Root cortical aerenchyma inhibits radial nutrient transport in maize (Zea mays)

Background and Aims

Formation of root cortical aerenchyma (RCA) can be induced by nutrient deficiency. In species adapted to aerobic soil conditions, this response is adaptive by reducing root maintenance requirements, thereby permitting greater soil exploration. One trade-off of RCA formation may be reduced radial transport of nutrients due to reduction in living cortical tissue. To test this hypothesis, radial nutrient transport in intact roots of maize (Zea mays) was investigated in two radiolabelling experiments employing genotypes with contrasting RCA.

Methods

In the first experiment, time-course dynamics of phosphate loading into the xylem were measured from excised nodal roots that varied in RCA formation. In the second experiment, uptake of phosphate, calcium and sulphate was measured in seminal roots of intact young plants in which variation in RCA was induced by treatments altering ethylene action or genetic differences.

Key Results

In each of three paired genotype comparisons, the rate of phosphate exudation of high-RCA genotypes was significantly less than that of low-RCA genotypes. In the second experiment, radial nutrient transport of phosphate and calcium was negatively correlated with the extent of RCA for some genotypes.

Conclusions

The results support the hypothesis that RCA can reduce radial transport of some nutrients in some genotypes, which could be an important trade-off of this trait.     

Effect of gibberellic acid on the ion ratios in a dwarf maize mutant (Zea mays L. d1)

The effect of gibberellic acid (GA₃) on ion ratios in a dwarf maize mutant (Zea mays L. d₁) exhibiting normal growth after hormone treatment has been investigated by electron microprobe analysis. Gibberellic-acid treatment increased the ion content in chloroplasts and vacuoles whereas no change of the ion content was found in the cytoplasm. The relation of these observations to the action of the hormone is discussed.Abbreviation GA₃ Gibberellic acid     

Zinc deficiency and pollen fertility in maize (Zea mays)

Zinc deficiency decreased pollen viability in maize (Zea mays L. cv. G2) grown in sand culture. On restoring normal zinc supply to zinc-deficient plants before the pollen mother cell stage of anther development, the vegetative yield of plants and pollen fertility could be recovered to a large extent, but the recovery treatment was not effective when given after the release of microspores from the tetrads. If zinc deficiency was induced prior to microsporogenesis it did not significantly affect vegetative yield and ovule fertility, but decreased the fertility of pollen grains, even of those which visibly appeared normal. If the deficiency was induced after the release of microspores from the tetrads, not only vegetative yield and ovule fertility but pollen fertility also remained unaffected.     

An integrated method for quantifying root architecture of field-grown maize

Background and Aims

A number of techniques have recently been developed for studying the root system architecture (RSA) of seedlings grown in various media. In contrast, methods for sampling and analysis of the RSA of field-grown plants, particularly for details of the lateral root components, are generally inadequate.

Methods

An integrated methodology was developed that includes a custom-made root-core sampling system for extracting intact root systems of individual maize plants, a combination of proprietary software and a novel program used for collecting individual RSA information, and software for visualizing the measured individual nodal root architecture.

Key Results

Example experiments show that large root cores can be sampled, and topological and geometrical structure of field-grown maize root systems can be quantified and reconstructed using this method. Second- and higher order laterals are found to contribute substantially to total root number and length. The length of laterals of distinct orders varies significantly. Abundant higher order laterals can arise from a single first-order lateral, and they concentrate in the proximal axile branching zone.

Conclusions

The new method allows more meaningful sampling than conventional methods because of its easily opened, wide corer and sampling machinery, and effective analysis of RSA using the software. This provides a novel technique for quantifying RSA of field-grown maize and also provides a unique evaluation of the contribution of lateral roots. The method also offers valuable potential for parameterization of root architectural models.     

