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

Modhi O. Alotaibi,Mohamed S. Sheteiwy,Hamada Abd Elgawad,Afrah E. Mohammed,Ahmed M. El-Sawah,Renato L. Sobrinho,Ahmed M. Saleh

doi:10.3390/jof7070531

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.

Highlights

IntroductionIn acidic soils (pH ≤ 6), Al3+ inters into plant roots, where it inhibits root cell division and elongation [5,6]
Mycorrhizal colonization was highly affected by different tested treatments in both plants; no mycorrhizal colonization was observed in the roots of plants that were not inoculated with Arbuscular mycorrhizal fungi (AMF) (Table 1)
Since Pro metabolism is closely connected to nitrogen metabolism [68], the present study suggested that increased nitrogen assimilation in lotus improved the ability to further elevate Pro levels to increase tolerance to Al stress, consistent with previous studies reporting that the external addition of nitrogen results in elevated Pro [69,70]

Summary

Introduction

In acidic soils (pH ≤ 6), Al3+ inters into plant roots, where it inhibits root cell division and elongation [5,6]. Al3+ -stressed plants experience a great reduction in the assimilation and uptake of water and nutrients, which reduces their productivity [7,8]. The accumulation of Al in leaves directly induces specific stress responses in developmental, physiological, and biochemical processes. It disrupts the cell wall and plasma membrane structure, signal transduction, and nucleotide/phosphate homeostasis and alters the antioxidant and osmo-regulation status of plants [9,10]. Despite the agricultural impact of Al in soils, knowledge of the mechanisms of plant tolerance against Al toxicity is rather fragmentary

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