Transcriptional regulation of host enzymes involved in the cleavage of sucrose during arbuscular mycorrhizal symbiosis

To investigate plant carbon metabolism in arbuscular mycorrhizas, we have analyzed expression of the tomato invertase (EC 3.2.1.26) gene family members and the sucrose synthase (EC 2.4.1.13) gene TOMSSF in roots of non-mycorrhizal, Glomus mosseae- and Glomus intraradices-colonized plants. Furthermore, root soluble carbohydrate contents have been determined. Gene expression analyses showed that the cell wall invertase Lin6, the vacuolar invertase TIV1 and TOMSSF were upregulated in mycorrhizal roots and that this effect was caused by a direct effect of the colonization by the arbuscular mycorrhizal (AM) fungi and not mediated by an improved phosphorus nutrition. This study shows for the first time upregulation of a cell wall invertase gene in an AM association, which supports the general assumption that carbon transfer across the symbiotic interface requires host sucrose hydrolysis by a cell wall invertase. Transcriptional upregulation of sucrose-splitting enzymes during early colonization development agrees with the decreased levels of sucrose detected in these roots. Mycorrhizal plants had lower root glucose and fructose concentrations, which indicate consumption of the products of sucrose breakdown. The promoter sequences of Lin6, TIV1 and TOMSSF were analyzed in silico to get insights into the causes of their transcriptional activation in mycorrhizal roots. Upregulation of Lin6, TOMSSF and TIV1 expressions by salicylic acid and of TOMSSF and TIV1 by abscisic acid suggest that these compounds might mediate upregulation of these genes in mycorrhizal roots.