Regulation of auxin response by the protein kinase PINOID

187

Plant biology Paper alert A selection of interesting papers that were published in the two months before our press date in major journals most likely to report significant results in plant biology. Current

167 166

Opinion

in Plant Biology

2000, 39 67-l 75

Growth and development (Schneitz and Berleth) Genome studies and molecular genetics (Lemieux Grossniklaus) Plant biotechnology (Dunwell) Physiology and metabolism (Hill and Sweetlove) Plant-microbe interactions (Metraux) Cell signalling (McAinsh and Palme) Cell biology (Berger)

170 170 171 172 174 .

..

Growth

and

of special interest of outstanding interest

and development

Selected by Kay Schneih Institute of Plant Biology, University

of Zurich, Zurich, Switzerland

Delayed activation of the paternal genome during seed development Vielle-Calzada J-P, Basker R, Grossniklaus U: Nature 2000,404:91-94. w Significance: This paper provides excellent evidence that, in contrast to previous assumptions, the maternal genome mainly Arabidopsis. Perhaps most, if not all, of the paternal genome is initially transcriptionally inactive and becomes active only after seed development has progressed for about three to four days following fertilization. Findings: The authors analysed the time-course of parental chromosome-specific expression of several genes during the early embryo development of seeds from reciprocal crosses. They made use of 19 different enhancer-detector or gene-trap lines harboring GUS-reporter constructs that exhibit GUS activity during early embryogenesis and/or endosperm development. For up to three to four days, expression was only detected if the reporter transgenes were of maternal origin. The differential expression was confirmed for the endogenous PROLlFER.4 gene by a RT-PCRlsingle nucleotide polymorphism (SNP) detection approach. Twelve of the trapped genes were cloned and sequence analysis suggests that they function in a variety of processes. In addition, they are scattered throughout the genome. Interestingly, mutations in EMB30IGNOM result in a previously unnoticed maternaleffect phenotype that can be rescued paternally. The male determinant of self-incompatibility in Brassica. Schopfer CR, Nasrallah ME, Nasrallah J: Science 1999, 288:1697-l 700. l * Significance: The male and female aspects of the sporophytic self-incompatibility (SI) response in Bras&a are regulated by the complex and polymorphic S locus. The S locus harbors a number of genes and one of them has now been identified as that encoding the long sought-after male component of the SI response.

Findings: Sequencing of the S locus revealed a novel gene termed S locus cysteine-rich protein gene (SC/?). The SCR gene is highly polymorphic between different S haplotypes and was shown to be specifically expressed in the anthers. Mutant analysis and transgenic experiments involving haplotype-specific lack- or gain-of-SC/?expression demonstrated that SCR is necessary and sufficient to eonfer pollen SI specificity. The different putative SCR proteins are likely to be secreted from developing a unicrospores, have a basic isoelectric point and to be of small size (74-77 aminoacids or about 8.4-8.6 kD). They constitute possible ligands for the S-locus receptor protein kinase (SRK), which acts as the female component of the SI response. The SCR proteins resemble a separate class of previously identified small, basic and cysteine-rich pollen coat proteins (PCPs). The S receptor kinase determines self-incompatibility in Brassica stigma. Takasaki T, Hatakeyama K, Suzuki G, Watanabe M, lsogai A, Hinata K: Nature 2000,403:913-916. AND A breakdown of Brassica self-incompatibility in ARC1 antisense transgenic plants. Stone SL, Arnold0 M, Goring DR: Science 1999, 286:1729-l 731. l * Significance: In addition to the SC/? gene, the S locus includes two genes that encode the S locus receptor kinase (SRK) and the S locus glycoprotein (SLG), respectively. Both SRK and SLG have been implicated in conferring female specificity in the SI response. The first paper shows that SRK is responsible for haplotype specificity and that SLG enhances the ability of SRK to reject self pollen. Work described in the second paper identifies an arm-repeat factor ARCl, which binds to the SRK kinase domain, as a positive downstream component of the SRK signaling pathway that is involved in the SI response. Findings: In the first paper, the authors directly test the effect of SRK and SLG of a given S haplotype on the specificity of pollen rejection. They circumvented co-suppression problems, which arise when using SRK or SLG transgenes that are too closely related to the endogenous SRK and SLG genes of the transformed haplotype, by choosing material from distantly related haplotypes. Plants of the Sso haplotype transformed with an SRKgene of the S2s haplotype (SRK28) reject only Sss pollen (or Sso pollen) but not pollen from other haplotypes. In a related experiment, the SLG2s gene failed to confer an Sss-specific SI response. Transformants carrying SRK2s and SLG2s showed a more complete @%pecific response. In the second paper, self-incompatible Srassica napus plants were transformed with an antisense ARC7 construct. The transgenic plants had fewer ARC7 transcripts coupled with a partial release of the SI response, which was measured by an elevated number of pollen adhering to the papillar cells and by an increase in seed set. Selected by Thomas Berleth University of Toronto, Toronto, Ontario, Regulation

of

auxin

PINOID. Christensen 2000,

100:469-476.

