Critical Reviews in Plant Sciences

Cutler, Horace G.; Wells, John M.

doi: 10.1080/07352688809382254pmid: N/A

Microorganisms produce a wide assortment of secondary metabolites which may be used to control growth and development of plants. These natural products are diverse in structure and range from oligopeptides to complex and simple nonamino acid molecules. Each has unique properties, but in general they have high specific activity, a narrow spectrum of activity, and are biodegradable. Each molecule has the potential for synthetic modification by two routes. One is to derivatize the natural product; the other is to synthesize simpler structures which are analogs of the natural product. Thus, the spectrum of activity may be suitably altered. Fermentation products and their derivatives are logical candidates for the next generation of agrochemicals. Examples of microbial secondary metabolites and their structures, relative to biological activity, are discussed.

Pedrosa, Fábio de Oliveira; Yates, M. G.

doi: 10.1080/07352688809382255pmid: N/A

The advent of the acetylene reduction assay for determination of nitrogenase activity in the mid‐1960s had a great impact on the development of nitrogen‐fixation research, as a whole, and of plant‐diazotrophic bacteria associations, in particular. The number of diazotrophic rhizocoenosis increases continuously. Azospirilla, Azotobacteraceae, Enterobacteriaceae, Pseudomonadaceae, Bacillaceae, and other diazotrophs have been found closely associated with plant roots and could contribute substantial amounts of fixed nitrogen to the associated plant. Additionally, these bacteria may benefit the plant through the production of growth substances. In this article, the relevant literature on physiology, biochemistry, and genetics of these diazotrophs is reviewed. An integrated knowledge of these disciplines is essential for carrying out suitable manipulation of the environment and of the rhizocoenotic diazotroph for crop improvement.

Rebeiz, Constantin A.; Montazer‐Zouhoor, Ahmad; Mayasich, Joseph M.; Tripathy, Baishnab C.; Wu, Shi‐Ming; Rebeiz, Carole C.; Friedmann, Herbert C.

doi: 10.1080/07352688809382256pmid: N/A

In 1984, we described the development of a new concept in the design of photodynamic herbicides and demonstrated the phenomenology of the process by using a harmless amino acid, δ‐aminoevulinic acid (see Reference 1 of this article). δ‐Aminolevulinic acid (ALA) is the precursor of all tetrapyrroles in plant and animal cells. By spraying plants with ALA, the latter is converted to tetrapyrroles which accumulate in the plant tissues. In light, the accumulated tetrapyrroles kill some plant species while other plant species are left unharmed. It has recently become apparent that susceptible plant species are divinyl or monovinyl dicotyledonous plants, while monovinyl mono‐cotyledonous plants are less susceptible. At night, monovinyl plant species accumulate mainly monovinyl tetrapyrroles, while divinyl plants accumulate mainly divinyl tetrapyrroles (see Reference 43 of this article). When divinyl plant species are induced to accumulate large amounts of divinyl tetrapyrroles or lesser amounts of monovinyl tetrapyrroles, they are killed by the photodynamic action of the tetrapyrroles in light. Monovinyl plant species appear to evade destruction by conversion of the divinyl tetrapyrroles to monovinyl tetrapyrroles. The latter are then readily converted to chlorophyll in light. Three groups of chemicals which can act in concert with ALA have now been identified, namely: (I) enhancers of ALA conversion to tetrapyrroles, (2) inducers of ALA formation by plant tissues, and (3) inhibitors of divinyl tetrapyrrole conversion to monovinyl tetrapyrroles. By combining ALA with a member(s) of one or more of the foregoing groups of chemicals, it has become possible to design herbicidal formulations which are effective against a wide range of field conditions.