spring equinox (Fig. 2a; 08 and 28 S), when the advances in sunset time are slightly smaller (7 and 7.6 min versus 5.9 and 5.4 min). Implicitly, cumulative changes in sunset time of 5–7 min over 20 days are sufficient to induce flowering at the Equator. In view of the small maximum changes in sunset time near the Equator (5–7 min over 20 days) it seems likely that signals suffi- ciently large to trigger flowering or vegetative bud break occur only during periods of maximum change (Fig. 1 d, f ), that is, around the equinoxes (Fig. 2, grey bars) or solstices (Fig. 2d). If so, the observed staggered flowering times (Fig. 2a) could be primarily attributable to differences in the duration of flower development induced by perception of the photoperiodic signal. Flowering during an induc- tive period might indicate rapid flower emergence from resting buds, as in several Miconia species (Fig. 2a; 2.5–3.88 S). In other species, trees may flower months after the inductive period because the flowering signal causes the vegetative shoot apex to change into a large, branched inflorescence supporting many flowers 4 (Fig. 2a; 48 N Montanoa). It remains to be explained why some species flower only during or shortly before one of the two induction periods and why many trees do not flower every year. This is the first study that both confirms synchronous flowering in rainforest tree species near the Equator and proposes a timing mechanism. Synchronous flowering at the same time each year has long been noticed in Amazonian trees such as Miconia (Fig. 2a; 2.58 S), but studies of the phenomenon focused on the evolutionary consequences of staggered synchronous fruiting, for example, for frugivorous birds 14 . Photoperiodic control of vegetative develop- ment and flowering in tropical trees evolved in response to different adaptive pressures 3,5,6 . In tropical rainforests with an equitable climate, it may have evolved in response to the need to synchronize flowering to achieve cross-pollination in spite of low population densities. A Received 10 September; accepted 8 December 2004; doi:10.1038/nature03259. 1. Condit, R. et al. Tropical tree diversity: Results from a worldwide network of large plots. Biol. Skr. Dan. Vid. Selsk. (Copenhagen) (in the press). 2. Thomas, B. & Vince-Prue, D. Photoperiodism in Plants (Academic, San Diego, 1997). 3. Rivera, G. et al. Increasing day length induces spring flushing of tropical dry forest trees in the absence of rain. Trees – Struct. Funct. 16, 445–456 (2002). 4. Rivera, G. & Borchert, R. Induction of flowering in tropical trees by a 30-min reduction in photoperiod: evidence from field observations and herbarium specimens. Tree Physiol. 21, 201–212 (2001). 5. Borchert, R. & Rivera, G. Photoperiodic control of seasonal development in tropical stem-succulent trees. Tree Physiol. 21, 213–221 (2001). 6. Borchert, R., Meyer, S. A., Felger, R. S. & Porter-Bolland, L. Environmental control of flowering periodicity in Costa Rican and Mexican tropical dry forests. Global Ecol. Biogeogr. 13, 409–425 (2004). 7. Borchert, R. Environmental control of tropical tree phenology khttp://www.biology.ku.edu/ tropical_tree_phenology/l (2004). 8. Calle, Z. Temporal variation in the reproductive phenology of Montanoa quadrangularis in the Andes of Colombia: declining photoperiod as a likely environmental trigger. Biotropica 34, 621–622 (2002). 9. National Maritime Museum. Timekeeping: the Equation of Time khttp://www.nmm.ac.uk/site/request/ setTemplate:singlecontent/contentTypeA/conWebDoc/contentId/351l. 10. Holttum, R. E. Periodic leaf-exchange and flowering of trees in Singapore (II). Gardens Bull. S.S 11, 119–175 (1940). 11. Imaizumi, T., Tran, H. G., Swartz, T. E., Briggs, W. R. & Kay, S. A. FKF1 is essential for photoperiodic- specific light signaling in Arabidopsis. Nature 426, 302–306 (2004). 12. Valverde, F. et al. Photoreceptor regulation of CONSTANS protein in photoperiodic flowering. Science 303, 1003–1006 (2004). 13. Dore, J. Response of rice to small differences in length of day. Nature 183, 413–414 (1959). 14. Snow, D. W. A possible selective factor in the evolution of fruiting seasons in tropical forest. Oikos 15, 274–281 (1965). 15. Gautier, L. Reproduction de la Strate Arbore ´e d’uneForeˆt Ombrophile d’Amazonie Pe´ruvienne M.S. thesis, Univ. Gene `ve (1985). 16. Gautier, L. & Spichiger, R. Ritmos de reproduccio ´ n en el estrato arbo ´ reo del Arboretum Jenaro Herrera (provincia de Requena, departamento de Loreto, Peru ´ ). Contribucio ´ n al estudio de la flora y de la vegetacio ´n de la Amazonı´a peruana. X. Candollea 41, 193–207 (1986). 