Publikationen - Molekulare Signalverarbeitung

Grain development of the maternal effect shrunken endosperm mutant seg8 was analysed by comprehensive molecular, biochemical and histological methods. The most obvious finding was de‐regulation of ABA levels, which were lower compared to wild‐type during the pre‐storage phase but higher during the transition from cell division/differentiation to accumulation of storage products. Ploidy levels and ABA amounts were inversely correlated in the developing endosperms of both mutant and wild‐type, suggesting an influence of ABA on cell‐cycle regulation. The low ABA levels found in seg8 grains between anthesis and beginning endosperm cellularization may result from a gene dosage effect in the syncytial endosperm that causes impaired transfer of ABA synthesized in vegetative tissues into filial grain parts. Increased ABA levels during the transition phase are accompanied by higher chlorophyll and carotenoid/xanthophyll contents. The data suggest a disturbed ABA‐releasing biosynthetic pathway. This is indicated by up‐regulation of expression of the geranylgeranyl reductase (GGR) gene, which may be induced by ABA deficiency during the pre‐storage phase. Abnormal cellularization/differentiation of the developing seg8 endosperm and reduced accumulation of starch are phenotypic characteristics that reflect these disturbances. The present study did not reveal the primary gene defect causing the seg8 phenotype, but presents new insights into the maternal/filial relationships regulating barley endosperm development.

The history of plant biology is inexorably intertwined with the conception and discovery of auxin, followed by the many decades of research to comprehend its action during growth and development. Growth responses to auxin are complex and require the coordination of auxin production, transport, and perception. In this overview of past auxin research, we limit our discourse to the mechanism of auxin action. We attempt to trace the almost epic voyage from the birth of the hormonal concept in plants to the recent crystallographic studies that resolved the TIR1-auxin receptor complex, the first structural model of a plant hormone receptor. The century-long endeavor is a beautiful illustration of the power of scientific reasoning and human intuition, but it also brings to light the fact that decisive progress is made when new technologies emerge and disciplines unite.

Natural isothiocyanates, produced during plant tissue damage from methionine‐derived glucosinolates, are potent inducers of mammalian phase 2 detoxification enzymes such as quinone reductase (QR). A greatly simplified bioassay for glucosinolates based on induction and colorimetric detection of QR activity in murine hepatoma cells is described. It is demonstrated that excised leaf disks of Arabidopsis thaliana (ecotype Columbia) can directly and reproducibly substitute for cell‐free leaf extracts as inducers of murine QR, which reduces sample preparation to a minimum and maximizes throughput. A comparison of 1 and 3 mm diameter leaf disks indicated that QR inducer potency was proportional to disk circumference (extent of tissue damage) rather than to area. When compared to the QR inducer potency of the corresponding amount of extract, 1 mm leaf disks were equally effective, whereas 3 mm disks were 70% as potent. The QR inducer potency of leaf disks correlated positively with the content of methionine‐derived glucosinolates, as shown by the analysis of wild‐type plants and mutant lines with lower or higher glucosinolate content. Thus, the microtitre plate‐based assay of single leaf disks provides a robust and inexpensive visual method for rapidly screening large numbers of plants in mapping populations or mutant collections and may be applicable to other glucosinolate‐producing species.

FQR1 is a novel primary auxin-response gene that codes for a flavin mononucleotide-binding flavodoxin-like quinone reductase. Accumulation of FQR1 mRNA begins within 10 min of indole-3-acetic acid application and reaches a maximum of approximately 10-fold induction 30 min after treatment. This increase in FQR1 mRNA abundance is not diminished by the protein synthesis inhibitor cycloheximide, demonstrating thatFQR1 is a primary auxin-response gene. Sequence analysis reveals that FQR1 belongs to a family of flavin mononucleotide-binding quinone reductases. Partially purified His-tagged FQR1 isolated fromEscherichia coli catalyzes the transfer of electrons from NADH and NADPH to several substrates and exhibits in vitro quinone reductase activity. Overexpression of FQR1 in plants leads to increased levels of FQR1 protein and quinone reductase activity, indicating that FQR1 functions as a quinone reductase in vivo. In mammalian systems, glutathione S-transferases and quinone reductases are classified as phase II detoxification enzymes. We hypothesize that the auxin-inducible glutathioneS-transferases and quinone reductases found in plants also act as detoxification enzymes, possibly to protect against auxin-induced oxidative stress.

Glucosinolates are a diverse class of nitrogen- and sulfur-containing secondary metabolites. They are rapidly hydrolyzed on tissue disruption to a number of biologically active compounds that are increasingly attracting interest as anticarcinogenic phytochemicals and crop protectants. Several glucosinolate-derived isothiocyanates are potent chemopreventive agents that favorably modulate carcinogen metabolism in mammals. Methylsulfinylalkyl isothiocyanates, in particular the 4-methylsulfinylbutyl derivative, are selective and potent inducers of mammalian detoxification enzymes such as quinone reductase (QR). Cruciferous plants including Arabidopsis thaliana (L.) Heyhn, synthesize methylsulfinylalkyl glucosinolates, which are derived from methionine. Using a colorimetric assay for QR activity in murine hepatoma cells and high performance liquid chromatography (HPLC) analysis of desulfoglucosinolates, we have demonstrated a strong positive correlation between leaf QR inducer potency and leaf content of methionine-derived glucosinolates in various A. thaliana ecotypes and available glucosinolate mutants. In a molecular genetic approach to glucosinolate biosynthesis, we screened 3000 chemically mutagenized M2 plants of the Columbia ecotype for altered leaf QR inducer potency. Subsequent HPLC analysis of progeny of putative mutants identified six lines with significant and heritable changes in leaf glucosinolate content and composition.

Phosphate (Pi) plays a central role as reactant and effector molecule in plant cell metabolism. However, Pi is the least accessible macronutrient in many ecosystems and its low availability often limits plant growth. Plants have evolved an array of molecular and morphological adaptations to cope with Pi limitation, which include dramatic changes in gene expression and root development to facilitate Pi acquisition and recycling. Although physiological responses to Pi starvation have been increasingly studied and understood, the initial molecular events that monitor and transmit information on external and internal Pi status remain to be elucidated in plants. This review summarizes molecular and developmental Pi starvation responses of higher plants and the evidence for coordinated regulation of gene expression, followed by a discussion of the potential involvement of plant hormones in Pi sensing and of molecular genetic approaches to elucidate plant signalling of low Pi availability. Complementary genetic strategies in Arabidopsis thaliana have been developed that are expected to identify components of plant signal transduction pathways involved in Pi sensing. Innovative screening methods utilize reporter gene constructs, conditional growth on organophosphates and the inhibitory properties of the Pi analogue phosphite, which hold the promise for significant advances in our understanding of the complex mechanisms by which plants regulate Pi‐starvation responses.