DISTRIBUTION AND CELLULAR LOCALIZATION OF ASCORBIC ACID OXIDASE IN THE MAIZE ROOT TIP

MERTZ, DAN. (U. Missouri, Columbia.) Distribution and cellular localization of ascorbic acid oxidase in the maize root tip. Amer. Jour. Bot. 48(5): 405-413. Illus. 1961.-Ascorbic acid oxidase has been identified by its various properties in homogenates of the maize root tip. Following centrifugal fractionation of the gross homogenate, approximately 30% of the total oxidase activity was found in the soluble fraction, while roughly 70% was bound to the cell wall debris. Quantitative measurements of serial segments revealed that the bound oxidase per cell increases with cell elongation. The very striking parallelism of the bound oxidase activity with that of cell elongation suggests that ascorbic acid oxidase may in some way participate in growth of the cell wall, and have very little to do with cellular respiration. MANY studies have appeared which demonstrate that there are marked physiological changes associated with the enlargement and subsequent differentiation of meristematic cells. Respiration measured as oxygen uptake on a protein-nitrogen or total-nitrogen basis often exceeds the respiration of embryonic cells. Goddard and Meeuse (1950) reported that the respiration, on a nitrogen basis, in roots of Zea mays is low in the meristematic zone, rises strongly and reaches a maximum in the extension zone and sinks to a constant level after the cessation of growth. Similar respiratory changes have been observed in bean roots by Brown and Broadbent (1950) and in barley roots by Eliasson (1955). Jensen (1955, 1958) has shown that in rootsof Vicia faba and Allium cepa the cells of the apical meristem are low in protein, ribonucleic acid (RNA), desoxyribonucleic acid (DNA), carbohydrates and oxygen uptake. The beginning of cell elongation is characterized by leveling off of DNA, a leveling or slight decrease in protein content, and an increase in carbohydrates and oxygen uptake. Cessation of growth is accompanied by a decrease in RNA, protein and oxygen uptake. Although many studies, such as those just enumerated, have appeared in recent years, there have been relatively few dealing with changes of specific enzymes or enzyme complexes associated with cell growth. Brown and Robinson (1955) have put forth a very stimulating interpretation of cell growth based upon a series of studies involving the growth of intact segments of the bean root in culture and the enzymatic changes associated with cellular differentiation. They showed that the activity of invertase, acid phosphotase, glycine oxidase, and dipeptidase increased during cell growth 1 Received for publication November 7, 1960. The investigation on which this report is based was a part of a dissertation submitted to the Graduate School of The University of Texas in partial fulfil]ment of the requirements for the degree of Doctor of Philosophy. The author wishes to express his appreciation to Dr. W. G. Whaley for his timely advice during the course of investigation. The study was supported by the Plant Research Institute of The University of Texas. to a maximum level and then decreased with the cessation of growth. They assume that the cells immediately cut off from the apical meristem are in such a metabolic state that growth cannot proceed. The cells, however, develop into another state which does support growth, and finally into one in which growth ceases. They consider that the several states are determined by the composition of the protein of the cell, and are each characterized by a particular enzyme complex which is different from the succeeding and preceding states. This interpretation of the induction of a state of potential growth, and the development of subsequent states in which growth proceeds and ultimately ceases find support in the recent works by Robinson (1956) and Robinson and Cartwright (1958). They have demonstrated that proteolytic and nucleolytic enzymes may play an important role in the regulation of protein content and the level of certain enzymes during the growth of the cell. The need for further studies concerning the enzymatic changes accompanying cellular differentiation prompted the following study. Ascorbic acid oxidase was selected because of its somewhat questionable role in plant metabolism and because Lascorbic acid, the naturally occurring substrate for ascorbic acid oxidase, is concentrated in regions of high metabolic activity, and particularly in regions undergoing rapid cell elongation. Some years ago, Reid (1941) found that in the root tips of cowpeas the transition from cells of the apical meristem was associated with a 7-fold increase in ascorbic acid. She also deduced from her studies that ascorbic acid was largely associated with the cell surface and made the interesting suggestion that this accumulation may in some way condition cell enlargement. More recently, Jensen and Kavaljian (1956) , using Vicia faba and Allium cepa, confirmed Reid's findings. Although several studies have appeared concerning the properties of ascorbic acid oxidase in the root tip and its possible role as a terminal oxidase in respiration (James and Boulter, 1955; Honda 1955a, 1957; Hallaway and Butt, see Mapson,