess Mn in rice and wheat (Lidon et al ., 2004; Macfie & Taylor, 1992). Moreover, in the hyperaccumulator species Phytolacca acinosa, Mn affects photosynthetic activity, attributing the hyperaccumulator capacity of the species to efficient Mn complexing and not to abruptly modifying the chloroplast structures (Weng et al ., 2013). Thus, in a comprehensive way, it can be assumed that Mn toxicity hampers the photosynthesis process in plants, thereby restricting the growth of the plant in a concerted manner (Figure 2).

Toxicity

Tolerance

• ✓    Chlorosis of leaf

^ Generation of brown roots

• ✓    Trigger oxidative stress

^ Disnipt photosynthesis

^ ABC transporter and GSH metabolic pathway activates

• ✓    Organic acid formation and translocation

S Antioxidant enzymes produce

МАРК signaling pathway activates

J Membrane lipid peroxidation

Figure 1 . Schematic representation of toxicity and strategies of tolerance adopted by plants towards Mn. Uptake of Mn from soil and thereby translocation and distribution of Mn towards aerial parts.

Figure 2. Effect of Mn toxicity in photosynthesis leading to decline plant growth.

Table 1: Tolerance mechanism adopted by Mn hyperaccumulator plant species.

Plants/family	Mechanism	References
Celosia argentea (Amarantheaceae)	Plant transport Mn from root to shoot, where ABC transporter and GSH metabolic pathway help in the detoxification.	Yu et al . (2023), Liang et al . (2024)
Chengiopanax sciadopylloide (Araliaceae)	Carboxylic acids, viz., malate or citrate, play an important role in the hyperaccumulation of Mn.	Fernando et al . (2010)
Eucalyptus grandis ⨰ Eucalyptus urophylla (Myrtaceae)	Plants detoxify Mn by forming complexes with high-molecular-weight proteins and low-molecular-weight organic acids.	Xie et al . (2015)
Phytolacca americana (Phytolaccaceae)	Mn is removed from the root surface by precipitation of the phosphate to form Mn phosphate crystals in rhizosphere.	Dou & Qi (2023)
Polygonum hydropiper (Polygonacceae)	Enzymes like sulfhydryl group (-SH) and glutathion (GSH) play a major role in detoxification.	Yang et al . (2016)
Polygonum lapathifolium (Polygonacceae)	Sulphate regulates Mn uptake and translocation in this plant.	Liu et al . (2021a)
Polygonum perfoliatum (Polygonacceae)	This plant tolerates Mn stress through the production and transportation of organic acids and membrane lipid peroxidation.	Xue et al . (2018)
Polygonum pubescens (Polygonacceae)	Antioxidant enzymes play a vital role in alleviating Mn stress.	Liu et al . (2021b)
Schima superba (Theaceae)	Under stress condition, mitogen-activated protein kinase (MAPK) signalling regulates Mn stress.	Liaquat et al . (2022)

Interaction of Mn with other elements

applications, which could lead to serious nutritional imbalance because Mg also interferes with Ca uptake. As compared to other nutrient elements, the absorption of K is slightly affected by increasing Mn concentrations (Heenan & Campbell, 1981). High K levels in the shoots of Mn-tolerant 'Lee' soybean alleviated the harmful effects of high internal Mn concentrations (Brown & Jones, 1977). Abundant evidence shows that a soluble source of Si in the growth medium can protect plants against Mn toxicity (Bowen, 1972). The higher absorption of Si by monocots than by dicots may help explain the higher tolerance of monocots to Mn toxicity (Foy et al ., 1978). Si reduced or prevented Mn toxicity in barley, rice, rye, ryegrass, and sorghum (Vlamis & Williams, 1967; Galvez et al ., 1989). Plant tolerance to soluble Mn may also be affected by the concentration of S, Al, Zn and Cu in the medium. Additional S may lower the pH of the growth medium and increase the availability of Mn to plants. Thus, all the elements behave diversely to increase the internal concentration of Mn in plants.

Mn detoxification mechanisms

Hyperaccumulator of Mn

Some rare plants accumulate trace elements in extreme concentrations and are known as hyperaccumulator plants. Cuba, an island country, has the highest number of plant hyperaccumulators, accounting for 128 species (Reeves et al ., 1999). Several Mn hyperaccumulator plant species, along with their specific tolerance strategies, have been enlisted in Table 1. In an experiment with Mn hyperaccumulators ( Phytolacca americana , P. perfoliatum, and P. hydropiper), it was found that P. perfoliatum has superior Mn accumulation and tolerance abilities (Liu et al ., 2010). However, P. americana is a common weedy species and has no specific association with high Mn soils. It was suggested that P. americana secretes acids into the rhizosphere as a means of acquiring P, which might coincidentally increase Mn uptake.

Conclusion and future prospects

Mn toxicity hampers plant growth and yield worldwide, particularly in acidic soils. Mn, however, is necessary for plant growth in trace amounts, but the surplus availability of this metal ion consequently leads to phtotoxicity. Thus, in the global scenario, it is imperative to study the response of diverse agroecosystems to surplus Mn concentrations. Furthermore, a clearer understanding of the mechanisms of Mn toxicity and tolerance among different plants is of the utmost necessity for future sustainable agriculture. In this context, the Mn hyperaccumulator species are likely to serve as an important genotype for understanding the tolerance strategies adopted by those plants to survive the Mn surplus condition. In addition, the interactive influence of Mn with other co-occurring factors in the acidic soils, such as other heavy metals as well as Al stress and surplus Fe concentrations, needs to be appropriately addressed. Moreover, cereal crops must be genetically engineered in order to develop Mn-tolerant crops that can be grown in soil with low pH conditions to maintain the production of food grains with low yield losses.

ACKNOWLEDGMENT

The authors are grateful to respective Department of Agricultural Biotechnology, Assam Agricultural University and Department of Botany, School of Life Sciences, Sikkim University. The award of Senior Research Fellowship to PC by Council of Scientific and Industrial Research (CSIR), New Delhi is also thankfully acknowledged.

CONFLICTS OF INTEREST

The authors declare that they have no potential conflicts of interest.