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Ethylene and the wound response
research news and bacteria? Intriguingly, Leu-x-Cys-x-Glu motifs are also present in someAgrobacterium proteins, but their significance remains untested. It should be emphasized that nearly 2000 entries in the current Swissprot database contain Leu-x-Cys-x-Glusequences, and these are unlikely all to be Rb-binding proteins.
Involvement in endoreduplication? Endoreduplication is important in various
plant developmental processes, involving S phases without an intervening mitosis. Endoreduplication initiates in maize endosperm 10-12 d after fertilization, and is accompanied by an increase in S phaseassociated ldnase activityi5 and the appearance ofRb (Ref. 2). This Rb can be phosphorylated in vitro, catalysed by kinase activity from endoreduplicating endosperm2. Although indicative of a role for Rb in maize endosperm endoreduplication, the interpretation of these experiments is complicated by the low specificity of the antibody used. It is interesting to note that maize Rb is not detected before endoreduplication begins, but the significance of this remains to be established. It is also unclear whether phosphorylation converts maize Rb from an active to an inactive state.
Evolution of G1-S controls in mulUcellular eukaryotes It is now apparent that the control of the G1-S phase of the cell cycle is far more similar in vertebrates and plants than was suspected only a year ago. In mammalian cells, Rb proteins play important roles in integrating positive and negative signals from proliferation and differentiation signals. It is attractive to draw direct parallels between the functions of Rb proteins in mammalian cells
and their possible roles in plants. What is still unclear is the extent to which this is likely to be true. The reiterative development of plants, characterized by the continuous differentiation of cells as they are produced by the meristem, would imply important roles for proteins that are at the interface between cell proliferation and differentiation. The reversibility under appropriate conditions of much of plant differentiation might also suggest the involvement of Rb proteins. Is there only a single plant Rb that is much smaller in size than its animal homologues? My bet is that we have only scratched at the surface of this story, and that plant pocket proteins will provide a rich seam for understanding the interplay of development, cell proliferation and pathogenrelated processes.
James A.H. Murray Instituteof Biotechnology, University of Cambridge, Tennis Court Road, Cambridge, UK CB2 1QT (tel +44 1223 334166; fax +44 1223 334162; e-mail j.murray @biotech.cam.ac.uk)
References 1 Yale,Q. et al. (1996) Plant-cells contain a novel
member of the retinoblastoma family of growth-regulatery proteins, EMBO J. 15, 4900-4908 2 Graft, G. et al. (1996) A maize cDNAencoding a member of the retinoblastoma protein family - involvementin endoreduplication, Proc. Natl. Acad. Sci. U. S. A. 93, 8962-8967 3 Du, W. et al. (1996) RBF, a novel RB-related gene that regulates E2F activity and interacts with cyclinE in Drosophila, Genes Dev. 10, 1206-1218 4 Weinberg, R.A. (1995) The retinoblastoma protein and cell cyclecontrol, Cell 81, 323-330
literature
5 Nevins, J.R. (1992) E2F: a link between the Rb tumour suppressor protein and viral oncoproteins,Science 258, 424-429 6 Weintraub, S.J. et al. (1995) Mechanism of active transcriptional repression by the retinoblastoma protein, Nature 375, 812~15 7 Johnson, D.G. et al. (1993) Expression of transcription factor E2F1 induces quiescent cells to enter S phase, Nature 365, 349-352 8 Pines, J. (1995) Cyclinsand cyclin-dependent kinases: theme and variations, Adv. Cancer Res. 66, 181-212 9 Jacks, T. etal. (1992) Effects of an Rb mutation in the mouse, Nature 359, 295-300 10 Soni, R. et al. (1995) A family of cyclinD homologsfrom plants differentially controlled by growth regulators and containing the conservedretinoblastema protein interaction motif,Plant Cell 7, 85-103 11 Xie, Q., Su~rez-L6pez,P. and Guti6rrez, C. (1995) Identification and analysis of a retinoblastoma binding motif in the replication protein of a plant virus: requirement for efficientviral DNA replication, EMBO J. 14, 4073-4082 12 Nagar, S. et al. (1995) A geminivirus induces expression of a host DNA synthesis protein in terminally differentiated plant cells, Plant Cell 7, 705-719 13 Shen, B. et al. (1994) Partial sequencing and mapping of clonesfrom two maize cDNA libraries, Plant Mol. Biol. 26, 1085-1101 14 Wang, J.Y.J., Knudsen, E.S. and Welch, P.J. (1994) The retinoblastema tumor suppressor protein, Adv. Cancer Res. 64, 25~5 15 Graft, G. and Lm'kins,B.A. (1995) Endoreduplicationin maize endosperm: involvement of M phase-promoting factor inhibition and induction of S phase-related kinases, Science 269, 1262-1264
