- Email: [email protected]
A third dimension in the control of plant development
49
T I B S - February 1984
a culture passage, possibly due to a wound response I~L To conclude, if it is accepted that the concentration of hormone binding sites is a reasonable criterion for assessing tissue sensitivity, the sparse literature available indicates that such sensitivity may be influenced by certain environmental factors and also by the actual hormone concentration itself; further research along these lines may well prove very informative.
References 1 Trewavas, A. J. and Cleland, R. E. (1983) Trends Biochem. Sci. 8, 354-357 2 Sinnott, E. W. (1946) Amer. Nat. 80, 497-505
3 Cuatrecasas, P. and Greaves, M. F. Receptors and Recognition (all volumes), Chapman and Hall 4 Cuatrecasas, P., Hollenberg, M. D., Chang, K.-J. and Bennett, V. (1977) Recent Adv. Cell Res. 31, 37-62 5 Dodds, J. H. and Hall, M. A. (1980) Sci. Prog. (Oxf.) 66, 513--535 6 Ray, P. M. (1977) Plant Physiol. 59, 594-599 7 Rayle, D. L. and Cleland, R. E. (1970) Plant Physiol. 46, 250-253 8 Marre, E. (1979) Annu. Rev. Plant Physiol. 30, 273-288 9 Dohrmann, U., Hertel, R. and Kowalik, H. (1978) Planta 140, 97-104 10 Murphy, G. J. P. (1980) Plant Sci. Lett. 19, 157-168 11 Dodds, J. H., Bengochea, T. and Starting, R. J. (1982) Proc. 5th Int. Con. Plant Tissue and Ceil Cult. (Fujiwara, A., ed.), pp. 5%58,
A third dimension in the control of plant development SIR: The title of your October 1983 Discussion Forum, 'Is plant development regulated by changes in the concentration of growth substances or by changes in sensitivity to growth substances?' seems to rule out any consideration of the possibility that some developmental responses might be dependent on neither changes in hormone concentration n o r changes in hormone sensitivity of the tissue. This type of hormone-independent response is clearly recognized by Trewavas and there is growing evidence of the importance of such phenomena, particularly in cases where the plant responds rapidly (< 300 s) to environmental stimuli. We would also consider shoot tropistic responses to be included in this category. The results of our recent studies ~.2.3 and those of others 4"5 on the rapid growth rate changes bringing about tropistic curvatures in shoots, suggest that these changes in growth rate are not caused by changes in the concentration of hormones. We would not accept Cleland's view that changes in the concentration of auxin can explain such growth rate changes - in our opinion the changes found are too small, they have not been shown to occur rapidly enough and the hormone changes do not correlate well with the complex growth patterns causing curvature. However, poor correlations have not in the past disturbed plant hormonologists as there is always the excuse, noted by Cleland, that changes in some particular fraction of the total hormone might correlate well. Unfortunately, Trewavas may have offered yet another straw for plant hormonologists to clutch at, as they can now evoke ~mewhat nebulous sensitivity arguments after analyses of even the smallest hormone compartment have
failed to reveal appropriate changes. After 50 years of analysing plant hormones, admittedly usually imperfectly, it is now accepted that changes in plant hormone levels do not provide an adequate explanation of many developmental changes. By all means let us consider whether hormone sensitivity might not provide us with a better explanation of some responses but let us also reconsider whether hormonal control need be the only way in which plant cells regulate basic processes.
References 1 Firn, R. D. and Digby, J. (1980) Annu. Rev.
Jap. Ass. Plant Tissue Culture, Tokyo 12 Graham, C. F. and Wareing, P. F. (1976) The Developmental Biology of Plants and Animals, Blackwell Scientific Publishers 13 Lefkowitz, R. J., ed. (1982) Receptors and Recognition, Series B, Vol. 13, Chapman and Hall 14 Trewavas, A. J. (1980) Phytochemistry 19, 1303-1308 15 Bhattacharyya, K. and Biswas, B. B. (1982) Phytochemistry 21, 1207-1211 16 Ostroom, H., Kulescha, Z., van Vliet, Th. B. and Libbenga, K. R. (1980) Planta 149, 44-47 ROBERT J. STARLING
Department of Plant Biology, University of Birmingham, PO Box 363, Birmingham B15 2T-F, UK.