Vittatidera zeaphila (Nematoda: Heteroderidae), a new genus and species of cyst nematode parasitic on corn (Zea mays)

A new genus and species of cyst nematode, Vittatidera zeaphila, is described from Tennessee. The new genus is superficially similar to Cactodera but is distinguished from other cyst-forming taxa in having a persistent lateral field in females and cysts, persistent vulval lips covering a circumfenestrate vulva, and subventral gland nuclei of the female contained in a separate small lobe. Infective juveniles (J2) are distinguished from all previously described Cactodera spp. by the short stylet in the second-stage juvenile (14-17 μm); J2 of Cactodera spp. have stylets at least 18 μm long. The new species also is unusual in that the females produce large egg masses. Known hosts are corn and goosegrass. DNA analysis suggests that Vittatidera forms a separate group apart from other cyst-forming genera within Heteroderinae.     

Evaluation of parameters describing the root system architecture of field grown maize plants (Zea mays L.)

The objective of this work was to study elongation curves of maize axile roots throughout their elongation period under field conditions. Relationships between their elongation rate and the extension rate of their branched region were also studied. Maize, early-maturing cultivar Dea, was grown on a deep, barrier-free clay loam (depth 1.80m). Trenches were dug during four periods until after silking and axile roots were excavated. Parameters measured were total length and the lengths of basal and apical unbranched zones. The rank of the bearing phytomer and general data about the carrying plant were also recorded. Results showed that axile roots from lower phytomers had similar elongation rates irrespective of the rank of the carrying phytomer. This elongation rate declined with root age. A monomolecular elongation model was fitted to the experimental data. Elongation was much slower in roots from upper phytomers. A rough linear relationship was found between the elongation rate of axile roots and the length of the apical unbranched zone. This result suggests that laterals appeared on a root segment a constant time after it was formed. Possible mechanisms with may account for the declining elongation rate with root age (increasing distance from aerial parts or adverse environmental conditions in deep soil layers) and variability between individual roots are also discussed.     

Release of the benzoxazinoids defense molecules during lateral- and crown root emergence in Zea mays

We observed the release of the benzoxazinoids defense molecules on the surface of the primary root and the coleoptilar node in Zea mays during the emergence of lateral- and crown-roots, respectively. At later stages of crown root and lateral root development, benzoxazinoids around the emerged roots were no longer observed. Specific mutants revealed that the developmental status of the emerged roots was not important for the release of benzoxazinoids, but the breakage of the epidermis by emerging roots was. This is the first report of benzoxazinoid-release during normal development controlled by endogenous developmental programs. Release of benzoxazinoids around the emerging roots supports the idea that defense molecules accumulate at the site of root emergence in order to reduce pathogenic infections. We discuss possible explanations for the evolution of two different developmental mechanisms of root emergence.     

Closure of plasmodesmata in maize (Zea mays) at low temperature: a new mechanism for inhibition of photosynthesis

Background and Aims

Photosynthesis is one of the processes most susceptible to low-temperature inhibition in maize, a tropical C4 crop not yet fully adapted to a temperate climate. C4 photosynthesis relies on symplasmic exchange of large amounts of photosynthetic intermediates between Kranz mesophyll (KMS) and bundle sheath (BS) cells. The aim of this study was to test the hypothesis that the slowing of maize photosynthesis at low temperature is related to ultrastructural changes in the plasmodesmata between KM and BS as well as BS and vascular parenchyma (VP) cells.

Methods

Chilling-tolerant (CT) KW 1074 and chilling-sensitive (CS) CM 109 maize (Zea mays) lines were studied. The effect of moderate chilling (14 °C) on the rate of photosynthesis, photosynthate transport kinetics, and the ultrastructure of plasmodesmata linking the KMS, BS and VP cells were analysed. Additionally, the accumulation of callose and calreticulin was studied by the immunogold method.

Key Results

Chilling inhibited photosynthesis, photosynthate transfer to the phloem and photosynthate export from leaves in both lines. This inhibition was reversible upon cessation of chilling in the CT line but irreversible in the CS line. Simultaneously to physiological changes, chilling induced swelling of the sphincters of plasmodesmata linking KMS and BS cells and a decreased lumen of the cytoplasmic sleeve of plasmodesmata at the BS/VP interface in the CS line but not in the CT line. Accumulation of calreticulin, which occurred near the neck region of the closed plasmodesmata was observed after just 4 h of chilling and over-accumulation of callose at the KMS/BS and BS/VP interfaces occurred after 28 h of chilling.

