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.     

The quiescent center in roots of maize: initiation,maintenance and role in organization of the root apical meristem

Summary Using roots of maize, we tested the hypothesis that the origin and maintenance of the quiescent center (QC) are a consequence of polar auxin supply. Exposing roots to the polar auxin transport inhibitor 2,3,5-triiodobenzoic acid (TIBA), or to low temperature (4 °C, with subsequent return to 24 °C), enhances mitotic frequency within the QC. In both treatments, the QC most typically is activated at its distal face, and the protoderm/dermatogen undergoes several periclinal divisions. As a result, the root body penetrates and ruptures the root cap junction and the characteristic closed apical organization changes to open. A QC persists during these changes in apical organization, but it is diminished in size. The data from the TIBA-treated roots suggest a role for auxin in the origin and maintenance of the QC, and further, that alterations in QC dimensions are a consequence of polar auxin supply. We hypothesize that the root cap, and specifically the root cap initials, are important in regulating polar auxin movements towards the root apex, and hence are important in determining the status of the QC.Abbreviations QC quiescent center - TIBA 2,3,5-triiodobenzoic acid Dedicated to the memory of Professor John G. Torrey     

The de novo origin of the quiescent center regenerating root apices of Zea mays

Summary In roots from which the root cap and quiescent center have been removed new apical tissues regenerated in line with the main axis of the root. Regeneration of these tissues occurred from the region of the proximal meristem, which extends for no more than 350 m from the cut surface. Accompanying the regeneration of new apical tissues is a change in the architecture of the root apex and initiation and enlargement of a new quiescent center. A possible role for the quiescent center in the establishment of pattern at the apex is considered. Regeneration of the original apex failed to occur in those roots from which the root cap, quiescent center and proximal meristem were excised.Abbreviation QC quiescent center     

Tissue-specific subcellular immunolocalization of a myosin-like protein in maize root apices

Summary Using a heterologous myosin antibody raised against the whole molecule of bovine muscle myosin, we have identified a myosin-like protein in maize. Immunoblots of subcellular fractions isolated from roots identified one distinct band at about 210 kDa in the microsomal protein fraction and one band at about 180 kDa in the soluble protein fraction. Indirect immunofluorescence was performed using maize root apex sections to reveal endocellular distributions of the myosin-like protein. Both diffuse and particulate labelling patterns were observed throughout the cytoplasm of all root cells. In mitotic cells, myosin-like protein was excluded from spindle regions. Amyloplast surfaces were labelled prominently in cells of the root cap statenchyma and in all root cortex cells. On the other hand, myosin-like protein was prominently enriched at cellular peripheries in cells of the pericycle and outer stele in the form of continuous peripheral labelling. From all root apex tissues, phloem elements showed the most abundant presence of myosinlike protein.Abbreviations AFs actin filaments - MTs microtubules Dedicated to Professor Walter Gustav Url on the occasion of his 70th birthday     

Modeling the formation of root apices

The formation of new root apices from small groups of cells with different cellular patterns has been simulated using an existing model based on growth tensors. To generate an apex, a steady growth field was used. The pattern of cells evolved to approach the steady state. Two extreme types of progressions have been obtained : one leading to an apex with a single or a few apical cells, and the other to an apex with a quiescent centre. The change of structure while applying a steady growth tensor indicates that development may involve a succession of discrete growth tensors.     