response S, Dagenais

Canada by the N, Chary

protein J, Weigel

kinase D: Cell

188

Paper alert

* Significance: In Arabidopsis, flowerless, pin-shaped inflorescence axes are often associated with defects in auxin transport and/or perception. Mutations in the P/NO/D (P/D) gene result in this type of inflorescence defect, but the involvement of the P/D gene in auxin-related processes has remained controversial. The cloning of the P/D gene enables tests in transgenic plants, which suggest that PID acts as a negative regulator of auxin signaling. Findings: The P/D gene encodes a serine-threonine protein kinase. P/D mRNA is expressed in a dynamic pattern, predominantly during the embryonic and reproductive stages. The strongest expression domains for this gene are at the flanks of lateral organ outgrowths in the embryo and inflorescence, and match the sites of mutant defects in the formation and positioning of cotyledons and flowers. Marker gene expression patterns suggest that pid mutant inflorescence meristems are primarily impaired in the elaboration rather than in the establishment of lateral primordia. Ubiquitous overexpression of P/D antagonizes auxin responses in major plant organs, suggesting that wild-type PID could act as a local negative regulator of auxin signaling. l

RPTS: A signal transducer of the phototropic response in Arabidopsis. Sakai T, Wada T, lshiguro S, Okada, K: Plant Cell 2000,12:225-236. l * Significance: Genes in short-wavelength phototropic responses have mainly been identified through mutant screens for non-phototropic hypocotyl(s). The products of four genes, NPH (nonphototropic hypocotyl) 7-4 seem to act in a signal transduction chain, with the NPHl product functioning as the UV-A/blue-light receptor. Mutant screens for abnormal root phototropism have identified another gene, RPTZ, which may define a sub-pathway in UV-A/blue-light signal transduction. Findings: The newly identified RPT2 (ROOT PHOTOTROPISM-2) gene is light inducible and encodes a protein with similarites to the NPH3 product. Although NPH3 seems to be essential for all types of UV-A/blue-light responses, RPT2 seems to be most important under high fluence rate. Conversely, the presumed sole UV-A/blue-light receptor, NPHl , seems to act preferentially at low fluence rate, suggesting the existence of a second, as yet unidentified, photoreceptor. These findings modify our understanding of blue-light signal transduction and point towards a model in which two-input and output branches are linked through the common signal transduction element NPH3.

Genome Selected ‘University *Friedrich

studies

and molecular

genetics

by Bertrand Lemieux’ and Ueli Grossniklaust of Delaware, Newark, Delaware, USA Miescher Institute, BaseE, Switzerland

DNA microsatellite analysis using ion-pair reversed-phase high-performance liquid chromatography. Devaney JM, Girard JE, Marino MA: Anal Cbem 2000, 72:858-864. l Signfficance: Simple sequence repeat (SSR) markers are commonly used by virtually all plant geneticists. Ion-pair reversed-phase high-perfor.mance liquid chromatography (IP-RP HPLC) offers an attractive substitute for gel electrophoresis for SSR typing because it provides reduced analysis time, easily automated chromatographic separations and low operation cost (typically $0.1 B/run). Findings: The authors report on the use of IP-RP HPLC to genetically type a simple sequence repeat (SSR) marker;

HUMTHOl (a human simple sequence repeat polymorphism marker). The IP-RP HPLC conditions (i.e. column temperature, flow rate and percent organic modifier per minute) were optimized for the separation of PCR products. Using the optimized separation conditions, the alleles of the HUMTHOl system were sized in their native state (i.e. double stranded) with the use of those internal markers. The typing results correlated well with those obtained by the accepted methods of SSR typing (i.e. gel electrophoresis). The analysis time for the HUMTHOl locus was less than 14 min, and the alleles could be peak-captured for further examination such as sequencing. Computational identification of k-regulatory elements associated with groups of functionally related genes in Saccharomycas carevisiae. Hughes JD, Estep PW, Tavazoie S, Church GM: J Mol Biol2000,296:1205-1214. l Significance: The program AlignACE finds motifs that often correspond to the DNA-binding preferences of transcription factors. This program can be used to identify putative cis-regulatory elements using whole genome mRNA expression data. Findings: The authors report on the use of AlignACE, a Gibb’s sampling algorithm, to identify motifs that are overrepresented in a set of DNA sequences and to search upstream of apparently coregulated genes. They present a detailed study of the effectiveness of this program in identifying putative regulatory motifs of genes in the Saccharomyces cerevisiae genome. Published functional catalogs of genes and sets of genes grouped by common name provided 248 groups, resulting in 331 1 regulatory motifs. In conjunction with this analysis, the authors measured the tendency of a motif to target a given set of genes within the genome and gauged the extent to which a motif is preferentially located a specific distance upstream of translational start sites. Many previously identified cis-regulatory elements were found as well as novel motifs; one of these was verified in the laboratory. TOGA: An automated parsing technology for analyzing expression of nearly all genes. Sutcliffe JG, Foye PE, Erlander MG, Hilbush ES, Bodzin U, Durham JT, Hasel KW: Proc Nat/ Acad Sci USA 2000, 97:1976-l 981. l Significance: This technique can detect gene expression differences in pattern, location or level (depending on the sample) as banding-pattern differences on DNA sequencing gels even if the DNA sequences of the genes is unknown. Findings: Total gene expression analysis (TOGA) is a systematic cDNA display method that utilizes first, 8-nucleotide (nt) sequences comprised of a 4-nt restriction endonuclease cleavage site and an adjacent 4-nt parsing sequence that allows longer fragments to be assigned to a given band, and second, the distances of these sites from the 3’ ends of mRNA molecules, to identify transcripts. The parsing sequences were used as parts of primer-binding sites in 256 PCR-based assays that were performed in parallel by a robotic liquid handling station. The products of these reactions were separated by electrophoresis. Software that operates via a Netscape browserrbased graphical user interface allows the user to compa;e the status of each mRNA (whether expressed or not) and to match each of the binding patterns with the sequences of known mRNAs compiled in databases.