Conclusions

Stronger chilling sensitivity of the CM 109 maize line compared with the KW 1074 line, shown by decreased photosynthesis and assimilate export from a leaf, is related to changes in the ultrastructure of leaf plasmodesmata at low temperature. The chain of reactions to chilling is likely to include calreticulin action resulting in rapid and efficient closure of the plasmodesmata at both KMS/BS and BS/VP interfaces. Callose deposition in a leaf was a secondary effect of chilling.     

Brassinosteroids are inherently biosynthesized in the primary roots of maize, Zea mays L

GC-MS analysis revealed that primary roots of maize contain 6-deoxocathasterone, 6-deoxoteasterone and 6-deoxotyphasterol. These brassinosteroids, and the previously identified campesterol, campestanol, 6-deoxocastasterone and castasterone, in the roots are members of a biosynthetic pathway to castasterone, namely the late C-6 oxidation pathway, suggesting that its biosynthetic pathway is operative in the roots. To verify this, a cell-free enzyme extract was prepared from maize roots, and enzymatic conversions from campesterol to castasterone through the aforementioned sterols and brassinosteroids were examined. The presence for the biosynthetic sequences, campesterol-->24-methylcholest-4-en-3beta-ol-->24-methylcholest-4-en-3-one-->24-methylcholest-5 alpha-cholestan-3-one-->campestanol and 6-deoxoteasterone-->6-deoxo-3-dehydroteasterone-->6-deoxotyphasterol-->6-deoxocastasterone-->castasterone were demonstrated. These results indicate that maize roots contain a complete set of enzymes involved in the late C-6 oxidation pathway, thereby demonstrating that endogenous brassinosteroids are biosynthesized in the roots.     

In Vitro Culture and Feeding Behavior of Belonolaimus longicaudatus on Excised Zea mays Roots

A greenhouse population of the sting nematode, Belonolaimus longicaudatus, obtained from an infested golf course in California''s Coachella Valley, was surface-decontaminated and cuhured on excised roots of Zea mays supported by Gamborg''s B5 medium. At 26-27 °C the females laid eggs, and newly emerged juveniles of the second generation completed three molts within 29 days after egg deposition. Sixty days after inoculation with 60 females and 40 males, an average of 529 nematodes and 83 eggs were recovered from the culture. The feeding process consisted of probing, stylet penetration, ingestion, and stylet retraction. Feeding seemed to be necessary before egg deposition or molting occurred. The sting nematode was observed feeding exclusively as an ectoparasite and preferably at the region of cell division and elongation. Vigorous feeding by many nematodes usually caused discoloration of root tips and termination of growth.     

Epistasis in maize (Zea mays L.)

Summary Three flint and three dent maize (Zea mays L.) inbred lines, their possible F₁ crosses, F₂ and backcross progenies, and all possible three-way crosses were evaluated in a three-year experiment for yield, ear moisture, and plant height. The purpose was to estimate genetic parameters in European breeding materials from (i) generation means analysis, (ii) diallel analysis of generation means, and (iii) analysis of F₁ and three-way cross hybrids. Method (i) was based on the F-metric model and methods (ii) and (iii) on the Eberhart-Gardner (1966) genetic model; both models extended for heterotic maternal effects.Differences among generation means for yield and plant height were mainly attributable to dominance effects. Epistatic effects were significantly different from zero in a few crosses and considerably reduced heterosis in both traits. Additive x additive and domiance x dominance effects for yield were consistently positive and negative, respectively. Significant maternal effects were established to the advantage of generations with a heterozygous seed parent. In the diallel analysis, mean squares for dominance effects were greater than for additive effects for yield and plant height but smaller for ear moisture. Though significant for yield and plant height, epistatic variation was small compared to additive and dominance variation. Estimates of additive x additive epistasis for yield were significantly negative in 11 of 15 crosses, suggesting that advantageous gene combinations in the lines had been disrupted by recombination in the segregating generations. The analysis of hybrids supported the above findings regarding the analysis of variance. However, the estimates of additive x additive epistasis for yield were considerably smaller and only minimally correlated with those from the diallel analysis. Use of noninbred materials as opposed to materials with different levels of inbreeding is considered the main reason for the discrepancies in the results.     