RNA and protein synthesis in sperm cells isolated from Zea mays L. pollen

Summary Sperm cells are thought to be quiescent in pollen and activated upon pollen germination. To test this hypothesis, protein, RNA and DNA synthesis were assessed in Zea mays sperm cells at different times after isolation from pollen. Protein synthesis changed with time; while some proteins were found to be constitutive in both 0 and 24 h cells, others were synthesized and some disappeared. Overall, the number of proteins detected at 24 h doubled compared with freshly isolated cells. Incorporation of [³H]leucine in 24 h cells was about 50 times that in freshly isolated cells, and that of [5, 6-³H]uridine, about 7 times. Very low incorporation of [6-³H]thymidine into the cells was detected; there was no difference between freshly isolated and 24 h cells. It is possible that the differences in synthetic activity between freshly isolated and 24-h-old cells might correspond to sperm cell activation during pollen tube growth. If so, these metabolic changes may play an important role in fertilization.Supported by funds from a Strategic Grant (D.D.C.) and an Operating Grant (D.J.G.) from the Natural Sciences and Engineering Research Council of Canada     

Shoot and root growth of hydroponic maize (Zea mays L.) as influenced by K deficiency

Potassium (K) has major biophysical and biochemical functions in plant physiology. However, plant responses to K deficiency at the whole plant level are not always clearly related to these well-known functions of K at the cellular level. The objective of this study was to investigate the morphological response of maize to increasing K deficiency and test to what extent this morphological response can be interpreted in the light of the simple model proposed by Leigh and Wyn Jones, suggesting that biophysical functions are affected first. Maize was grown in a greenhouse under hydroponic conditions. For half of the plants, K was removed from the nutrient solution from the 4th visible leaf stage. The K content in the starved plants dropped from 100 to 30 mM, and was not fully compensated by an increase in other cations. Leaf elongation rates were reduced on K-deprived plants, whereas axile root elongation rates were slightly increased between 45°C days and 75°C days after starvation, and reduced thereafter. During the first part of the starvation period, i.e. under moderate K deficiency (K concentration above 40 mM), all measured variables suggest that the whole plant response may be interpreted as the consequence of the reduced leaf growth, probably due to insufficient turgor pressure or cell-wall extensibility. This general pattern of response is in agreement with the model of Leigh and Wyn Jones. However, during the second part of the starvation period, i.e. under more severe K deficiency (K concentration below 40 mM), malfunction of additional physiological processes (mostly related to biochemical functions like photosynthetic processes) must be considered to explain the plant morphological response.     

Relationship between gene expression and hybrid vigor in primary root tips of young maize (Zea mays L.) plantlets

Summary To provide an insight into the molecular basis of heterosis, we investigated gene expression in primary root tips of a heterotic maize hybrid (B73 × Mo17) and its parental lines (B73 and Mo17). This analysis was carried out (i) by differential plaque hybridization of a recombinant cDNA library made to poly(A) RNA isolated from B73 × Mo17 primary root tips, and (ii) by comparing with two-dimensional gel electrophoresis proteins synthesized in vitro in the rabbit reticulocyte system by poly(A) RNA isolated, at different stages of development, from the three genotypes. The results showed that there are sets of proteins and mRNAs that are differentially synthesized and expressed in the F₁ primary root tips in comparison to the parental lines. Moreover, results from the survey of 21 major in-vitrosynthesized polypeptide variants, from mRNAs of primary root tips of the parental lines and their F₁ hybrid, indicated that in seven instances hybrid proteins translated in vitro were more abundant or possibly new. In most of the remaining cases, hybrid spots were similar in intensity to the same protein produced by one of the two parental lines.     

