Activities of Transaminases, Amylases and Proteases during Endosperm Degradation in Normal and Opaque-2 Zea mays L. cv. Maya

Transaminase, amylase and protease activities were comparedin seedlings of normal and Opaque-2 (o2) maize. Transaminaseactivity, greater in normal maize, was highest in the scutellumfrom which it decreased rapidly in activity from day 2 afterimbibition; only low activity was observed in endosperm andaxis tissue. Amylolytic activity, optimal around pH 5, was greater in normalmaize at all stages of endosperm degradation. Activity whichwas low on day 2, rose to a peak on day 6 and declined afterwards.The level of free sugars in the endosperm of normal was higherthan in o2 maize, and in both varieties was highly correlatedwith amylolysis. Protease activity, optimal at pH 3.6, was also greater in normalendosperms and increased up to day 6 and activity was maintainedat this level until around day 14. Although the activity ofall three enzyme systems examined was greater in normal maizethere were no apparent differences in the overall growth ofnormal and o2 seedlings during this period. Zea mays L, maize, corn, endosperm, enzyme activity, transaminase, amylase, protease     

Relationships Between Seed Respiration During Imbibition and Subsequent Seedling Growth in Zea mays L

Corn (Zea mays L.) seed respiration rates during the first 30 hours of germination were compared with seedling growth 3 to 5 days after planting. Significant positive correlations were observed between rates of O₂ uptake during imbibition and later stages of germination and seedling growth. Glutamic acid decarboxylase activity also was positively correlated with seedling growth. The highly significant correlations between respiratory quotients and seedling growth were negative.

Seed metabolism during the initial hours of germination is evidently related somehow to seedling growth rates several days later. Whether this relationship is due to the dependence of synthetic processes and growth on respiratory energy, the fact that high respiration rates reflect high levels of metabolic activity, or to some other cause, remains to be determined.

     

QUANTITATIVE ULTRASTRUCTURE AND PROTEIN COMPOSITION OF DATE PALM (PHOENIX DACTYLIFERA) SEEDS: A COMPARATIVE STUDY OF ENDOSPERM VS. EMBRYO

Comparative studies on the ultrastructure and protein composition of the embryo and endosperm of date palm (Phoenix dactylifera L.) were conducted. Cells of the embryo cotyledon and endosperm function in reserve storage and contained cell walls, nuclei, and cytoplasm rich in lipid and protein bodies. Morphometric analysis from light and electron micrographs showed that the cell walls of the endosperm occupied 65% of the total cell volume, but only 6% in the embryo. The protein bodies of the endosperm accounted for 11%, whereas those of the embryo occupied more than half of the total cell volume. The volume of organelles and organelle-free cytoplasm in the endosperm was negligible, suggesting that most of the extractable endosperm proteins are localized in the protein bodies. Extractable proteins in the embryo may come from cytoplasm, protein bodies, and other organelles. The endosperm contains relatively lower amounts of proteins than does the embryo. Proteins extracted from both tissues were compared using SDS-polyacrylamide gel electrophoresis, tube gel isoelectric focusing, and two-dimensional electrophoresis. Proteins of both the tissues were heterogeneous in molecular mass and charge. The majority of the proteins were similar in molecular mass and charge in the two tissues, suggesting that most of the storage proteins are probably the same. However, there were also several embryo- and endosperm-specific proteins apparent in both the first- and second-dimension gels. The endosperm-specific proteins may play an important role in germination and seedling development.     

Nucleotide Metabolism in Salt-Stressed Zea mays L. Root Tips: I. Adenine and Uridine Nucleotides

