The maize endosperm-specific gene shrunken2 (Sh2) MATERIALS AND METHODSIsolation of Sh2 Isoalleles from sh2-ml. The inbred stock containing sh2-ml, the closely linked al-m3, and the activating element, Ac was described (5). Plants were self-pollinated and plump or nonshrunken seed were collected. Remnant mutant seed from ears of corn containing at least one plump seed were saved for planting in subsequent generations. Most revertant Abbreviations: AGP, adenosine diphosphoglucose pyrophosphorylase; 3-PGA, 3-phosphoglyceric acid. tPresent address:

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1990. A light and electron microscope investigation of the transfer cell region of maize caryopses. Can. J. Bot. 68: 471-479.The transfer cell zones from 23-day postpollination corn caryopses were examined using light and electron microscopes and X-ray elemental analysis. The transfer cells were sectioned in cross and longitudinal planes and were characterized by having numerous cell-wall extensions in the form of anastomosing lamellae. The most basal transfer cells had more cell-wall extensions than those that were successively deeper in the endosperm. Cytoplasm, rich with mitochondria, filled the interstices of cell-wall extensions, and many vesiculate areas could be found along the plasma membrane. Some transfer cells contained crystals within plastids. The crystals were composed of magnesium, phosphorus, calcium, and zinc. Other cells had large aggregations of endoplasmic reticulum that were often in close association with mitochondria or unidentified, single membrane bounded organelles. When viewed in cross section, the cell-wall extensions of contiguous cells tended to originate from common loci. Plasmodesmata were absent in the bottom parts of the basal transfer cells where they contacted the maternal tissue but were abundant in the upper parts of these cells and in the transfer cells found deeper in the endosperm. The plasmodesmata were found in clusters and alternated with the wall extension areas.

Seedlings of alcohol dehydrogenase 7 null mutants (Adhl-) of Zea mays L., which fail to synthesize alcohol dehydrogenase 1 (ADH 1) isozymes, were hypoxically acclimated by 18 h of exposure to an atmosphere of 4% (v/v) O2 in N2 at 25'C. Their ability to tolerate subsequent anoxia by exposure to anaerobic (Oz-free) conditions was compared with that of unacclimated seedlings that were transferred immediately from an atmosphere of 40% (v/v) Oz to anaerobic conditions. Only 10% of the root tips of unacclimated seminal roots survived 6 h of anoxia, whereas 70% of the hypoxically acclimated root tips were viable at 24 h. During anoxia, acclimated root tips had enhanced ADH activity compared with unacclimated root tips, through induction of Adh2. Despite this, enzyme activity was still only about 5% that of acclimated, wild-type root tips and about half that of unacclimated, wild-type root tips. During anoxia, acclimated Adhl-root tips showed a higher rate of anaerobic respiration and ethanol production, greater concentrations of ATP and total adenylates, and a greater adenylate energy charge compared with unacclimated root tips. These results suggest that although enhanced ADH activity may have raised fermentation rates in acclimated Adhl-tissues and thereby contributed to energy metabolism and viability, the high levels of ADH activity inducible in acclimated, wild-type maize root tips appear to be in excess of that required to increase rates of fermentation.

When root tips of fully aerobic, intact maize (Zea mays L.) seedlings are made anaerobic, viability normally is only 24 hours or less at 250C. We find that viability can be extended to at least 96 hours if seedlings are given a hypoxic pretreatment for 18 hours by sparging the solution with 4% 02 in nitrogen (v/v) before anoxia. Fully aerobic root tips (sparged with 40% 02) had very low alcohol dehydrogenase (ADH) activity (per gram root fresh weight), and the level remained low under anoxia. In hypoxically pretreated roots, however, high levels of ADH activity were induced, and activity rose further during the initial 24 hours of anoxia, and then remained high at about 20 times that of controls in 40% 02. ADH activity in roots in solution sparged with air (21% 02) was about three times that in 40% 02. Improved viability of hypoxically pretreated root tips was associated with maintenance of a high energy metabolism (ATP concentration, total adenylates, and adenylate energy charge). Roots that were not pretreated lost 94% of the total adenylates and ATP at 24 hours of anoxia. The relation between induced ADH activity, energy metabolism, and improved anoxia-tolerance in acclimated maize root tips is discussed.Many dryland plant species, including maize, sustain injury or even death when the soil becomes excessively wet through irrigation, poor drainage of the soil profile, or transient flooding (7,8). Flooding injury is initiated when the concentration of molecular oxygen in the rooting zone is too low to maintain aerobic respiration throughout the root tissue. Below this COP,2 energy metabolism in cells progressively declines (28) with inhibition of root growth, ion transport, and alterations in water and hormone relations (5,8).A critical feature for plant survival under these conditions may be the ability of root cells to maintain their energy status anaerobically. Some native wetland species and rice can survive extended periods of anoxia (2,12,14,15,17). Survival and a relatively high AEC are associated with sustained ethanolic fermentation which results in a net synthesis of ATP during glycolysis (15,17,23,24). Most dryland species do not display a comparable degree of anoxia tolerance.