Plant Physiol. 31, 131-148 2 Franssen, J. M., Cooke, S. A., Digby, J. and Fire, R . D . (t981) Z. Pflanzenphysiol. 103, 207-216 3 Digby, J., Firn, R. D. and Carrington, S. C. M. (1982) in Plant Growth Substances 1982' (Wareing, P. F., ed.), pp. 519-528, Academic Press 4 Hart, J. W., Gordon, D. C. and MacDonald, I. R. (1982) Plant Cell Envir. 5, 361-366 5 MacDonald, I. R., Hart, J. W. and Gordon, D. C. (1983) Plant Cell and Envir. 6, 401-406 RICHARD FIRN a n d JOI~IN DIGBY
Department of Biology, University of York, York YO1 5DD, UK.
Antizymes: inhibitor proteins or regulatory subunits? SIR: It has been recognized for over half a century that an important factor influencing the activity of many enzymes in vivo is their association with other proteins. As early as 1930, Haldane ~ described 'antienzymes' as a class of proteins encompassing both immunoglobulins and other types of inhibitory proteins; since that time, the number and types of such known interactions have grown impressively. Recently, the discovery of different regulatory proteins that inhibit a specific target enzyme has been accompanied by changing terminology that accentuates the uniqueness of each regulatory protein, while obscuring its general membership in the class of regulatory subunits. It has become apparent that the use of regulatory subunits to govern the function of structural proteins, transport proteins, and enzymes is widespread. From the extant characterized examples, some generalizations can be made:
(1) The association of a protein with its regulatory subunit affects the affinity and/or specificity of ligand binding either by alteration in tertiary structure or by steric impedance resulting from the aggregation itself. (2) In many cases, aggregation of a protein with its regulatory subunit is mediated by a small molecule effector(s) since most regulatory subunits are themselves controlled either by chemical modification or by small metabolites. A considerable spectrum is apparent in the types of regulatory, subunits, examples of which are given in Table I. The list of proteins is in no way comprehensive, but was chosen to illustrate diversity. At one extreme is aspartate carbamoyltransferase, where the regulatory subunit could be viewed as a genetically independently produced allosteric site. At the other extreme is the complex of two enzymes, ornithine
T I B S - February 1984
a culture passage, possibly due to a wound response I~L To conclude, if it is accepted that the concentration of hormone binding sites is a reasonable criterion for assessing tissue sensitivity, the sparse literature available indicates that such sensitivity may be influenced by certain environmental factors and also by the actual hormone concentration itself; further research along these lines may well prove very informative.
References 1 Trewavas, A. J. and Cleland, R. E. (1983) Trends Biochem. Sci. 8, 354-357 2 Sinnott, E. W. (1946) Amer. Nat. 80, 497-505
3 Cuatrecasas, P. and Greaves, M. F. Receptors and Recognition (all volumes), Chapman and Hall 4 Cuatrecasas, P., Hollenberg, M. D., Chang, K.-J. and Bennett, V. (1977) Recent Adv. Cell Res. 31, 37-62 5 Dodds, J. H. and Hall, M. A. (1980) Sci. Prog. (Oxf.) 66, 513--535 6 Ray, P. M. (1977) Plant Physiol. 59, 594-599 7 Rayle, D. L. and Cleland, R. E. (1970) Plant Physiol. 46, 250-253 8 Marre, E. (1979) Annu. Rev. Plant Physiol. 30, 273-288 9 Dohrmann, U., Hertel, R. and Kowalik, H. (1978) Planta 140, 97-104 10 Murphy, G. J. P. (1980) Plant Sci. Lett. 19, 157-168 11 Dodds, J. H., Bengochea, T. and Starting, R. J. (1982) Proc. 5th Int. Con. Plant Tissue and Ceil Cult. (Fujiwara, A., ed.), pp. 5%58,
A third dimension in the control of plant development SIR: The title of your October 1983 Discussion Forum, 'Is plant development regulated by changes in the concentration of growth substances or by changes in sensitivity to growth substances?' seems to rule out any consideration of the possibility that some developmental responses might be dependent on neither changes in hormone concentration n o r changes in hormone sensitivity of the tissue. This type of hormone-independent response is clearly recognized by Trewavas and there is growing evidence of the importance of such phenomena, particularly in cases where the plant responds rapidly (< 300 s) to environmental stimuli. We would also consider shoot tropistic responses to be included in this category. The results of our recent studies ~.2.3 and those of others 4"5 on the rapid growth rate changes bringing about tropistic curvatures in shoots, suggest that these changes in growth rate are not caused by changes in the concentration of hormones. We would not accept Cleland's view that changes in the concentration of auxin can explain such growth rate changes - in our opinion the changes found are too small, they have not been shown to occur rapidly enough and the hormone changes do not correlate well with the complex growth patterns causing curvature. However, poor correlations have not in the past disturbed plant hormonologists as there is always the excuse, noted by Cleland, that changes in some particular fraction of the total hormone might correlate well. Unfortunately, Trewavas may have offered yet another straw for plant hormonologists to clutch at, as they can now evoke ~mewhat nebulous sensitivity arguments after analyses of even the smallest hormone compartment have
failed to reveal appropriate changes. After 50 years of analysing plant hormones, admittedly usually imperfectly, it is now accepted that changes in plant hormone levels do not provide an adequate explanation of many developmental changes. By all means let us consider whether hormone sensitivity might not provide us with a better explanation of some responses but let us also reconsider whether hormonal control need be the only way in which plant cells regulate basic processes.
