The chemical structures of the primary cell walls of the grasses and their progenitors differ from those of all other flowering plant species. They vary in the complex glycans that interlace and cross-link the cellulose microfibrils to form a strong framework, in the nature of the gel matrix surrounding this framework, and in the types of aromatic substances and structural proteins that covalently cross-link the primary and secondary walls and lock cells into shape. This review focuses on the chemistry of the unique polysaccharides, aromatic substances, and proteins of the grasses and how these structural elements are synthesized and assembled into dynamic and functional cell walls. Despite wide differences in wall composition, the developmental physiology of grasses is similar to that of all flowering plants. Grass cells respond similarly to environmental cues and growth regulators, exhibit the same alterations in physical properties of the wall to allow cell growth, and possess similar patterns of wall biogenesis during the development of specific cell and tissue types. Possible unifying mechanisms of growth are suggested to explain how grasses perform the same wall functions as other plants but with different constituents and architecture.

Structure and biogenesis of the cell walls of grasses

INTRODtTCTION ... .. .... . . ........ . . . . ....... . . . ... .. .. . .. . . .... . . . .. ... ....... .. . .. .. . . .... .. . .. .. . . ... . . . . 74 OXYGEN ACTIVATION AND REACTIVE OXYGEN SPECIES ... ..... . . . . . ..... . .. ... 75 Reductive Activation ........ . . ......... ... . . . ..... . ..... .. ....... . . . . .. ....... . . . . 75 Oxygen Addition to Organic Radicals or Reduced Metal Complexes 75 Photodynamic Reactions ,.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...... . . . . . . . 76 REACTIVITY AND DETECTION OF ACTIVE OXYGEN SPECIES 77 Superoxide . . . . . ... . . . . ....... . . . . . . . . ....... . . . . . . . . ..... ......... ..... 77 Hydrogen Peroxide (H20JJ 77 The OR Radical ....... . . 78 Singlet Oxygen 78 Organic Peroxides and Peroxy Radicals ......... . . . . . 79 OXYGEN ACTIVATION IN DIFFERENT COMPARTMENTS AND ORGANELLES OF PLANT CELLS 79 Peroxides in Cell Walls . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Microsomal Oxygen Activation .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Peroxisomes ........ . . . . . . . . . . . . . . . . . . . . . . ....... . . . . . . . . . . . . . ..... ...... 80 Oxygen Activation and Detoxification in Mitochondria . ...... 80 Chloroplasts ....... .... ...... ..... .. . . . ..... . . . ...... . . . . . . . . ..... . . . . . ......... . ... 81 Physiological observations and the biological significance of oxygen reduction 8 1 Mechanisms o f oxygen activation in the chloroplast .... ..... 82 Functions of activated oxygen species in chloroplasts . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . 84 ENDOGENOUS MECHANISMS OF PROTECTION AGAINST DELETERIOUS OXYGEN SPECIES ...... 84 PATHOLOGICAL, ENVIRONMENTAL, AND COMMERCIAL ASPECTS OF OXYGEN TOXICITy 86 CONCLUDING REMARKS AND FUTURE TRENDS 88

Oxygen activation and oxygen toxicity

Cytochrome P450-dependent monooxygenases are a large group of heme-containing enzymes, most of which catalyze NADPH- and O2-dependent hydroxylation reactions. The cloning of plant P450s has been hampered because these membrane-localized proteins are typically present in low abundance and are often unstable to purification. Since the cloning of the first plant P450 gene in 1990, there has been an explosion in the rate at which genes encoding plant P450s have been identified. These successes have largely been the result of advances in purification techniques, as well as the application of alternative methods such as mutant- and PCR-based cloning strategies. The availability of these cloned genes has made possible the analysis of P450 gene regulation and may soon reveal aspects of the evolution of P450s in plants. This new knowledge will significantly improve our understanding of many metabolic pathways and may permit their manipulation in the near future.

Molecular-genetic analysis of plant cytochrome p450-dependent monooxygenases.

Phytoalexins: enzymology and molecular biology.

Plant systems utilize a diverse array of cytochrome P450 monooxygenases (P450s) in their biosynthetic and detoxification pathways. The classic forms of these enzymes are heme-dependent mixed function oxidases that utilize NADPH or NADH and molecular oxygen to produce functionalized organic products. The nonclassical forms are monooxygenases that either do not utilize flavoproteins for dioxygen activation or fail to incorporate molecular oxygen into their final product. Biosynthetic P450s play paramount roles in the synthesis of lignin intermediates, sterols, terpenes, flavonoids, isoflavonoids, furanocoumarins, and a variety of other secondary plant products. Other catabolic P450s metabolize toxic herbicides and insecticides into nontoxic products or, conversely, activate nontoxic substances into toxic products. Biochemical and molecular characterizations on a number of plant P450s have indicated that the relationships between these heme proteins and their substrates are at least as complex as th...

J. Rowell M. Potts

Papers

The 4-Hydroxylation of Cinnamic Acid by Sorghum Microsomes and the Requirement for Cytochrome P-450