17. Justiniano, M. J. & Fredericksen, T. S. Phenology of tree species in Bolivian dry forests. Biotropica 32, 276–286 (2000). Supplementary Information accompanies the paper on www.nature.com/nature. Competing interests statement The authors declare that they have no competing financial interests. Correspondence and requests for materials should be addressed to R.B. (borchert@ku.edu). .............................................................. Gene transfer to plants by diverse species of bacteria Wim Broothaerts*†, Heidi J. Mitchell†, Brian Weir†, Sarah Kaines*, Leon M. A. Smith, Wei Yang, Jorge E. Mayer*, Carolina Roa-Rodrı´guez* & Richard A. Jefferson CAMBIA (An Affiliated Research Centre of Charles Sturt University), G.P.O. Box 3200, Canberra, ACT 2601, Australia *Present addresses: European Commission, Joint Research Centre, Institute for Reference Materials and Measurement, Retieseweg 111, B-2440 Geel, Belgium (W.B.); Research School of Biological Sciences, Australian National University, ACT 2601, Australia (S.K.); Campus Technologies Freiburg, University of Freiburg, Stefan Meier Str. 8, Freiburg D-79104, Germany (J.E.M.); RegNet, Research School of Social Sciences, Australian National University, ACT 2601, Australia (C.R.-R.) † These authors contributed equally to this work ............................................................................................................................................................................. Agrobacterium is widely considered to be the only bacterial genus capable of transferring genes to plants. When suitably modified, Agrobacterium has become the most effective vector for gene transfer in plant biotechnology 1 . However, the complexity of the patent landscape 2 has created both real and perceived obstacles to the effective use of this technology for agricultural improvements by many public and private organizations worldwide. Here we show that several species of bacteria outside the Agrobacterium genus can be modified to mediate gene transfer to a number of diverse plants. These plant-associated symbiotic bacteria were made competent for gene transfer by acquisition of both a disarmed Ti plasmid and a suitable binary vector. This alterna- tive to Agrobacterium-mediated technology for crop improve- ment, in addition to affording a versatile ‘open source’ platform for plant biotechnology, may lead to new uses of natural bac- teria–plant interactions to achieve plant transformation. Agrobacterium tumefaciens is a ubiquitous soil bacterium that induces galls on plants. The discovery that this gall formation is due to integration into the plant genome of bacterial DNA (T-DNA) laid the foundations for plant biotechnology 3 . The T-DNA is part of the ,200 kb Ti (tumour-inducing) plasmid, which also encodes func- tions for Ti plasmid conjugation, opine metabolism and the initiation, transfer and processing of the T-DNA 4,5 . Before the discovery of the Ti plasmid, gall-inducing ability was shown to be transferable to non-virulent Agrobacteria and to Rhizobium leguminosarum 6 . Ti plasmid transfer to Rhizobium trifolii and Phyllobacterium myrsinacearum resulted in strains that caused galls on some plants 7,8 , but a Sinorhizobium meliloti strain contain- ing a Ti plasmid was not tumorigenic 9 . Although these experiments showed that close relatives of Agrobacterium could harbour the Ti plasmid, no direct molecular evidence of gene transfer to plants by these bacteria was reported, leaving open the possibility that gall formation may have resulted from hormonal perturbations in the host plant unrelated to DNA transfer 10 . Indeed, a disarmed Ti plasmid and binary vector were introduced into a bacterial isolate apparently related to Phyllobacterium spp. for the purpose of tobacco inoculation 11 , and although galls resulted from production of auxin by Phyllobacterium, these galls were morphologically different from those produced by an Agrobacterium-transformed plant through gene transfer; moreover, evidence of gene transfer was sought but not found. Accordingly, the scientific community has focused on Agrobacterium as a vehicle for gene transfer; the vast majority of patent claims regarding biological plant transformation explicitly refer to Agrobacterium 2 . A recent proposal suggesting that A. tumefaciens be reclassified as Rhizobium radiobacter has been widely disputed 12 , although Agrobacterium is clearly closely related to Rhizobium. However there is little doubt that Agrobacterium, Sinorhizobium and Mesorhizobium are in distinct phylogenetic clades and their genomic organization differs considerably 4,13 . letters to nature NATURE | VOL 433 | 10 FEBRUARY 2005 | www.nature.com/nature 629 © 2005 Nature Publishing Group