Turnover and distribution of root exudates of Zea mays

Decomposition and distribution of root exudates of Zea mays L. were studied by means of ¹⁴CO₂ pulse labeling of shoots on a loamy Haplic Luvisol. Plants were grown in two-compartment pots, where the lower part was separated from the roots by monofilament gauze. Root hairs, but not roots, penetrated through the gauze into the lower part of the soil. The root-free soil in the lower compartment was either sterilized with cycloheximide and streptomycin or remained non-sterile. In order to investigate exudate distribution, 3 days after the ¹⁴C labeling, the lower soil part was frozen and sliced into 15, one-mm thick layers using a microtome. Cumulative ¹⁴CO₂ efflux from the soil during the first 3 days after ¹⁴C pulse labeling did not change during plant growth and amounted to about 13–20% of the total recovered ¹⁴C (41–55% of the carbon translocated below ground). Nighttime rate of total CO₂ efflux was 1.5 times lower than during daytime because of tight coupling of exudation with photosynthesis intensity. The average CO₂ efflux from the soil with Zea mays was about 74 g C g^–1 day^–1 (22 g C m^–2 day^–1), although, the contribution of plant roots to the total CO₂ efflux from the soil was about 78%, and only 22% was respired from the soil organic matter. Zea mays transferred about 4 g m^–2 of carbon under ground during 26 days of growth. Three zones of exudate concentrations were identified from the distribution of the ¹⁴C-activity in rhizosphere profiles after two labeling periods: (1) 1–2 (3) mm (maximal concentration of exudates) 2) 3–5 mm (presence of exudates is caused by their diffusion from the zone 1); (3) 6–10 mm (very insignificant amounts of exudates diffused from the previous zones). At the distance further than 10 mm no exudates were found. The calculated coefficient of exudate diffusion in the soil was 1.9 × 10^–7 cm² s^–1.     

Origin of the quiescent center in the root of Capsella bursa-pastoris (L.) Medik

The origin of the quiescent center in the embryonic radicle of Capsella bursa-pastoris was investigated by in-situ hybridization to cellular polyadenylic-acid-containing RNA using [³H]polyuridylic acid as a probe. In the globular embryo, autoradiographic silver grains were localized in all cells of the presumptive root apex except in the hypophysis. As the inner cell formed by a transverse division of the hypophysis cut off new cells toward the central procambial cylinder of the embryo, these cells remained characteristically unlabeled, in contrast to the labeled cells of the rest of the embryo. In the embryonic radicles of mature seeds and of seedlings, cells derived from the hypophysis appeared as a nonmeristematic, unlabeled, hemispherical group, bounded by the procambium to the inside and the root epidermis to the outside. When root tips excised from 2-d-old seedlings were incubated in [methyl-³H]thymidine, sectioned, and autoradiographed, cells derived from the inner cell of the hypophysis were found to be unlabeled, thus showing that they constitute the specific cells of the quiescent center. These results present evidence for the single-cell origin of the quiescent center in an angiosperm root and a role for the hypophysis in it.Abbreviations poly(A)⁺RNA polyadenylicacid-containing RNA - [³H]poly(U) [³H]polyuridylic acid - QC quiescent center This work was supported in part by National Science Foundation grants PCM-7902898 and DCB-8709092.     

Occurrence of spermine in chromatin of Zea mays

Chromatin prepared from maize shoot tips using as extraction medium including quinacrine as an inhibitor of polyamine oxidase, contained 1.6 pmol spermidine g DNA^-1 and 14.8 pmol spermine g DNA^-1, respectively. This represented 0.1% spermidine and 3.7% spermine as compared with the content of those amines in the whole tissue. No putrescine was detectable in the chromatin preparation. When contamination of polyamines in the preparation was determined by the addition of labeled polyamines to the extraction medium, the ratio of the polyamines in the preparation to those in the extraction medium was 0.1% spermidine and 0.7% spermine, respectively. Spermine in the chromatin preparation was almost fully solubilized by a DNase-treatment, but spermidine was less easily solubilized. Most of the spermine associated with the chromation is chromatin-specific.     

Biochemical selection of immature,haploid embryos of Zea mays L.

Summary A method was devised for the biochemical selection of immature, haploid Zea mays embryos using Adh1 ^– and either the Stock 6 or indeterminate gametophyte (ig in W23) high haploid-inducing systems. Haploid (Adh1 ^–) embryos survived exposure to levels of allyl alcohol which killed diploid (Adh1 ⁺/Adh1 ^–) embryos. Of the total surviving embryos which were examined cytologically 15% (using ig) and 22% (using Stock 6) were haploid. In two experiments with Stock 6, 100% of the surviving embryos were haploid. To obtain maximum effectiveness of Stock 6 and ig, Adh1 ^– was transferred to stock 6 and W23 backgrounds. Immature, haploid embryos are being used to develop haploid, morphogenic tissue cultures of Zea mays.     

