Arbuscular Mycorrhizae Mitigate Aluminum Toxicity and Regulate Proline Metabolism in Plants Grown in Acidic Soil

Abstract

Arbuscular mycorrhizal fungi (AMF) can promote plant growth and induce stress tolerance. Proline is reported to accumulate in mycorrhizal plants under stressful conditions, such as aluminum (Al) stress. However, the detailed changes induced in proline metabolism under AMF-plant symbiosis has not been studied. Accordingly, this work aimed to study how Al-stressed grass (barley) and legume (lotus) species respond to AMF inoculation at growth and biochemical levels. The associated changes in Al uptake and accumulation, the rate of photosynthesis, and the key enzymes and metabolites involved in proline biosynthesis and degradation pathways were studied. Soil contamination with Al induced Al accumulation in tissues of both species and, consequently, reduced plant growth and the rate of photosynthesis, while more tolerance was noticed in lotus. Inoculation with AMF significantly reduced Al accumulation and mitigated the negative impacts of Al on growth and photosynthesis in both species; however, these positive effects were more pronounced in barley plants. The mitigating action of AMF was associated with upregulation of proline biosynthesis through glutamate and ornithine pathways, more in lotus than in barley, and repression of its catabolism. The increased proline level in lotus was consistent with improved N metabolism (N level and nitrate reductase). Overall, this study suggests the role of AMF in mitigating Al stress, where regulation of proline metabolism is a worthy mechanism underlying this mitigating action.

Keywords: Rhizophagus irregularis; barley; lotus; nitrogen assimilation; photosynthesis; proline metabolism.

Figure 1

Proline biosynthesis (glutamate pathway). Changes in metabolic level and related enzyme activities: impact of arbuscular mycorrhizal fungi (AMF), Al (25 mg Al kg⁻¹ soil), or their combination. Panels show concentrations of glutamate (A), glutamine (B), and the activity of glutamate synthase (GOGAT; (C)), glutamine synthetase (GS; (D)), P5C synthase (P5CS; (E)), and D1-pyrroline-5-carboxylate reductase (ProDH) (F). Different letters in each graph represent significant differences between the three treatments in each species (Tukey’s test; p < 0.05; n = 5).

Figure 2

Proline biosynthesis (ornithine pathway). Changes in metabolic level and related enzyme activity in barley and lotus plants: impact of arbuscular mycorrhizal fungi (AMF), Al (25 mg Al kg⁻¹ soil), or their combination. Panels show concentrations of ornithine (A), arginine (B), and the activity of arginase (C) and ornithine aminotransferase (OAT; (D)). Different letters in each graph represent significant differences between the three treatments in each species (Tukey’s test; p < 0.05; n = 5).

Figure 3

Pro biosynthesis (P5C–Pro metabolism cycle). Changes in metabolic level and related enzyme activities: impact of arbuscular mycorrhizal fungi (AMF), Al (25 mg Al kg⁻¹ soil), or their combination. Panels show concentrations of proline (A) and its immediate precursor P5C (B) and the activity of pyrroline-5-carboxylate reductase (P5CR) (C) and proline dehydrogenase (ProDH) (D). Different letters in each graph represent significant differences between the three treatments in each species (Tukey’s test; p < 0.05; n = 5).

Figure 4

Hierarchical clustering of proline-metabolism-related parameters. The levels patterns are relatively demonstrated on the heatmap based on the mean value (n = 5) for each parameter. Red and blue color gradients indicate higher and lower levels, respectively. Full names are indicted in the legends of Figure 1, Figure 2 and Figure 3 and Table 1, Table 2 and Table 3.