Macronutrients mediated plant disease management
NPK and his role
Nitrogen
In the context of plant disease management, nitrogen (N) is an essentially important macronutrient required for the normal growth and development of the plant (Scheible et al., 2004). N plays a prominent role in varying metabolic and physiological processes, such as photosynthesis, amino acid synthesis, respiration, and tricarboxylic acid (TCA) cycle (Foyer et al., 2011). The N availability can restrict pathogen growth by alleviation and deployment of different plant defense mechanisms, and the different forms of N (NH4+ and NO3 form) are reported to have diverse effects on plant disease resistance (Bolton and Thomma, 2008; Mur et al., 2017). Several instances have been reported wherein N fertilization increased the plant disease incidence, for example, downy mildew, powdery mildew, leaf rust, stem rot, and rice blast diseases (Ballini et al., 2013; Devadas et al., 2014; Huang et al., 2017) while contrary results have been reported for diseases, such as take-all, gray mold, and leaf spot (Krupinsky et al., 2007; Lecompte et al., 2010). The excessive use of N fertilization in plants promotes succulent tissue growth and alleviates apoplastic amino acid concentration along with improving the plant canopy, which ultimately favors the growth of pathogenic spores (Neumann et al., 2004; Dordas, 2008).
The impact of N limitation on Pseudomonas syringae pv. syringae B728a when studied through an extensive transcriptomic assessment revealed the prominence of virulence-associated features, such as swarming motility, type three secretion system (T3SS), and metabolic pathways involved in gamma-aminobutyric acid (GABA) and polyketide metabolism (Bolton and Thomma, 2008). N starvation studies confirm its importance in initiating pathogenesis by stimulating the pathogen effector genes, such as the hypersensitive response and pathogenicity (hrp), avirulence (avr), and hydrophobin MPG1 genes in Magnaporthe oryzae (Pérez-García et al., 2001) while opposite results were documented for effectors from Magnaporthe oryzae (Huang et al., 2017) and Passalora fulva (ex Cladosporium fulvum) (Thomma et al., 2005). Defense enzymes are also an important arsenal possessed by plants in fighting the invading pathogen and N is observed to be involved in the stimulation of these enzymes during the host-pathogen interaction (Ngadze et al., 2012). The genes encoding the key regulatory enzymes of the defense pathway, such as phenyl ammonia lyase (PAL), cinnamate-4-hydroxylase (C4H), and 4-coumarate: CoA ligase (4CL), are all upregulated by N deficiency (Camargo et al., 2014) while a reduction in PAL activity has been observed with N fertilization (Sun et al., 2018). However, the relationship between N fertilization and plant disease is still unclear, but the understanding of the fundamental mechanism is noteworthy in crop production.
Phosphorus
Phosphorus (P) is thought to be the second most commonly applied nutrient after nitrogen in crops but its role in resistance is seemingly inconsistent and variable. P is a part of many cell organic + molecules, such as deoxyribonucleic acid (DNA), ribonucleic acid (RNA), adenosine triphosphate (ATP), and is also involved in many metabolic processes taking place both in the plant and in the pathogen. During pathogen infection, the extracellular ATP is also received as a damage-associated molecular pattern (DAMP) since it is sensed by the plant when cellular damage is caused during pathogen colonization (Tanaka et al., 2014), considering it as a signaling molecule for the defense response activation in the plant (Cao et al., 2014). In recent reports, the role of extracellular ATP has been also proposed in jasmonic acid (JA)-induced defense response through direct activation of JA signaling in the Arabidopsis plant (Tripathi et al., 2018; Jewell et al., 2019). The beneficial effects of P application are also observed in controlling seedling and fungal diseases wherein the prolific root growth enables the plant to escape the disease (Huber and Graham, 1999). Various researchers have shown the significant effect of P fertilization in managing Pythium root rot in wheat (Huber, 1980) and reducing bacterial leaf blight in rice, downy mildew, blue mold, leaf curl virus disease in tobacco, pod and stem blight in soybean, yellow dwarf virus disease in barley, brown stripe disease in sugarcane, and blast disease in rice (Potash and Phosphate Institute [PPI], 1988; Reuveni et al., 1998; Huber and Graham, 1999; Kirkegaard et al., 1999; Reuveni et al., 2000). Campos-Soriano et al. (2020) reported overexpression of miR399 resulting in high Pi content and enhanced susceptibility to infection by the rice blast fungus Magnaporthe oryzae due to high phosphate fertilization.
Potassium
Potassium (K) is an essential nutrient and the most plentiful inorganic cation found in plants (Shabala and Pottosin, 2010). K plays essential roles in enzyme activation, protein synthesis, photosynthesis, osmoregulation, stomatal movement, energy transfer, phloem transport, cation-anion balance, stress resistance (Marschner, 2012) crop yield, and quality improvement (Marschner, 2012; Oosterhuis et al., 2014). The plants with K starvation symptoms are observed to be more susceptible to disease in comparison to those having adequate K supply. A reduction in the incidence of fungal diseases (70%), bacterial diseases (69%), viral diseases (41%), and nematodes (33%) due to the profound K use was reported by Perrenoud (1990). Though K fertilization decreased the disease incidence in most of the cases, contrary results were also reported in some instances thereby categorizing the K impact on plant disease as “increased,” “decreased,” and having “no effect” or “variable effect” (Prabhu et al., 2007). The increased susceptibility in strawberries grown under K concentration excess toward Colletotrichum gloeosporioides and resistance alleviation in K fertilization absence due to starvation-induced synthesis of ROS and phytohormones were reported by Nam et al. (2006) which lead to enhanced plant stress tolerance (Amtmann et al., 2008). The increased K+ concentrations also decrease the prevailing intra-plant pathogen competition for nutrients (Holzmueller et al., 2007) and thereby enabling the plant to divert more resources to build the physical defense barrier and damage repair (Mengel, 2001). K is also an important facet in regulating the plant enzyme function by regulating the plants’ metabolite pattern and eventually varying its metabolite concentrations (Marschner, 2012). The synthesis of high-molecular-weight compounds (such as proteins, starches, and cellulose) and phenol concentration was significantly increased in plants with an adequate supply of K, which depressed the concentrations of low-molecular-weight compounds (soluble sugars, organic acids, amino acids, and amides) essential for diseases development in plant tissues, thereby making the plant less prone toward disease incidence (Prasad et al., 2010).