Effects of air-filled soil porosity and aeration on the initiation and growth of secondary roots of maize (Zea mays)

We studied the possibility whether the initiation of secondary roots is regulated by the air-filled porosity in soil, i.e. the availability of oxygen in the soil. Maize plants were grown in long PVC tubes (1 m long and 12 cm diameter) and were unwatered for different numbers of days so that variations of soil water content with depth were achieved on the same date with plants at the same age. The plants were harvested when their root systems were established in the whole soil column and watering had been withheld for 0, 15, 20, 25 days. A decrease of soil water content was significantly correlated with an increase of air-filled porosity in soil. The number of secondary lateral roots from segments of primary adventitious roots increased dramatically when soil water content decreased from field capacity to about 0.05 g water g^-1 dried soil. The total dried mass of roots at different soil depths was also positively correlated with soil air-filled porosity. It was observed that the elongation of the initiated secondary roots responded differently to the variations of soil air-filled porosity. The length of secondary roots increased initially when the soil was dried from field capacity to 0.18 g g^-1 dried soil (water potential at about−0.2 MPa, air-filled porosity 0.26 cm³ cm^-3), but was drastically reduced when the soil was dried further. Obviously elongation of secondary roots was inhibited when soil water potential began to deviate substantially from an optimum value. The present results suggested that the initiation of secondary roots was greatly promoted by the increase of air-filled soil porosity, i.e. availability of oxygen. This conclusion was further verified in a separate experiment where solution-cultured maize seedlings were subjected to different aeration treatments. An obvious increase in secondary root initiation was found in plants which were aerated with normal air (21% O₂) than in plants which were either not aerated or aerated with 5% O₂ air. ei]Section editor: B E Clothier     

Chlorophyll-deficient gene and morphological variations in Korean populations of maize (Zea mays)

We studied cultivated and naturalized Korean maize populations to determine the extent to which the chlorophylldeficient mutation and the phenotypic variations of two morphological characters (i.e., red coleoptiles and epicotyls, and the number of the first root hairs) are maintained. The frequency of the chlorophyll-deficient mutant gene (2.73% on average) was highly variable. Frequencies of red coleoptiles and epicotyls also were higher than expected from a mutation-selection balance. The average number of hairy phenotypes within populations was 1.8, ranging from 0.0 to 4.0. Naturalized populations were closely related to with cultivated communities. Most striking, however, was the more significant difference among populations than within populations with regard to both the frequency of chlorophyll-deficient mutant genes and the phenotypic variations of our two morphological characters. On a per-gene basis, the majority of the phenotypic variation (mean of 73.3%) resided among populations.     

The effects of aflatoxin B1 on immature germinating maize (Zea mays) embryos

Immature maize (Zea mays L.) embryos were treated with aflatoxin B₁ concentrations, ranging from 0.1 g ml^–1 to 25 g ml^–1. Below 5 g ml^–1 aflatoxin B₁, root and shoot elongation was not significantly inhibited. Ultrastructurally, root tip cells showed little deterioration, except a possible diffused clearing in mitochondria and plastids. As the toxin concentration was increased above 5 gml^–1, shoot, and particularly root elongation, was progressively inhibited. Associated with this, there was an apparent decrease in the ribosome population. Furthermore, membranes, particularly the vacuolar membrane, became abnormal and vacuolar distension occurred. At 20 and 25 g ml^–1, these effects were exacerbated, and mitochondria and plastid structure was disrupted. At these concentrations, there was evidence of a disruption in lipid metabolism. The results are discussed in the context of known aflatoxin effects on cellular control mechanisms and ultrastructure in animal systems.     

Phytoliths as indicators of prehistoric maize (Zea mays subsp.mays,Poaceae) cultivation

Maize (Zea mays L. subsp.mays) has been identified in archaeological contexts by a high proportion of large cross-shaped phytoliths. Given the numerous races of maize, this study was undertaken to determine if differences below the species level could be noted. It was also designed to see if phytoliths differed in various plant parts at various stages of growth. Several races were grown under experimental conditions. No significant differences were found. Furthermore, few phytoliths alleged to be diagnostic of maize were discovered. Systemic studies of maize and analyses of prehistoric cultivation by means of phytoliths seem not to be as promising as some researchers have argued.     

