Rapid activation of phospholipase A (PLA) by auxin or plant-pathogen interaction suggests a function in signal transduction for this enzyme, but the molecular identification of a cytosolic PLA carrying out this function remains open. We isolated four cDNA sequences from Arabidopsis (ecotype Columbia), AtPLA I, AtPLA IIA, AtPLA IVA, and AtPLA IVC, which are members of the patatin-related PLA gene family in plants and which are homologous to the animal Ca 2ϩ -independent PLA 2 gene family. Expression was measured by reverse transcriptase-polymerase chain reaction, and AtPLA I transcripts were found preferentially in shoots, AtPLA IIA and AtPLA IVA in roots, and AtPLA IVC in flowers. Transient expression of the four PLA-green fluorescent protein fusion proteins in tobacco (Nicotiana tabacum) leaves showed they were located in the cytosol and not in the vacuoles. Surprisingly, AtPLA::green fluorescent protein was also localized to chloroplasts. The enzymatic activity of the purified recombinant AtPLA IVA toward phosphatidylcholine was dependent on Ca 2ϩ , saturated at 0.5 mm, and had a pH optimum of about 7.0. It had both PLA 1 and PLA 2 specificity. The enzyme showed in vitro highest sensitivity toward the PLA 2 inhibitors palmitoyltrifluoromethyl ketone (PACOCF 3 , K i approximately 30 nm), arachidonyltrifluoromethyl ketone (AACOCF 3 , K i approximately 25 m), and tetrahydro-3-(1-naphtalenyl)-2H-pyran-2-one (K i approximately 200 nm) and was also sensitive to other previously used inhibitors 5,8,11,14-eicosatetraynoic acid (K i approximately 3 m) and nordihydroguajaretic acid (K i approximately 15 m). The influence of these PLA 2 inhibitors on elongation in etiolated Arabidopsis seedlings was tested, and tetrahydro-3-(1-naphtalenyl)-2H-pyran-2-one and 5,8,11,14-eicosatetraynoic acid inhibited hypocotyl elongation maximally at concentrations close to their K i in vitro.

The beneficial element silicon (Si) may affect radial oxygen loss (ROL) of rice roots depending on suberization of the exodermis and lignification of sclerenchyma. Thus, the effect of Si nutrition on the oxidation power of rice roots, suberization and lignification was examined. In addition, Si-induced alterations of the transcript levels of 265 genes related to suberin and lignin synthesis were studied by custom-made microarray and quantitative Real Time-PCR. Without Si supply, the oxidation zone of 12 cm long adventitious roots extended along the entire root length but with Si supply the oxidation zone was restricted to 5 cm behind the root tip. This pattern coincided with enhanced suberization of the exodermis and lignification of sclerenchyma by Si supply. Suberization of the exodermis started, with and without Si supply, at 4–5 cm and 8–9 cm distance from the root tip (drt), respectively. Si significantly increased transcript abundance of 12 genes, while two genes had a reduced transcript level. A gene coding for a leucine-rich repeat protein exhibited a 25-fold higher transcript level with Si nutrition. Physiological, histochemical, and molecular-biological data showing that Si has an active impact on rice root anatomy and gene transcription is presented here.

The expression of a citrate transporter MATE gene is crucial for citrate exudation in common bean. However, although the expression of the citrate transporter is a prerequisite for citrate exudation, genotypic Al resistance in common bean particularly depends on the capacity to sustain the synthesis of citrate for maintaining the cytosolic citrate pool that enables exudation.

Auxin increases phospholipase A(2) activity within 2min (Paul, R., Holk, A. and Scherer, G.F.E. (1998) Fatty acids and lysophospholipids as potential second messengers in auxin action. Rapid activation of phospholipase A(2) activity by auxin in suspension-cultured parsley and soybean cells. Plant J. 16, 601-611) and the phospholipase A inhibitors, ETYA and HELSS, inhibit elongation growth of etiolated Arabidopsis hypoctyls (Holk, A., Rietz, S., Zahn, M., Quader, H. and Scherer, G.F.E. (2002) Molecular identification of cytosolic, patatin-related phospholipases A from Arabidopsis with potential functions in plant signal transduction. Plant Physiol. 130, 90-101). To identify the mode of action, rapid auxin-regulated gene expression was tested for sensitivity to these PLA(2) inhibitors using seedlings expressing beta-glucuronidase (GUS) under the control of the synthetic auxin-responsive promoter DR5. ETYA and HELSS inhibited the auxin-induced increases in GUS activity, the steady-state level of the corresponding GUS mRNA and the mRNAs encoded by four other auxin-induced genes, IAA1, IAA5, IAA19 and ARF19. Factors that bind to the auxin response elements of the DR5 promoter and thereby regulate gene expression are regulated by a set of proteins such as Aux/IAA1 whose abundances are, in part, under control of E3 ubiquitin ligase SCF complexes. To investigate this mechanism further, the effect of ETYA on Aux/IAA1 degradation rate was examined using seedlings expressing Aux/IAA1:luciferase fusion proteins. In the presence of cycloheximide and excluding synthesis of IAA1:luciferase, ETYA had no apparent effect on degradation rates of IAA1, either with or without exogenous auxin. Therefore, the E3 ubiquitin ligase SCF(TIR1) complex is an unlikely direct target of the PLA inhibitor. When cycloheximide was omitted, however, the inhibitors ETYA and HELSS blocked a sustained auxin-induced decrease in its steady-state level, indicating an unknown target capable to regulate Aux/IAA protein levels and, hence, transcription.

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