1 Considerations in Searching for New Plant Growth Regulators E R N E S T G. JAWORSKI
Downloaded by HARVARD UNIV on May 25, 2014 | http://pubs.acs.org Publication Date: May 1, 1977 | doi: 10.1021/ba-1977-0159.ch001
Monsanto Agricultural Products Co., St. Louis, Mo. 63166
Interest is strong in the study of plant growth regulators as reflected by the increasing number of research publications and the fact that at least 29 major companies are now actively engaged i n this type of research. Nevertheless, advances to date have been limited by complications confounding straightforward elucidation of the relationships between chemical structure and plant response. Plant growth regulation is expressed by an integrated series of reactions and interactions. The ability to influence these reactions toward some desired goal represents a more complex situation than one where a simple lethal effect (herbicide) is desired. Structure-activity data for plant regulators are not very abundant in comparison with similar information on pesticides. This stems partly from the relative novelty of plant regulators and partly from the difficulty in obtaining biological test data. W h i l e research in plant regulation has been active for scores of years, identification of specific regulators—more precisely, plant hormones—has been generated within the past 35-40 yrs. Auxins, and more specifically indoleacetic acid, have probably received the greatest attention, having been the first type of plant hormone to be characterized. Research i n this area led to such auxin analogs as naphthaleneacetic acid and the phenoxyacetic acids and to their commercial application i n fruit set, fruit thinning, and other regulatory (non-herbicidal) uses. The subsequent discoveries of other natural plant hormones like gibberellins, cytokinins, and abscisic acid led to a similar evolution of analogs although the commercial applications were, w i t h the possible exception of gibberellic acid, on a much more modest scale than the auxins. Ethylene and ethylene-generating chemicals such as ethephon have probably received the most intensive scrutiny i n recent times where structure-activity relations are concerned. E v e n so, relatively little published data are available regarding structure and biological activities for all hormone classes presently recognized. 3
In Plant Growth Regulators; Stutte, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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GROWTH REGULATORS
Downloaded by HARVARD UNIV on May 25, 2014 | http://pubs.acs.org Publication Date: May 1, 1977 | doi: 10.1021/ba-1977-0159.ch001
The discovery of naturally occurring hormones has also generated considerable interest i n the synthesis of chemicals that would either block the endogenous synthesis of the hormone or interfere with its transport from the site of formation to the target site of action. Such approaches have resulted in the development of a number of commercial or near commercial chemicals such as Alar, which is used to promote increased flowering and fruit set. Thus the search for structures analogous to known plant hormones or their precursors has served as a fundamental approach to finding plant regulators over the past three decades. Knowledge of the pathways of hormone biosynthesis and the mechanisms for their regulation would be especially useful in developing chemical modifiers that could alter hormone levels i n desired places i n the plant and during appropriate temporal stages of its development. F o r example, it is conceivable that fruit retention i n cotton and soybeans would enhance yield. Enhancement of auxin transport from leaves to newly developing reproductive organs could result i n such retention by preventing abscission, as suggested by Addicott some time ago. Possibly one of the best cases that exemplify the difficulties i n defining structure—and what we perhaps should now refer to as commercial activity or agronomically significant activity—is one that could be called the T I B A story. T I B A , 2,3,5-triiodobenzoic acid, was felt to have great potential i n increasing soybean yields. Numerous analogs and homologs were studied, and this particular compound appeared to be the best—at least under the test conditions used. F i e l d results, unfortunately, were erratic and ranged from highly positive to highly negative effects on yield. Large differences i n the responses of different cultivars were noted, and strong interactions with management and cultural practices as well as with environmental effects, especially early season moisture levels, were all encountered i n seven years of field research with T I B A . One wonders whether some other derivative, though perhaps of lesser activity under the standard test conditions, might have been more effective under varying field conditions. The T I B A story highlights at least two major considerations that need more attention i n future structure-activity evaluations of plant regulators. (1) W e must recognize that plants, unlike animals, cannot move about. They are literally stuck with whatever environment they find themselves i n . Natural selection over tens of millions of years has therefore resulted i n the remarkable adaptability we see i n plants today. Their ability to adjust to adverse environmental shifts, however, could also result in internal changes that either work indirectly to counteract a desired plant regulator effect or directly to correct for the elicited response. In either event, the plant regulator effect might then be shortlived. More sophisticated determinations of endogenous biochemical and
In Plant Growth Regulators; Stutte, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
Downloaded by HARVARD UNIV on May 25, 2014 | http://pubs.acs.org Publication Date: May 1, 1977 | doi: 10.1021/ba-1977-0159.ch001
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Searching for New Plant Growth Regulators
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physiological activities w i l l probably be required to develop a greater understanding of our plant regulator candidate. (2) The environmentally elicited responses within the plant w i l l have to be superimposed on plant regulator studies under controlled conditions if useful structure-activity correlations are to be made. Plant regulators must be examined from the point of view of the plant. There is interest i n improving photosynthesis and yield by blocking photorespiration—a process considered by some research workers to be unnecessary and even wasteful for the plant. In soybeans, we have found that the leaves can increase their photosynthesis rates by 5 0 % simply i n response to the development of the bean and the bean pod. In other words, the plant contains endogenous mechanisms for enhancing photosynthesis in response to a sink demand by a reproductive organ. This example of inherent endogenous response capability i n the plant illustrates how critical the timing of an application of a plant regulator could be in seeking to find a photorespiratory inhibitor to enhance photosynthesis. Thus the stage of plant development, the timing of chemical treatment, and the environment all require consideration i n structure-activity studies. W h e n we superimpose genotypic and phenotypic variations upon this complex of variables, it is apparent that plant regulator research and development is considerably more complex and requires more rigorous and critical evaluation criteria than has been customary i n pesticide research. However, attention to such detail should promote discovery of useful chemicals which w i l l play a highly significant role i n our future efforts to improve food quality and increase crop productivity. RECEIVED
November 2, 1976.
In Plant Growth Regulators; Stutte, C.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