Low-P tolerance by maize (Zea mays L.) genotypes: Significance of root growth,and organic acids and acid phosphatase root exudation

We investigated some mechanisms, which allow maize genotypes to adapt to soils which are low in available P. Dry matter production, root/shoot-ratio, root length and root exudation of organic acids and acid phosphatase were investigated in four maize genotypes grown under P-deficient and P-sufficient conditions in sterile hydroponic culture. A low-P tolerant, an acid-tolerant and a low-P susceptible genotype of maize were compared with a Swiss commercial cultivar. The study found increased root development and increased exudation of acid phosphatase under P-deficient conditions in all maize genotypes, except for the Swiss cultivar. Effects on root formation and acid phosphatase were greater for the low-P tolerant than for the low-P susceptible, and the acid soil tolerant genotypes. Organic acid contents in root tissues were increased under P deficiency and related to increased PEPC activity. However, the increase in contents was associated with an increase in exudation for the low-P tolerant genotype only. The low-P susceptible genotype was characterized by high organic acid content in roots and low organic acid exudation. The organic acids content in the phloem exudates of shoots was related to root exudation under different P supply, to the difference between lines in organic acids root content, but not to the low-P tolerance or susceptibility of maize genotypes.     

Regulation of flower development in cultured ears of maize (Zea mays L.)

Summary Young ears of maize were cultured in two different liquid media containing either kinetin (KN) or kinetin + gibberellic acid (KN + GA₃) in order to manipulate stamen and gynoecium development. In KN medium, stamens developed and gynoecia aborted in the flowers of the cultured immature ears. In the KN + GA₃ medium, however, ovaries with silks developed and stamens aborted. These differential morphological events were recorded with SEM photomicrographs at regular intervals after excision of ear inflorescences. In addition, the mitotic activity in the developing or aborting organs was determined over a 75-h period. It increased from 6% to 14% in developing organs (i.e. stamens in KN medium, and gynoecia in KN + GA₃ medium) and gradually decreased to 1% in the degenerating organs (i.e. gynoecia in KN medium, and stamens in KN + GA₃ medium) by 45 h of culture. The mitotic activity reached zero in degenerating flower organs by 75 h of culture. Whether these differential sensitivities to the exogenously applied members of these two plant growth regulator classes are unique to our in vitro system or reflect a more general control feature of in vivo inflorescences must await further clarification.     

Effects of organic acid fractions extracted from Eucalyptus camaldulensis leaves on root elongation of maize (Zea mays) in the presence and absence of aluminium

Complexes of aluminium (Al) with organic ligands are believed to represent an important detoxification mechanism in acid soils. However, relatively little is known about the particular ligands produced by decomposing vegetation or about their effects on plant growth in the presence or absence of toxic Al. This paper reports an experiment on the effects of decomposition products of Eucalyptus camaldulensis leaves on the root elongation of maize (Zea mays) cv. DK687 in the presence or absence of Al. The static solution culture experiment used fulvic acid (FA) and humic acid (HA), extracted from E. camaldulensis leaves, at three nominal concentrations, viz. 40, 120 and 360 mg C L^-1, replicated 4 times in the presence and absence of 30 µM Al. In the absence of Al, root elongation was increased by 30% by HA at 40 mg C L^-1 and by 36% by FA at 120 mg C L^-1. In the presence of 30 µM Al, the effects of toxic Al on root elongation were negated by FA and HA at all concentrations. Aluminium was totally complexed in all treatments except FA at 40 mg C L^-1 in which treatment only 2.7 µM Al was present in the monomeric form. The E. camaldulensis FA and HA at concentrations of 40 and 120 mg C L^-1, either in the presence or absence of Al, stimulated maize root elongation. Aluminium was strongly complexed by the E. camaldulensis FA and HA. The present results, in which FA and HA alleviated Al toxicity limitations on root elongation of maize, are relevant to the protection afforded to plant growth in acid soils amended with organic materials. They highlight the need to focus more on the role of FA and HA.