focus
Ethylene and the wound response O'Donnell, P.J., Calvert, C., Atzorn, R., Wasternack, C., Leyser, H.M.O. and Bowles, D.J. (1996) Ethylene as a signal mediating the wound response of tomato plants, Science 274, 1914-1917 When tomato plants are wounded, one of a multitude of responses is the upregulation of pin genes, which encode proteinase inhibitor proteins - these have been shown to inhibit insect feeding. However, there is a gulf in our understanding of the processes involved in the pin gene activation pathway. Elucidating this model system will shed light on a variety of other signalling pathways in plants. Positive regulators of pin expression include systemin, jasmonic acid and oligogalacturonide fragments of pectin polysaccharides, but previous analysis indicated that ethylene was not involved. However, in the present study, preliminary results showed that systemin, jasmonic acid and the oligogalacturonide fragments induce ethylene when applied to intact tomato plants. 841-
March1997,Vol.2, No.3
Furthermore, aspirin (acetylsalicylic acid), which is a negative regulator of pin expression, was shown to prevent the effects of wounding and the three positive elicitors on bothpin expression and ethylene production. Following on from these results, O'Dounell et al. were able to perturb ethylene perception and biosynthesis, and so demonstrate beyond doubt that ethylene is required forpin expression. For example, the ethylene-receptor blocker silver thiosulphate, and a competitive inhibitor of ethylene, norbornadiene, were found to inhibit the induction of pins. Moreover, plants transformed using the antisense sequence of the gene encoding 1aminocyclopropane-l-carboxylase oxidase an enzyme involved in ethylene biosynthesis - did not express pin genes when wounded.
Previous studies had already established that jasmonic acid is required for pin gene expression, and that it acts downstream of the other elicitors - so where does ethylene interact? A crucial result came from the analysis of aspirin-treated plants, because it was found that only when both jasmonic acid and ethylene were present was pin expression restored. Such a co-requirement also explained an earlier result, that gassing plants with ethylene alone had no effect onpin expression. But the interaction ofjasmonic acid and ethylene is yet more complex, as further experiments suggest that ethylene also influencesjasmonic acid levels in wounded plants. It is important that this interactionis fully tmderstoed, because wounding will almost certainly not be the only response in which ethylene and jasmouic acid interact.
© 1997 Elsevier Science Ltd
Involvement in endoreduplication? Endoreduplication is important in various
plant developmental processes, involving S phases without an intervening mitosis. Endoreduplication initiates in maize endosperm 10-12 d after fertilization, and is accompanied by an increase in S phaseassociated ldnase activityi5 and the appearance ofRb (Ref. 2). This Rb can be phosphorylated in vitro, catalysed by kinase activity from endoreduplicating endosperm2. Although indicative of a role for Rb in maize endosperm endoreduplication, the interpretation of these experiments is complicated by the low specificity of the antibody used. It is interesting to note that maize Rb is not detected before endoreduplication begins, but the significance of this remains to be established. It is also unclear whether phosphorylation converts maize Rb from an active to an inactive state.
Evolution of G1-S controls in mulUcellular eukaryotes It is now apparent that the control of the G1-S phase of the cell cycle is far more similar in vertebrates and plants than was suspected only a year ago. In mammalian cells, Rb proteins play important roles in integrating positive and negative signals from proliferation and differentiation signals. It is attractive to draw direct parallels between the functions of Rb proteins in mammalian cells
and their possible roles in plants. What is still unclear is the extent to which this is likely to be true. The reiterative development of plants, characterized by the continuous differentiation of cells as they are produced by the meristem, would imply important roles for proteins that are at the interface between cell proliferation and differentiation. The reversibility under appropriate conditions of much of plant differentiation might also suggest the involvement of Rb proteins. Is there only a single plant Rb that is much smaller in size than its animal homologues? My bet is that we have only scratched at the surface of this story, and that plant pocket proteins will provide a rich seam for understanding the interplay of development, cell proliferation and pathogenrelated processes.
