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Abstract
The mutualistic interaction of plants with arbuscular mycorrhizal (AM) fungi is characterized by an exchange of nutrients. The plant provides sugars in the form of hexoses to the heterotrophic fungus in return for phosphate as well as nitrogen, water, and micronutrients. Plant sucrose-cleaving enzymes are predicted to play a crucial role in hexose mobilization as these enzymes appear to be absent in the fungal partner. Here, recent findings concerning the function of plant apoplastic invertases in the AM symbiosis are discussed. Plants with modulated enzyme activity in roots and leaves provide additional insight on the complexity of the regulation of the AM interaction by apoplastic invertases as mycorrhization could be reduced or stimulated depending on the level of invertase activity and its tissue-specific expression.
Figures and Tables
Figure 1 Regulation of arbuscular mycorrhization by modulated invertase activity in the plant apoplast. (A) A decreased mycorrhization can be achieved either by reduced activity of apoplastic invertase in the root or strongly increased activity in the leaf (up to 25-fold). In both cases, the root is characterized by an undersupply of carbohydrates. Moreover, the strong invertase activity in the source leaves results in sugar accumulation and activation of defense mechanisms in the leaves, that include the accumulation of PR transcript levels and phenolic compounds like chlorogenic acid isomers, scopolin and scopoletin.Citation22 This increased defense status of the plant may contribute additionally to the inhibition of AM. (B) An increased mycorrhization cannot, however, be obtained by increased invertase activity in the root apoplast despite the higher root hexose levels. Instead, stimulated mycorrhization was observed in plants with slightly (2- to 4-fold) enhanced invertase activity in the leaf apoplast. Such plants showed an altered defense and hormone status in the root as characterized by a reduction in phenolic compounds (chlorogenic acid, scopolin and scopoletin), amines and some amino acids as well as increased abscisic acid levels. These changes are potentially triggered by slightly reduced sugar levels in the leaves.
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