Corn plants (Zea mays L. cv Pioneer 3906) were grown in a glass house on control and saline nutrient solutions, in winter and summer. There were two saline treatments, both with osmotic potential = −0.4 megapascal but with different Ca²⁺/Na⁺ ratios: 0.03 and 0.73. Root tips and shoot meristems (culm tissue) of 26 day-old plants were analyzed for nucleotides to ascertain if there were correlations between nucleotide pool size and the reduced growth on saline cultures. Several other cell components also were determined. Plants grown in winter were only half as large as those grown in summer mainly because of the lower light intensity and lower temperature. But the relative yield reduction on salt treatment compared to the control was similar in winter and summer. The two different salt treatments caused similar yield reductions. Neither salt treatment affected nucleotide pools in culm tissue, with the possible exception of UDPG in winter. In the case of root tips, salt treatment had little or no effect on nucleotide pool sizes in winter when many already seemed near a critical minimum, but in summer it reduced several pools including ATP, total adenine nucleotide, UTP, total uridine nucleotide, and UDP-glucose. The reductions were greatest on the salt treatment with low Ca²⁺/Na⁺. There was no simple correlation between the effects of salt stress on growth and on nucleotide pool size. The nucleotide pools of culm tissue indicated that in some respects this tissue was effectively insulated from the salt stress. Roots that were in direct contact with the saline solution indicated significant reductions in nucleotide pools only in the summer whereas growth was reduced both summer and winter. It is possible that the nucleotide concentrations of root cells in winter were already near a critical minimum so that nucleotide synthesis and growth were tightly linked. Significant reductions in nucleotide pools that would be expected to affect growth were more evident in summer when pools were larger and growth was more rapid. But even where ATP and total adenine nucleotides were reduced, the ratio of ATP:ADP and the adenylate energy charge remained unchanged indicating an active adenylate kinase that had access to most of the adenine nucleotide pools, and possible catabolism of excess AMP.     

Auxin and Ethylene Control of Growth in Seedlings of Zea mays L. and Avena sativa L

The effects of applied ethylene on the growth of coleoptilesand mesocotyls of etiolated monocot seedlings (oat and maize)have been compared with those on the epicotyl of a dicot seedling(the etiolated pea). Significant inhibition of elongation by ethylene (10 µll^–1for 24 h) was found in intact seedlings of all three species,but lateral expansion growth was observed only in the pea internodeand oat mesocotyl tissue. The sensitivity of the growth of seedlingparts to ethylene is in the decreasing order pea internode,oat coleoptile and oat mesocotyl, with maize exhibiting theleast growth response. Although excised segments of mesocotyland coleoptile or pea internode all exhibit enhanced elongationgrowth in IAA solutions (10^–6–2 ? 10^–5 moll^–1), no consistent effects were found in ethylene. Ethyleneproduction in segments was significantly enhanced by applicationof auxin (IAA, 10^–5 mol l^–6 or less) in all tissuesexcept those of the eat mesocotyl. Segments of maize show a slow rate of metabolism of applied[2-¹⁴C]IAA (30 per cent converted to other metabolites within9 h) and a high capacity for polar auxin transport. Ethylene(10 µl l^–1 for 24 h) has little effect on eitherof these processes. The oat has a smaller capacity for polartransport than maize and the rate ef metabolism of auxin isas fast as in the pea (90 per cent metabolized in 6 h). Althoughethylene pretreatment does not change the rate of auxin metabolismin oat, there is a marked reduction in auxin transport. It is proposed that the insensitivity of maize seedlings toethylene is related to the supply and persistence of auxin whichcould protect the seedling against the effects of applied orendogenously produced ethylene. Although the mesocotyl of oatis sensitive to applied ethylene it may be in part protectedagainst ethylene in vivo by the absence of an auxin-enhancedethylene production system. The results are discussed in relationto a model for the auxin and ethylene control of cell growthin the pea.     

Chilling Damage to Photosynthesis in Young Zea mays: I. EFFECTS OF LIGHT AND TEMPERATURE VARIATION ON PHOTOSYNTHETIC CO2 ASSIMILATION

Carbon dioxide and water vapour exchanges of the second leafof Zea mays in controlled environment cuvettes were measuredin an open gas-exchange system, during and following subjectionto low temperature stress. Photosynthetic CO2 assimilation (Fc)decreased markedly with decrease in leaf temperature so thatFc at 5 °C was c. 7% of Fc at 20 °C. Fc continued todecline if leaf temperature was maintained at 5 °C, andwhen returned to 20 °C the leaf could not regain its previousFc. This chill-induced reduction in the capacity of the leafto assimilate CO2 was proportional to the duration of the chilland increased with water vapour pressure deficit and photonflux density (I_n). Six hours at 5 °C decreased Fc on returnto 20 °C, relative to Fc prior to treatment, by c. 10% indarkness and by c. 50% in a photon flux density approachingfull-sunlight (I_p = 1.5 mmol m^–2 s^–1). The degreeof reduction in Fc following chill treatment showed an almostlinear dependence on both the length and temperature of thechill. Chill treatments resulted in a decrease in both stomataland mesophyll conductances. Examination of the responses ofF_c to light and CO₂ concentration suggested that chill damageto the capacity for CO₂ assimilation resulted from effects onboth the light and CO₂ limited processes within photosynthesis. Key words: Chilling, Photosynthesis, Zea mays, Light-temperature interaction     

Growth dynamics and cytoskeleton organization during stem maturation and gravity-induced stem bending in Zea mays L.