References 1 Firn, R. D. and Digby, J. (1980) Annu. Rev.
Jap. Ass. Plant Tissue Culture, Tokyo 12 Graham, C. F. and Wareing, P. F. (1976) The Developmental Biology of Plants and Animals, Blackwell Scientific Publishers 13 Lefkowitz, R. J., ed. (1982) Receptors and Recognition, Series B, Vol. 13, Chapman and Hall 14 Trewavas, A. J. (1980) Phytochemistry 19, 1303-1308 15 Bhattacharyya, K. and Biswas, B. B. (1982) Phytochemistry 21, 1207-1211 16 Ostroom, H., Kulescha, Z., van Vliet, Th. B. and Libbenga, K. R. (1980) Planta 149, 44-47 ROBERT J. STARLING
Department of Plant Biology, University of Birmingham, PO Box 363, Birmingham B15 2T-F, UK.
Plant Physiol. 31, 131-148 2 Franssen, J. M., Cooke, S. A., Digby, J. and Fire, R . D . (t981) Z. Pflanzenphysiol. 103, 207-216 3 Digby, J., Firn, R. D. and Carrington, S. C. M. (1982) in Plant Growth Substances 1982' (Wareing, P. F., ed.), pp. 519-528, Academic Press 4 Hart, J. W., Gordon, D. C. and MacDonald, I. R. (1982) Plant Cell Envir. 5, 361-366 5 MacDonald, I. R., Hart, J. W. and Gordon, D. C. (1983) Plant Cell and Envir. 6, 401-406 RICHARD FIRN a n d JOI~IN DIGBY
Department of Biology, University of York, York YO1 5DD, UK.
Antizymes: inhibitor proteins or regulatory subunits? SIR: It has been recognized for over half a century that an important factor influencing the activity of many enzymes in vivo is their association with other proteins. As early as 1930, Haldane ~ described 'antienzymes' as a class of proteins encompassing both immunoglobulins and other types of inhibitory proteins; since that time, the number and types of such known interactions have grown impressively. Recently, the discovery of different regulatory proteins that inhibit a specific target enzyme has been accompanied by changing terminology that accentuates the uniqueness of each regulatory protein, while obscuring its general membership in the class of regulatory subunits. It has become apparent that the use of regulatory subunits to govern the function of structural proteins, transport proteins, and enzymes is widespread. From the extant characterized examples, some generalizations can be made:
(1) The association of a protein with its regulatory subunit affects the affinity and/or specificity of ligand binding either by alteration in tertiary structure or by steric impedance resulting from the aggregation itself. (2) In many cases, aggregation of a protein with its regulatory subunit is mediated by a small molecule effector(s) since most regulatory subunits are themselves controlled either by chemical modification or by small metabolites. A considerable spectrum is apparent in the types of regulatory, subunits, examples of which are given in Table I. The list of proteins is in no way comprehensive, but was chosen to illustrate diversity. At one extreme is aspartate carbamoyltransferase, where the regulatory subunit could be viewed as a genetically independently produced allosteric site. At the other extreme is the complex of two enzymes, ornithine