Hydroxy proline and gamma-aminobutyric acid: markers of susceptibility to vine decline disease caused by the fungus Monosporascus cannonballus in melons (Cucumis melo L.)

0 hours

The means of amino acids concentrations are displayed in Table 1. The concentration of glutamine, citrulline, serine, asparagine, glycine, beta-alanine, methionine, tyrosine, phenylalanine, isoleucine, leucine, hydroxy proline, valine, threonine, gamma-aminobutyric acid were significantly higher in the susceptible genotype TAM-Uvalde while the concentration of alanine was significantly higher in the resistant genotype USDA PI 124104.

24 hours

The means of the amino acid concentrations are displayed in Table 1. The amino acids glutamine, citrulline, serine, glycine, beta-alanine, alanine, hydroxy proline, isoleucine, and valine presented significant differences among treatments due to the interaction genotype × inoculation. The amino acids asparagine and methionine presented differences due to genotypes. Lastly, the amino acids threonine and gamma-aminobutyric acid presented significant differences between treatments due to inoculations.

48 hours

The means of the amino acid concentrations are displayed in Table 1. The amino acids serine, asparagine, threonine, glycine, beta-alanine, gamma-aminobutyric acid, and hydroxy proline presented significant differences among the treatments due to the interaction genotype × inoculation. The amino acids glutamine and alanine presented significant differences between the treatments due to genotypes. Finally, the amino acids citrulline and methionine presented significant differences between the treatments due to inoculations.

72 hours

The means of the amino acid concentrations are displayed in Table 1. The amino acids glutamine, citrulline, threonine, glycine, gamma-aminobutyric acid, methionine, tyrosine, and hydroxy proline presented differences among the treatments due to the interaction genotype × inoculation. Finally, the amino acids beta-alanine and alanine presented differences between the treatments due to genotypes.

It is commonly known that amino acids are part of all living organisms, and, in some cases, the concentration of amino acids was higher due to inoculations with the fungus, M. cannonballus. Thus, they may have been produced by the fungus. For example, Gong et al. (2007) indicated that asparagine, which is an essential amino acid derived from citrulline, is needed by fungus Coniothyrium minitans for conidiation. In other words, it needs it to reach its reproductive stage. Likewise, Canonica et al. (1979) reported that the fungus Cochliobolus miyabeanus requires methionine to produce metabolites derived from cochlioquinone. Therefore, it can be thought that the fungus M. cannonballus may have produced amino acids to carry out physiological functions. Also, the presence of the pathogen may have triggered the production of these amino acids in the plants regardless of the genotype used, which can be interpreted as a response of the plant that was reflected in significant differences due to the inoculation. Hence, further research, for instance, gene expression studies may be useful in determining their origin (Joshi et al., 2019). Similarly, the concentration of amino acids is linked to several factors. For instance, genotype, environment, and cultural practices (Bernillon et al., 2013). Therefore, the presence of the pathogen along with a defense response of the plant to counter the infection may have caused an increase in their concentration.

Citrulline is commonly present in plants belonging to the Cucurbitaceae family. Moreover, its presence is associated with responses to abiotic stresses. Interestingly, citrulline is an amino acid that is not translocated long distances within the plant and is found in higher concentrations within fruits of the same family (Joshi et al., 2019). In addition, citrulline is a precursor of arginine that is a precursor of nitric oxide, which has been reported to be produced by plants as a defensive compound (Vitor et al., 2013). However, it was not present in the roots (Table 1). Nevertheless, it is notable that citrulline was only present in the susceptible and resistant genotypes whose plants were inoculated (Table 1), which suggests that it is not involved in resistance to vine decline disease. However, citrulline may have been used as a signaling molecule. For example, molecules such as hydrogen sulfide and nitric oxide are important signaling molecules in plants. Therefore, citrulline may have been involved in the activation of a physiological process within the plant (Zang & Xie, 2021).

Gamma-aminobutyric acid is reported as a signaling molecule because it is produced rapidly when a plant is wounded or suffered from mechanical damage (Arçay et al., 2012; Busch & Fromm, 1999; Kinnersley & Turano, 2000). Remarkably, the concentration of this amino acid increased in both genotypes in the same proportion 24 h after inoculating the fungus, M. cannonballus (Table 1). However, the susceptible genotype TAM-Uvalde, exhibited higher quantities 48 and 72 h after inoculating the fungus M. cannonballus (Table 1), which suggests that the roots of the susceptible genotype suffered from more penetration of the fungus, M. cannonballus, than the roots of the resistant genotype USDA PI 124104.

Hypersensitive responses of plants have been related to the presence of calcium, which is involved in the production of GABA (Kinnersley & Turano, 2000). Xu & Heath (1998) reported that the concentration of calcium in cells of resistant cowpea plants rises to generate a hypersensitive response to counter the infection of cowpea rust fungus, Urumices vignae. Thus, the surge in the concentration of GABA previously described may have been an indication of a similar response of the plants against the infection of the fungus M. cannonballus.

Gamma-aminobutyric acid (GABA) is a 4 carbon non-protein amino acid that is involved in several metabolic functions of plants. For instance, it regulates pH and osmotic potential as well as the growth of pollen tubes and more remarkably, it prevents the accumulation of reactive oxygen species when plants are under stress. Hence, the spike in the concentration of this amino acid may have been related to the stress underwent by the plant due to pathogen infection. Similarly, Kinnersley & Turano (2000) documented that the production of GABA is linked to low pH concentrations. Thus, the use of peat moss as a medium, may have contributed to its production.

The existence of higher concentrations of GABA may be indicative of the synthesis of secondary metabolites that might have been used as defensive compounds. Kinnersley & Turano (2000) documented that GABA could be a potential source of carbon to replenish intermediaries in the Krebs cycle that are used to produce secondary metabolites with antimicrobial properties such as phytoalexins, coumesterol and coumarin. Furthermore, its presence has been related to increased tolerance to heat stress in mung beans plants (Priya et al., 2019).

Hydroxy proline was also detected in this study. Generally, the presence of this amino acid is linked to abiotic stress responses related to salinity and drought. Moreover, its presence in cells regulates osmotic potential, protects enzymes as well as membranes. It also captures and retains reactive oxygen species to prevent oxidative damage (Rabisa et al., 2021). Thus, its presence may be associated with a defensive response of the plant. However, it may have not been helpful in countering the infection of the pathogen since it was up regulated in higher quantities in the susceptible genotype. Hence, it could be linked to its susceptibility (Table 1) .

Amino acids such as valine, threonine, methionine, phenylalanine, isoleucine, and leucine are commonly found in melons. Moreover, they are linked to characteristics related to quality. For example, fruit aroma, nutritional value, and health-promoting properties (Singh et al., 2020).

Current knowledge indicates that the catabolism of glycine and serine is energetically expensive. Hence, they usually remain in the plant without any change. On the other hand, glutamine is metabolized to aspartate and glutamate, which is a precursor of GABA (Hildebrandt et al., 2015).

It is notable that some amino acids such as lysine and tryptophan were not present in the roots (Table 1). Lysine is required for the synthesis of L-pipecolate, which is a regulator of inducible plant immunity and tryptophan is a precursor of auxins and secondary metabolites such as phytoalexins, glucosinolates and alkaloids (Hildebrandt et al., 2015; Navarova et al., 2012; Radwanski & Last, 1995).