Characterization of gravitropic bending in the maize stem pulvinus, a tissue that functions specifically in gravity responses, demonstrates that the pulvinus is an ideal system for studying gravitropism. Gravistimulation during the second of three developmental phases of the pulvinus induces a gradient of cell elongation across the non-growing cells of the pulvinus, with the most elongation occurring on the lower side. This cell elongation is spatially and temporally separated from normal internodal cell elongation. The three characterized growth phases in the pulvinus correspond closely to a specialized developmental sequence in which structural features typical of cells not fully matured are retained while cell maturation occurs in surrounding internodal and nodal tissue. For example, the lignification of supporting tissue and rearrangement of transverse microtubules to oblique that occur in the internode when cell elongation ceases are delayed for up to 10 d in the adjacent cells of the pulvinus, and only occurs as a pulvinus loses its capacity to respond to gravistimulation. Gravistimulation does not modify this developmental sequence. Neither wall lignification nor rearrangement of transverse microtubules occurs in the rapidly elongating lower side or non-responsive upper side of the pulvinus until the pulvinus loses the capacity to bend further. Gravistimulation does, however, lead to the formation of putative pit fields within the expanding cells of the pulvinus. Received: 18 April 1998 / Accepted: 2 July 1998     

CO2 gas exchange and phosphoenolpyruvate carboxylase activity in leaves of Zea mays L.

Important gas exchange characteristics of C4 plants depend on the properties of phophoenolpyruvate carboxylase (PEPC), the enzyme catalysing the primary fixation of CO2 during C4 photosynthesis. In this study, the relationship between intracellular resistance for CO2 fixation (r_i) at high photosynthetically active photon flux densities (PPFD) and maximum PEPC activity in vitro (V_pm) was examined in leaves of Zea mays L. The analysis allowed the estimation of the Michaelis constant K_p of the enzyme for CO2 (or the equivalent number for bicarbonate) in vivo. At low PPFD (below 100 mol m-2 s-1) the initial slopes of the curves describing net CO2 uptake rate A as a function of intercellular CO2 concentration c_i increased with increasing PPFD. The increase (i. e. a decrease in r_i) was interpreted as due to a reversible activation of PEPC by light. Including this assumption into a model of C4 photosynthesis enabled us to reproduce A(c_i) response curves measured at low levels of PPFD. Fitting the model to experimental data resulted in values for K_I, the PPFD at which PEPC reaches half of its full activation, of about 200 mol m-2 s-1. Similar results were derived from the dependence of r_i on PPFD. The analysis of the relationships between r_i and V_pm and between r_i and PPFD, as well as fitting of the model to gas exchange data all gave rise to estimates for the resistance for CO2 transfer within mesophyll cells that are comparable with those known from C3 plants.     

Root Growth, Secondary Root Formation and Root Gravitropism in Carotenoid-deficient Seedlings of Zea mays L.

The effect of ABA on root growth, secondary-root formation androot gravitropism in seedlings of Zea mays was investigatedby using Fluridone-treated seedlings and a viviparous mutant,both of which lack carotenoids and ABA. Primary roots of seedlingsgrown in the presence of Fluridone grew significantly slowerthan those of control (i.e. untreated) roots. Elongation ofFluridone-treated roots was inhibited significantly by the exogenousapplication of 1 mM ABA. Exogenous application of 1 µMand 1 nM ABA had either no effect or only a slight stimulatoryeffect on root elongation, depending on the method of application.The absence of ABA in Fluridone-treated plants was not an importantfactor in secondary-root formation in seedlings less than 9–10d old. However, ABA may suppress secondary-root formation inolder seedlings, since 11-d-old control seedlings had significantlyfewer secondary roots than Fluridone-treated seedlings. Rootsof Fluridone-treated and control seedlings were graviresponsive.Similar data were obtained for vp-9 mutants of Z. mays, whichare phenotypically identical to Fluridone-treated seedlings.These results indicate that ABA is necessary for neither secondary-rootformation nor for positive gravitropism by primary roots. Zea mays, gravitropism, carotenoid-deficient, Fluridone, root growth, vp-9 mutant     