A comparison between minirhizotron and monolith sampling methods for measuring root growth of maize (Zea mays L.)

Transparent plastic minirhizotron tubes have been used to evaluate spatial and temporal growth activities of plant root systems. Root number was estimated from video recordings of roots intersecting minirhizotron tubes and of washed roots extracted from monoliths of the same soil profiles at the physiological maturity stage of a maize (Zea mays L.) crop. Root length was measured by the line intercept (LI) and computer image processing (CIP) methods from the monolith samples.There was a slight significant correlation (r=0.28, p<0.005) between the number of roots measured by minirhizotron and root lengths measured by the LI method, however, no correlation was found with the CIP method. Using a single regression line, root number was underestimated by the minirhizotron method at depths between 0–7.6 cm. A correlation was found between root length estimated by LI and CIP. The slope of estimated RLD was significant with depth for these two methods. Root length density (RLD) measured by CIP showed a more erratic decline with distance from the plant row and soil surface than the LI method.     

Induction of lytic enzymes by the interaction of Ustilago maydis with Zea mays tissues

The nonpathogenic (FB-2) and pathogenic (FB-D12) strains of Ustilago maydis were grown in medium supplemented with different carbon sources including monosaccharides, polysaccharides, and plant tissues. Both strains were able to grow on all substrates, with doubling times varying from 2 to 25 h depending on the carbon source. Plant tissues supplied as carbon source induced lytic enzymes differentially; pectate lyase and cellulase activities were induced preferentially by apical stem meristem in strain FB-D12, whereas leaves preferentially induced xylanase and cellulase activities in strain FB2. Stems induced polygalacturonase activity in both strains. All enzyme activities, except cellulase in the FB-D12 strain, were detected at a low level when U. maydis was grown on glucose. In planta, chlorosis and production of teliospores were paralleled by an increase in pectate lyase activity. Anthocyanin production and formation of galls and teliospores correlated with polygalacturonase expression whereas cellulase activity increased only during the stage of anthocyanin production and gall formation. Expression of xylanase activity coincided with the last stage of teliospore formation.     

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.     

Arylsulphatase activity of corn (Zea mays L.) seedling roots

The crude lysosomal fraction of corn seedling root tips contains an arylsulphatase (E.C. 3.1.6.1) which hydrolysed p-nitrophenyl sulphate at pH 8.0 but had no activity towards p-nitrocatechol sulphate. The Km value for p-nitrophenyl sulphate was 1.24 mM. The hydrolysis of p-nitrophenyl sulphate was linear up to 2 h and the rate was proportional to the amount of enzyme added. The enzyme was strongly inhibited by cyanide, fluoride and phosphate ions and did not resemble the arylsulphatases of bacterial and animal origin.     

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.     

The effect of auxins (IAA and 4-Cl-IAA) on the redox activity and medium pH of Zea mays L. root segments

Indole-3-acetic acid (IAA) and 4-chloroindole-3-acetic acid (4-Cl-IAA) were tested at different concentrations and times for their capacity to change the redox activity and medium pH of maize root segments. The dose-response surfaces (dose-response curves as a function of time) plotted for redox activity and changes in medium pH (expressed as ΔpH) had a similar shape for both auxins, but differed significantly at the optimal concentrations. With 4-Cl-IAA, the maximal values of redox activity and medium pH changes were observed at 10⁻¹⁰ M, which was a 100-fold lower concentration than with IAA. Correlations were observed between redox activity and medium pH changes at the optimal concentrations of both IAA and 4-Cl-IAA. The results are discussed herein, taking into account both the concentration of the auxins and the effects produced by them.