Regulation of Arbuscular Mycorrhization by Carbon. The Symbiotic Interaction Cannot Be Improved by Increased Carbon Availability Accomplished by Root-Specifically Enhanced Invertase Activity

Abstract

The mutualistic interaction in arbuscular mycorrhiza (AM) is characterized by an exchange of mineral nutrients and carbon. The major benefit of AM, which is the supply of phosphate to the plant, and the stimulation of mycorrhization by low phosphate fertilization has been well studied. However, less is known about the regulatory function of carbon availability on AM formation. Here the effect of enhanced levels of hexoses in the root, the main form of carbohydrate used by the fungus, on AM formation was analyzed. Modulation of the root carbohydrate status was performed by expressing genes encoding a yeast (Saccharomyces cerevisiae)-derived invertase, which was directed to different subcellular locations. Using tobacco (Nicotiana tabacum) alcTcwINV plants, the yeast invertase was induced in the whole root system or in root parts. Despite increased hexose levels in these roots, we did not detect any effect on the colonization with Glomus intraradices analyzed by assessment of fungal structures and the level of fungus-specific palmitvaccenic acid, indicative for the fungal carbon supply, or the plant phosphate content. Roots of Medicago truncatula, transformed to express genes encoding an apoplast-, cytosol-, or vacuolar-located yeastderived invertase, had increased hexose-to-sucrose ratios compared to b-glucuronidase-transformed roots. However, transformations with the invertase genes did not affect mycorrhization. These data suggest the carbohydrate supply in AM cannot be improved by root-specifically increased hexose levels, implying that under normal conditions sufficient carbon is available in mycorrhizal roots. In contrast, tobacco rolCTppa plants with defective phloem loading and tobacco pyk10TInvInh plants with decreased acid invertase activity in roots exhibited a diminished mycorrhization.

Plant-Derived Sucrose Is a Key Element in the Symbiotic Association between Trichoderma virens and Maize Plants

“…For instance, contrary to the effect observed in plants with a strong increase in Inv activity, mycorrhization was stimulated in plants with a slight increase in leaf Inv activity (Schaarschmidt et al, 2007b). Unexpectedly, arbuscular mycorrhization was not improved in transgenic plants with increased Inv activity in roots that produced high levels of monosaccharides for the fungal cells (Schaarschmidt et al, 2007a). These observations suggest a very tight and complex process that regulates carbohydrate accumulation, distribution, and root colonization by arbuscular mycorrhizae that we do not fully understand.…”

“…Carbohydrate flow to the symbiotic partner is controlled by the plant through the hydrolysis of Suc. For instance, the importance of plant apoplastic Inv in controlling carbon delivery and root colonization by arbuscular mycorrhiza fungi has been demonstrated in transgenic Nicotiana tabacum plants (Schaarschmidt et al, 2007a(Schaarschmidt et al, , 2007b. Transgenic plants with strongly enhanced Inv activity in leaves resulted in an accumulation of hexoses in source leaves, a decrease of Suc availability for roots, and a reduction of mycorrhization.…”

Enhancement of Drought Tolerance in Trifoliate Orange by Mycorrhiza: Changes in Root Sucrose and Proline Metabolisms

“…Effects of Funneliformis mosseae and Paraglomus occultum on activities of ornithine-δ-amino transferase (OAT) (a), Δ 1 -pyrroline-5-carboxylate reductase (P5CR) (b), proline dehydrogenase (ProDH) (c), and Δ 1 -pyrroline-5-carboxylate synthetase (P5CS) (d) in roots of Poncirus trifoliata seedlings under well-watered (WW) and drought stressed (DS) conditions. Data (means ± SD, n = 5) followed by the same letter above the bars are not significantly different among treatments at P <0.05 gradient between the source and the sink (Schaarschmidt et al, 2007). In contrast, the seedlings inoculated with F. mosseae revealed higher root AI and NI activities relative to non-AM seedlings under WW condition, suggesting that the responses of root AI and NI activities to mycorrhization are mycorrhizal fungal dependent .…”

Laser Microdissection Unravels Cell-Type-Specific Transcription in Arbuscular Mycorrhizal Roots, Including CAAT-Box Transcription Factor Gene Expression Correlating with Fungal Contact and Spread

“…Due to the fact that hexoses derived from the plant metabolism are supplied to mycorrhizal fungi, AM roots represent strong carbon sinks. Whereas hexoses generated from Suc via the activity of apoplastic invertases are directly available to the fungus (Schaarschmidt et al, 2007), hexoses provided by cytoplasmic invertases or Suc synthases (Hohnjec et al, 1999) first have to be exported to the extracellular space (Baier et al, 2010). Up to now, little is known about the carbohydrate transporters involved in the translocation of symplastic hexoses to the apoplastic interface.…”