Biochemical genetics of Zea mays L. I. Isozymes and the linkage relationship between AP-1 and AP-2 loci

Following starch gel electrophoresis and a modified staining method, three distinct groups of acid phosphatase isozymes (AP-1, AP-2, and AP-3) have been identified in scutellum extracts of different inbred maize lines. The AP-2 group includes three electrophoretic variants: a fast (F), an intermediate (I), and a slow (S) type. Genetic analysis showed that AP-2 variants are under the control of three codominant alleles at the AP-2 locus. A linkage test between AP-1 and AP-2 indicated that a close linkage between these two loci is ruled out.     

Nitrate Reductase Activity in Maize (Zea mays L.) Leaves: I. Regulation by Nitrate Flux

The roles that leaf nitrate content and nitrate flux play in regulating the levels of nitrate reductase activity (NRA) were investigated in 8- to 14-day old maize (Zea mays L.) plants containing high nitrate levels while other environmental and endogenous factors were constant. The nitrate flux of intact plants was measured from the product of the transpiration rate and the concentration of nitrate in the xylem. NRA decreased when the seedlings were deprived of nitrate. The nitrate flux and the leaf nitrate content also decreased. When nitrate was resupplied to the roots, all three parameters increased.     

Auxin Transport in Zea mays L. Coleoptiles I. Influence of Gravity on the Transport of Indoleacetic Acid-2-C

¹⁴C-methylene labeled IAA was used to determine the influence of reorientation with respect to gravity on auxin transport in Zea mays L. coleoptile segments. It was observed that inversion of the segments leads to a decrease in the capacity to transport ¹⁴C-IAA basipetally, as well as, in certain instances, the linear velocity of that transport. Segments were also reoriented horizontally, and the transport velocity and capacity of the upper and lower tissue halves compared with vertical halves. There was no significant change in the velocity, but the transport capacity of lower halves was higher than that of the vertical halves, which in turn was higher than the capacity of the horizontal upper halves. It is suggested that the geocurvature of horizontally placed coleoptiles may be caused primarily by the effect of reorientation on auxin transport.     

Nitrogen Enhancement of Phosphate Transport in Roots of Zea mays L. : I. Effects of Ammonium and Nitrate Pretreatment

The effect of nitrogen status on phosphorous uptake and translocation was examined in 6-day-old dark-grown decapitated maize seedlings exposed to 25 micromolar phosphorous. Transfer to complete solutions containing 1 millimolar ammonium resulted in an increase in phosphorous uptake rate after 6 to 8 hours. The stimulus remained effective for at least 5.5 hours upon subsequent transfer to nitrogen-free solutions. Pretreatments for 16 hours with either nitrate or ammonium resulted in enhanced rates of subsequent phosphorous uptake and in enhanced translocation to the xylem of the exogenously supplied phosphorous. Both processes reached a plateau following pretreatment with 0.1 to 1.0 millimolar concentrations of either nitrogen ion. Further enhancement occurred with 10 millimolar nitrate, but not with 10 millimolar ammonium pretreatment. Although nitrogen pretreatments slightly increased the quantity of exogenous phosphorous retained in the root tissue, most of the extra phosphorous taken up by the nitrogen-pretreated seedlings was translocated to the xylem. The enhanced translocation, however, did not totally account for the increase in uptake implying a specific stimulation of the uptake process.     

Leaf Extension in Zea mays: I. LEAF EXTENSION AND WATER POTENTIAL IN RELATION TO ROOT-ZONE AND AIR TEMPERATURES

The temperature of the roots and shoots of Zea mays plants werevaried independently of each other and the rates of leaf extensionand leaf water potentials were measured. Restrictions of leafextension occurred when root temperatures were lowered from35 to 0 °C, but leaf water potentials were lowered onlyat root temperatures below 5 °C. Similar changes in ratesof leaf extension were measured at air temperatures from 30to 5 °. Between 30 and 35 °C air temperature, in anunsaturated atmosphere, restrictions of leaf extension wereassociated with low leaf water potentials. It was concluded that, at root temperatures 5 to 35 °C,and shoot temperatures 5 to 30 °C, water stress was notthe main factor restricting the extension of Zea mays leaves.     