Calcium
Calcium is an essential element, serving as one of the cell wall and membrane constituents and thereby contributing to the cell structure along with upholding the physical barriers against invading pathogens (White and Broadley, 2003). Owing to its significance in the structural role, the plants showing Ca deficiency are observed to be more prone to disease infection, and element exogenous supply has been shown to alleviate the plant’s resistance response toward the pathogen. A reduction in the Ca concentration within the plant increases susceptibility toward the fungi preferentially invading the xylem tissue and dissolving the cell wall of the conducting vessels increases, leading to wilting of the plant (Hirschi, 2004). In addition, Ca also plays an important role in serving as a secondary messenger for a variety of metabolic processes carried out within the plant during biotic stresses (Lecourieux et al., 2002). The Ca2+ signal is observed to be one of the earliest responses in the basal defense response triggering the signaling cascade required for the pathogen-associated molecular patterns (PAMPs) or host-derived damage-associated molecular patterns (DAMPs) that are recognized by surface-localized pattern-recognition receptors (PRRs) eventually leading to PAMP-triggered immunity (PTI) (Dodds and Rathjen, 2010).
Sulfur
Sulfur (S) is an essential plant macronutrient having a pivotal role in plant disease resistance. The sulfur-containing defense compounds (SDCs) play versatile roles both in pathogen perception and initiating signal transduction pathways that are interconnected with various defense processes regulated by plant hormones (salicylic acid, JA, and ethylene) and ROS (Kunstler et al., 2020). The sulfur-containing amino acid (SAA) cysteine acts as a precursor of a large number of biomolecules, having major roles in plant disease resistance. Cysteine mediates spore germination and mycelial growth inhibition in a concentration-dependent manner in Phaeomoniella chlamydospora and Phaeoacremonium minimum, the two main causal agents of grapevine trunk disease (Roblin et al., 2018). The other important SAA in plants playing a central role in different defense reactions to biotic stresses is methionine (Met). A drastic reduction in the disease severity of Met-treated susceptible pearl millet cultivar (Pennisetum glaucum) infected by Sclerospora graminicola was reported by Sarosh et al. (2005). The Met treatment induces the generation of hydrogen peroxide (H2O2), a key element in plant defense signaling, leading to an upregulation in different defense-related gene expressions in grapevine (Vitis vinifera) (Boubakri et al., 2013). Sulfur-containing secondary metabolites play an important role in plant disease resistance and based on their mode of action can be classified into phytoalexins and phytoanticipins (Nwachukwu et al., 2012). In sulfur-deficient plants, there is a general gene downregulation responsible for sulfur-containing secondary metabolites synthesis and therefore the biosynthesis of S-containing phytoalexin (Camalexin) is also inhibited. Elemental sulfur (S0) can also be regarded as the only inorganic phytoalexin in plants that is accumulated during the xylem-invading fungal infection and bacterial pathogens infection, and its accumulation is faster and greater in disease-resistant genotypes than in susceptible lines (Cooper and Williams, 2004). The reactive sulfur species (RSSs) also play an important role in defense metabolism due to their participation in cellular signaling and regulatory processes. Two RSSs, hydrogen sulfide and sodium sulfite, have been shown to play important roles in plant disease resistance (Gao et al., 2012; Chen et al., 2014).
Magnesium
Magnesium (Mg) is a vital cation, which influences an array of in planta physiological functions when the plant presents deficient or excess concentrations (Wang et al., 2020). It can also affect the pathogen invasion way into a plant by colonizing the plant phloem tissues, as it is present within the young phloem tissues under high Mg concentration and outside the cells under Mg deficit conditions. A low Mg concentration was detected in maize plants infected with corn stunt spiroplasma, which occurs due to the competition for Mg between the plant and the pathogen, thereby causing pronounced symptoms in the plant deficient in Mg (Nome et al., 2009). Mg deficiency during plant growth can also reduce the structural integration within the middle lamella and may also lower the energy production necessary for defense functions eventually leading to pathogen metabolites inactivation. A nutrient-rich environment favoring several phytopathogens occurs in the leaf tissue under the Mg deficiency condition due to sucrose and starch deposition in the leaf tissue (Huber and Jones, 2013). A higher clubroot disease incidence was also reported in soils showing lower Mg concentrations (Young et al., 1991). A drastic increase in the rate of disease infection and severity of peanut leaf spots caused by Mycosphaerella arachidicola was observed during the Mg deficient conditions (Bledsoe et al., 1945). An increase in pepper and tomato bacterial spot disease incidence caused by Xanthomonas campestris pv. vesicatoria was observed due to alleviated Mg levels (Woltz and Jones, 1979).
