Jasmonatfunktion & Mykorrhiza

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) alc∷cwINV 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 yeast-derived invertase, had increased hexose-to-sucrose ratios compared to β-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 rolC∷ppa plants with defective phloem loading and tobacco pyk10∷InvInh plants with decreased acid invertase activity in roots exhibited a diminished mycorrhization.

The alc promoter system, derived from the filamentous fungi Aspergillus nidulans, allows chemically regulated gene expression in plants and thereby the study of gene function as well as metabolic and developmental processes. In addition to ethanol, this system can be activated by acetaldehyde, described as the physiological inducer in A. nidulans. Here, we show that in contrast to ethanol, acetaldehyde allows tissue-specific activation of the alc promoter in transgenic tobacco plants. Soil drenching with aqueous acetaldehyde solutions at a concentration of 0.05% (v/v) resulted in the rapid and temporary induction of the alc gene expression system exclusively in roots. In addition, the split root system allows activation to be restricted to the treated part of the root. The temporary activation of the alc system by soil drenching with acetaldehyde could be prolonged over several weeks by subsequent applications at intervals of 7 d. This effect was demonstrated for the root-specific induction of a yeast-derived apoplast-located invertase under the control of the alcohol-inducible promoter system. In leaves, which exhibit a lower responsiveness to acetaldehyde than roots, the alc system was induced in the directly treated tissue only. Thus, acetaldehyde can be used as a local inducer of the alc gene expression system in tobacco plants.