Pyruvate Decarboxylase from Zea mays L. : I. Purification and Partial Characterization from Mature Kernels and Anaerobically Treated Roots

Pyruvate decarboxylase (PDC) was purified from mature, dry maize kernels and from roots of anaerobically treated maize seedlings and partially characterized. PDC was purified to a specific activity of 96 units per milligram protein from kernels and to 41 units per milligram protein from root. The subunit molecular masses were estimated to be 61,000 and 60,000 for kernel PDC and 59,000 and 58,000 for root PDC. The pH optimum for each enzyme was 5.8. Since the pH optimum is nearly one pH unit below the value reported for the cytoplasm of anaerobically metabolizing maize roots (pH 6.7 ± 0.2), we investigated the effects of pH 5.8 and 6.6 on the cooperative kinetics observed for PDC from each source. The maximum Hill coefficients (n_H) were much greater at each pH for the kernel PDC (pH 5.8, n_H = 2.5 and pH 6.6, n_H = 3.2) than for the root PDC (pH 5.8, n_H = 1.4 and pH 6.6, n_H = 1.8). The cooperative kinetics observed with respect to pyruvate were asymmetric. Potassium inhibited maize PDC and was competitive with pyruvate (root PDC K_i = 16 millimolar and kernel PDC K_i = 10 millimolar).     

Movement of C-Labeled Assimilates into Kernels of Zea mays L: I. Pattern and Rate of Sugar Movement

Carbon-14, photosynthetically fixed in leaves of Zea mays L. and translocated to developing kernels, passed through specialized basal endosperm cells prior to movement into the starchy endosperm and embryo. Radioactivity migrated in the endosperm at a maximum rate of 2.7 millimeters per hour, and there was no difference in the rate of movement in kernels treated 14 to 30 days after pollination.     

Population genetics of duplicated disease-defense genes,hm1 and hm2, in maize (Zea mays ssp. mays L.) and its wild ancestor (Zea mays ssp. parviglumis)

Plant defense genes are subject to nonneutral evolutionary dynamics. Here we investigate the evolutionary dynamics of the duplicated defense genes hm1 and hm2 in maize and its wild ancestor Zea mays ssp. parviglumis. Both genes have been shown to confer resistance to the fungal pathogen Cochliobolus carbonum race 1, but the effectiveness of resistance differs between loci. The genes also display different population histories. The hm1 locus has the highest nucleotide diversity of any gene yet sampled in the wild ancestor of maize, and it contains a large number of indel polymorphisms. There is no evidence, however, that high diversity in hm1 is a product of nonneutral evolution. In contrast, hm2 has very low nucleotide diversity in the wild ancestor of maize. The distribution of hm2 polymorphic sites is consistent with nonneutral evolution, as indicated by Tajima's D and other neutrality tests. In addition, one hm2 haplotype is more frequent than expected under the equilibrium neutral model, suggesting hitchhiking selection. Both defense genes retain >80% of the level of genetic variation in maize relative to the wild ancestor, and this level is similar to other maize genes that were not subject to artificial selection during domestication.     

THE ORIGIN OF PROTEIN AND FATTY YOLK IN RANA PIPIENS : I. Phase Microscopy

Study of living frog oocytes with the phase microscope has shown that the early yolk appears in two forms. One of these, the protein yolk, consists of thin, dense, plate-like bodies which in face view are almost always regular hexagons. The other form, the fatty yolk, occurs as clusters of globules of varying sizes. The plate-like bodies occur both singly and in clusters. As the oocytes mature these plate-like bodies grow in size while retaining their hexagonal outline. Mitochondria have been observed to increase in length and numbers as the oocytes mature; they are rods or filaments at all stages of growth up to an oocyte diameter of 300 microns. The oocyte cytoplasm gradually becomes packed with long mitochondria, plate-like bodies, and clusters of globules.