James A.H. Murray Instituteof Biotechnology, University of Cambridge, Tennis Court Road, Cambridge, UK CB2 1QT (tel +44 1223 334166; fax +44 1223 334162; e-mail j.murray @biotech.cam.ac.uk)
References 1 Yale,Q. et al. (1996) Plant-cells contain a novel
member of the retinoblastoma family of growth-regulatery proteins, EMBO J. 15, 4900-4908 2 Graft, G. et al. (1996) A maize cDNAencoding a member of the retinoblastoma protein family - involvementin endoreduplication, Proc. Natl. Acad. Sci. U. S. A. 93, 8962-8967 3 Du, W. et al. (1996) RBF, a novel RB-related gene that regulates E2F activity and interacts with cyclinE in Drosophila, Genes Dev. 10, 1206-1218 4 Weinberg, R.A. (1995) The retinoblastoma protein and cell cyclecontrol, Cell 81, 323-330
literature
5 Nevins, J.R. (1992) E2F: a link between the Rb tumour suppressor protein and viral oncoproteins,Science 258, 424-429 6 Weintraub, S.J. et al. (1995) Mechanism of active transcriptional repression by the retinoblastoma protein, Nature 375, 812~15 7 Johnson, D.G. et al. (1993) Expression of transcription factor E2F1 induces quiescent cells to enter S phase, Nature 365, 349-352 8 Pines, J. (1995) Cyclinsand cyclin-dependent kinases: theme and variations, Adv. Cancer Res. 66, 181-212 9 Jacks, T. etal. (1992) Effects of an Rb mutation in the mouse, Nature 359, 295-300 10 Soni, R. et al. (1995) A family of cyclinD homologsfrom plants differentially controlled by growth regulators and containing the conservedretinoblastema protein interaction motif,Plant Cell 7, 85-103 11 Xie, Q., Su~rez-L6pez,P. and Guti6rrez, C. (1995) Identification and analysis of a retinoblastoma binding motif in the replication protein of a plant virus: requirement for efficientviral DNA replication, EMBO J. 14, 4073-4082 12 Nagar, S. et al. (1995) A geminivirus induces expression of a host DNA synthesis protein in terminally differentiated plant cells, Plant Cell 7, 705-719 13 Shen, B. et al. (1994) Partial sequencing and mapping of clonesfrom two maize cDNA libraries, Plant Mol. Biol. 26, 1085-1101 14 Wang, J.Y.J., Knudsen, E.S. and Welch, P.J. (1994) The retinoblastema tumor suppressor protein, Adv. Cancer Res. 64, 25~5 15 Graft, G. and Lm'kins,B.A. (1995) Endoreduplicationin maize endosperm: involvement of M phase-promoting factor inhibition and induction of S phase-related kinases, Science 269, 1262-1264
focus
Ethylene and the wound response O'Donnell, P.J., Calvert, C., Atzorn, R., Wasternack, C., Leyser, H.M.O. and Bowles, D.J. (1996) Ethylene as a signal mediating the wound response of tomato plants, Science 274, 1914-1917 When tomato plants are wounded, one of a multitude of responses is the upregulation of pin genes, which encode proteinase inhibitor proteins - these have been shown to inhibit insect feeding. However, there is a gulf in our understanding of the processes involved in the pin gene activation pathway. Elucidating this model system will shed light on a variety of other signalling pathways in plants. Positive regulators of pin expression include systemin, jasmonic acid and oligogalacturonide fragments of pectin polysaccharides, but previous analysis indicated that ethylene was not involved. However, in the present study, preliminary results showed that systemin, jasmonic acid and the oligogalacturonide fragments induce ethylene when applied to intact tomato plants. 841-
March1997,Vol.2, No.3
Furthermore, aspirin (acetylsalicylic acid), which is a negative regulator of pin expression, was shown to prevent the effects of wounding and the three positive elicitors on bothpin expression and ethylene production. Following on from these results, O'Dounell et al. were able to perturb ethylene perception and biosynthesis, and so demonstrate beyond doubt that ethylene is required forpin expression. For example, the ethylene-receptor blocker silver thiosulphate, and a competitive inhibitor of ethylene, norbornadiene, were found to inhibit the induction of pins. Moreover, plants transformed using the antisense sequence of the gene encoding 1aminocyclopropane-l-carboxylase oxidase an enzyme involved in ethylene biosynthesis - did not express pin genes when wounded.
Previous studies had already established that jasmonic acid is required for pin gene expression, and that it acts downstream of the other elicitors - so where does ethylene interact? A crucial result came from the analysis of aspirin-treated plants, because it was found that only when both jasmonic acid and ethylene were present was pin expression restored. Such a co-requirement also explained an earlier result, that gassing plants with ethylene alone had no effect onpin expression. But the interaction ofjasmonic acid and ethylene is yet more complex, as further experiments suggest that ethylene also influencesjasmonic acid levels in wounded plants. It is important that this interactionis fully tmderstoed, because wounding will almost certainly not be the only response in which ethylene and jasmouic acid interact.
© 1997 Elsevier Science Ltd