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Diese Seite wurde zuletzt am 25 Jul 2012 25 Jul 2012 geändert.
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Publikation
Adenosine monophosphate kinase (AMK; adenylate kinase) catalyses the reversible formation of ADP by the transfer of one phosphate group from ATP to AMP, thus equilibrating adenylates. The Arabidopsis (Arabidopsis thaliana) genome contains 10 genes with an adenylate/cytidylate kinase signature; seven of these are identified as putative adenylate kinases. Encoded proteins of at least two members of this Arabidopsis adenylate kinase gene family are targeted to plastids. However, when the individual genes are disrupted, the phenotypes of both mutants are strikingly different. Although absence of AMK2 causes only 30% reduction of total adenylate kinase activity in leaves, there is loss of chloroplast integrity leading to small, pale-looking plantlets from embryo to seedling development. In contrast, no phenotype for disruption of the second plastid adenylate kinase was found. From this analysis, we conclude that AMK2 is the major activity for equilibration of adenylates and de novo synthesis of ADP in the plastid stroma.
Publikation
The first step of the plastidial methylerythritol phosphate (MEP) pathway is catalyzed by two isoforms of 1‐deoxy‐d‐ xylulose 5‐phosphate synthase (DXS1 and DXS2). In Medicago truncatula , MtDXS1 and MtDXS2 genes exhibit completely different expression patterns. Most prominently, colonization by arbuscular mycorrhizal (AM) fungi induces the accumulation of certain apocarotenoids (cyclohexenone and mycorradicin derivatives) correlated with the expression of MtDXS2 but not of MtDXS1. To prove a distinct function of DXS2, a selective RNAi approach on MtDXS2 expression was performed in transgenic hairy roots of M. truncatula. Repression of MtDXS2 consistently led to reduced transcript levels in mycorrhizal roots, and to a concomitant reduction of AM‐induced apocarotenoid accumulation. The transcript levels of MtDXS1 remained unaltered in RNAi plants, and no phenotypical changes in non‐AM plants were observed. Late stages of the AM symbiosis were adversely affected, but only upon strong repression with residual MtDXS2‐1 transcript levels remaining below approximately 10%. This condition resulted in a strong decrease in the transcript levels of MtPT4 , an AM‐specific plant phosphate transporter gene, and in a multitude of other AM‐induced plant marker genes, as shown by transcriptome analysis. This was accompanied by an increased proportion of degenerating and dead arbuscules at the expense of mature ones. The data reveal a requirement for DXS2‐dependent MEP pathway‐based isoprenoid products to sustain mycorrhizal functionality at later stages of the symbiosis. They further validate the concept of a distinct role for DXS2 in secondary metabolism, and offer a novel tool to selectively manipulate the levels of secondary isoprenoids by targeting their precursor supply.
Publikation
A crucial step in the biosynthesis of jasmonic acid (JA) is the formation of its stereoisomeric precursor, cis-(+)-12-oxophytodienoic acid (OPDA), which is catalyzed by allene oxide cyclase (AOC, EC 5.3.99.6). A cDNA of AOC was isolated from Humulus lupulus var. Nugget. The ORF of 765 bp encodes a 255 amino acid protein, which carries a putative chloroplast targeting sequence. The recombinant protein without its putative chloroplast target sequence showed significant AOC activity. Previously we demonstrated that wounding induces organogenic nodule formation in hop. Here we show that the AOC transcript level increases in response to wounding of internodes, peaking between 2 and 4 h after wounding. In addition, Western blot analysis showed elevated levels of AOC peaking 24 h after internode inoculation. The AOC increase was accompanied by increased JA levels 24 h after wounding, whereas OPDA had already reached its highest level after 12 h. AOC is mostly present in the vascular bundles of inoculated internodes. During prenodule and nodule formation, AOC levels were still high. JA and OPDA levels decreased down to 10 and 118 pmol (g FW)–1, respectively, during nodule formation, but increased during plantlet regeneration. Double immunolocalization analysis of AOC and Rubisco in connection with lugol staining showed that AOC is present in amyloplasts of prenodular cells and in the chloroplasts of vacuolated nodular cells, whereas meristematic cells accumulated little AOC. These data suggest a role of AOC and jasmonates in organogenic nodule formation and plantlet regeneration from these nodules.
Diese Seite wurde zuletzt am 25 Jul 2012 25 Jul 2012 geändert.