1 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 Opinion in Plant Biology 2000, 3:1–9 Contents (chosen by) 1 Growth and development (Scheres and Höfte) 2 Genome studies and molecular genetics (Lemieux and Grossniklaus) 4 Plant biotechnology (Dunwell) 4 Physiology and metabolism (Hill and Sweetlove) 5 Plant–microbe interactions (Metraux) 6 Cell signalling (McAinsh and Palme) 8 Gene regulation (Weisshaar) 8 Cell biology (Berger) • of special interest •• of outstanding interest Growth and development Selected by Ben Scheres* and Herman Höfte † *Utrecht University, Utrecht, The Netherlands † INRA, Versailles, France Co-ordinated polar localisation of auxin efflux carrier PIN1 by GNOM ARF GEF. Steinmann T, Geldner N, Grebe M, Mangold S, Jackson CL, Paris S, Gälweiler L, Palme K, Jürgens G: Science 1999, 286:316-318. •• Significance: Arabidopsis gnom mutants display defects in the establishment of a polar axis. Biochemical evidence in this paper implicates the GNOM protein in membrane trafficking. The failure of gnom mutants to localize the PIN FORMED 1 (PIN1) transmembrane protein, a component of an auxin efflux carrier, in a consistent polar fashion may explain how gnom polarity defects result from altered auxin distribution. Findings: The polar localization of PIN1 was coordinated between cells in wild-type but not gnom mutant embryos. The GNOM protein displayed brefeldin A (BFA)-sensitive guano- sine-exchange activity on mammalian ADP ribosylation factor G protein (ARF), consistent with a role in protein trafficking, and GNOM co-purified with membrane fractions in a BFA-depen- dent fashion. BFA-disrupted polar PIN1 protein localization in emerging lateral roots, suggesting that ARF guanosine- nucleotide exchange factors (GEFs), like GNOM, are continuously required for the polar localization of auxin trans- porters. An epigenetic mutation responsible for natural variation in floral symmetry. Cubas P, Vincent C, Coen E: Nature 1999, 401:157-161. AND Control of organ asymmetry in flowers of Antirrhinum. Luo D, Carpenter R, Copsey C, Vincent C, Clark J, Coen E: Cell 1999, 99:367-376. • Significance: The snapdragon cycloidea (cyc) gene is involved in the establishment of floral asymmetry. In the first paper, it is shown that a naturally occurring radial-symmetric flo- ral mutant of toadflax (Linaria vulgaris) is caused by the transcriptional silencing of a cyc homologue. Thus, epigenetic mutations may play a significant role in natural morphological variation. In the second paper, it is shown that the snapdragon cyc gene cooperates with the homologous dichotoma (dich) gene to establish dorsal fates, and that the dich gene specifies a dorsal sub-domain. Findings: Wild-type flowers of toadflax display adaxial–abaxial (dorsoventral) asymmetry. Natural mutants, which are radially symmetrical, show methylation-dependent RFLPs within a cyc homologue, Lcyc. Lcyc is expressed in adaxial regions of wild- type flowers but is absent from mutant flowers. Nevertheless, sequencing revealed no nucleotide differences between the wild-type and the mutant. The snapdragon dich gene encodes a protein with 77% homology to cyc. It is expressed, like cyc, in the dorsal domain, but becomes restricted to the most dorsal part of this domain. Ectopic expression of cyc dorsalizes lateral and ventral petals in the backpetals mutant, but these petals do not acquire the dich-associated dorsal-most characteristics. These findings support the idea that cyc and dich specify dis- tinct dorsal subdomains. Distinct mechanisms promote polarity establishment in carpels of Arabidopsis. Eshed Y, Baum SF, Bowman JL: Cell 1999, 99:199-209. •• Significance: At least three Arabidopsis YABBY genes are involved in the specification of abaxial cell fate in above-ground organs. This function was not, however, evident for the found- ing member of this family of transcription factor-encoding genes, CRABS CLAW (CRC), which is required for proper carpel development. In this paper, a role for CRC in adaxial–abaxial polarity is revealed through the analysis of a genetic enhancer, the GYMNOS (GYM) gene. Findings: Arabidopsis gym ; crc mutants display ectopic ovule formation and other phenotypes that are indicative of adaxial- ization of carpels. gym mutations alone delay maturation of the carpels and other tissues, and so CRC and GYM are redun- dantly required for abaxialization. The additional observation that CRC over-expression promotes abaxialization is consistent with these findings. GYM encodes a member of the SWI2/SNF1 chromatin-remodeling factor family and is expressed in all dividing cells. GYM seems to prevent carpel adaxialization in a crc background by repression of AINTEGU- MENTA, which is thought to be required for primordium outgrowth and thus required for the formation of ectopic adax- ial ovules in crc mutants. A plant regulator controlling development of symbiotic root nodules. Schauser L, Roussis A, Stiller J, Stougaard J: Nature 1999, 402:191-195. •• Significance: This is the first example of the cloning of a central regulator of symbiotic nitrogen-fixing root nodules. This paper also demonstrates the feasibility of the use of transposon mutagenesis to clone genes from Lotus japonicus. Findings: Nodule-less (nin, or nodule inception) mutants were identified in an Ac transposon-mutagenised Lotus population. Plant biology Paper alert