Jasmonatfunktion & Mykorrhiza

Roots are highly plastic organs enabling plants to adapt to a changing below-ground environment. In addition to abiotic factors like nutrients or mechanical resistance, plant roots also respond to temperature variation. Below the heat stress threshold, Arabidopsis thaliana seedlings react to elevated temperature by promoting primary root growth, possibly to reach deeper soil regions with potentially better water saturation. While above-ground thermomorphogenesis is enabled by thermo-sensitive cell elongation, it was unknown how temperature modulates root growth. We here show that roots are able to sense and respond to elevated temperature independently of shoot-derived signals. This response is mediated by a yet unknown root thermosensor that employs auxin as a messenger to relay temperature signals to the cell cycle. Growth promotion is achieved primarily by increasing cell division rates in the root apical meristem, depending on de novo local auxin biosynthesis and temperature-sensitive organization of the polar auxin transport system. Hence, the primary cellular target of elevated ambient temperature differs fundamentally between root and shoot tissues, while the messenger auxin remains the same.

Jasmonates are important signals in plant stress responses and plant development. An essential step in the biosynthesis of jasmonic acid (JA) is catalysed by ALLENE OXIDE CYCLASE (AOC) which establishes the naturally occurring enantiomeric structure of jasmonates. In Arabidopsis thaliana, four genes encode four functional AOC polypeptides (AOC1, AOC2, AOC3, and AOC4) raising the question of functional redundancy or diversification. Analysis of transcript accumulation revealed an organ-specific expression pattern, whereas detailed inspection of transgenic lines expressing the GUS reporter gene under the control of individual AOC promoters showed partially redundant promoter activities during development: (i) In fully developed leaves, promoter activities of AOC1, AOC2, and AOC3 appeared throughout all leaf tissue, but AOC4 promoter activity was vascular bundle-specific; (ii) only AOC3 and AOC4 showed promoter activities in roots; and (iii) partially specific promoter activities were found for AOC1 and AOC4 in flower development. In situ hybridization of flower stalks confirmed the GUS activity data. Characterization of single and double AOC loss-of-function mutants further corroborates the hypothesis of functional redundancies among individual AOCs due to a lack of phenotypes indicative of JA deficiency (e.g. male sterility). To elucidate whether redundant AOC expression might contribute to regulation on AOC activity level, protein interaction studies using bimolecular fluorescence complementation (BiFC) were performed and showed that all AOCs can interact among each other. The data suggest a putative regulatory mechanism of temporal and spatial fine-tuning in JA formation by differential expression and via possible heteromerization of the four AOCs.

Among the plant hormones jasmonic acid and related derivatives are known to mediate stress responses and several developmental processes. Biosynthesis, regulation, and metabolism of jasmonic acid in Arabidopsis thaliana are reviewed, including properties of mutants of jasmonate biosynthesis. The individual signalling properties of several jasmonates are described.