Lateral root development in the maize (Zea mays) lateral rootless1 mutant

Background and Aims

The maize lrt1 (lateral rootless1) mutant is impaired in its development of lateral roots during early post-embryonic development. The aim of this study was to characterize, in detail, the influences that the mutation exerts on lateral root initiation and the subsequent developments, as well as to describe the behaviour of the entire plant under variable environmental conditions.

Methods

Mutant lrt1 plants were cultivated under different conditions of hydroponics, and in between sheets of moist paper. Cleared whole mounts and anatomical sections were used in combination with both selected staining procedures and histochemical tests to follow root development. Root surface permeability tests and the biochemical quantification of lignin were performed to complement the structural data.

Key Results

The data presented suggest a redefinition of lrt1 function in lateral roots as a promoter of later development; however, neither the complete absence of lateral roots nor the frequency of their initiation is linked to lrt1 function. The developmental effects of lrt1 are under strong environmental influences. Mutant primordia are affected in structure, growth and emergence; and the majority of primordia terminate their growth during this last step, or shortly thereafter. The lateral roots are impaired in the maintenance of the root apical meristem. The primary root shows disturbances in the organization of both epidermal and subepidermal layers. The lrt1-related cell-wall modifications include: lignification in peripheral layers, the deposition of polyphenolic substances and a higher activity of peroxidase.

Conclusions

The present study provides novel insights into the function of the lrt1 gene in root system development. The lrt1 gene participates in the spatial distribution of initiation, but not in its frequency. Later, the development of lateral roots is strongly affected. The effect of the lrt1 mutation is not as obvious in the primary root, with no influences observed on the root apical meristem structure and maintenance; however, development of the epidermis and cortex are impaired.     

Redundant segments inZea mays detected by translocations of monoploid origin

Twenty-two independently occurring spontaneous reciprocal translocations were isolated from monoploid X diploid crosses in maize and their breakpoints were determined. As 12 of the translocations involved the same two chromosomes and had breakpoints at approximately the same positions (6L. 2–3, 7L. 2–3) and two other translocations appeared to be identical with breakpoints at 2L. 9, 6L. 4, 14 of the 22 translocations probably arose by crossing over within duplicate segments of nonhomologous chromosomes. Thus, at least part of the bivalents seen at diakinesis and chromatid bridges seen at anaphase I in monoploid plants appear to be generated by recombination between redundant chromosome segments. The other eight translocations each occurred once. Because our evidence indicates that recombination between nonhomologous illegitimately synapsed chromosome segments does not occur in maize, these were probably also produced by recombination between redundant segments. If one assumes that their breakpoints also mark regions of interchromosomal redundancy, other conclusions can be reached: A) corn does not contain detectable homoeologous chromosomes, thus it is precently a true diploid, and B) as exchanges giving rise to translocations did not occur in the centromeres or proximal heterochromatin, these regions either do not possess redundancy or are rarely involved in chiasma formation. Furthermore, the duplicated segments in the genome giving rise to translocations in haploid microsporocytes probably have the same serial order with respect to the centromere.This work was partially supported by U.S. Atomic Energy Commission Contract AT(11-1)-2121.     

An evaluation of the stoichiometry ofin vitro nitrate assimilation inZea mays

Summary Reported non-stoichiometry in the reduction of nitrate to nitrite was investigated in leaves ofZea mays L.. The nitrogen balance sheet forin vitro nitrate assimilation was influenced by enzyme protectants in the extraction media and by the method employed to terminate the reaction. A number of limitations were found in the generally acceptedin vitro nitrate reductase assay, in particular the presence of endogenous components which interfered with the assay of nitrite were considered. A stoichiometric balance for thein vitro reduction of nitrate to nitrite was obtained when interfering factors were minimized. The absence of back reactions from ammonia in the assay was confirmed.     

Auxin causes oscillations of cytosolic free calcium and pH inZea mays coleoptiles

In epidermal cells of maize (Zea mays L.) coleoptiles, cytosolic pH (pH_c), cytosolic free calcium, membrane potential and changes thereof were monitored continuously and simultaneously (pH_c/, _m, Ca²⁺/ _m) using double-barrelled ion-sensitive microelectrodes. In the resting cells the cytosolic pH was 7.3–7.5 and the concentration of free calcium was 119±24 nM. One-micromolar indole-3-acetic acid (IAA), added to the external medium at pH 6.0 triggered oscillations in _m, pH_c and free calcium with a period of 20 to 30 min. Acidification of the cytosolic pH increased the cytosolic free calcium. The _m oscillations are attributed to changes in activity of the H⁺-extrusion pump at the plasmalemma, triggered off by pH and controlled by pH regulation (pH oscillation). The origin of the pH_c and Ca²⁺ changes remains unclear, but is possibly caused by auxin-receptor-induced lipid breakdown and subsequent second-messenger formation. It is suggested that the observed cytosolic pH and Ca²⁺ changes are intrinsically interrelated, and it is concluded that this onset of regulatory processes through the phytohormone IAA is indicative of calcium and protons mediating early auxin action in maize coleoptiles. It is further concluded that the double-barrelled ion-sensitive microelectrode is an invaluable tool for investigating in-vivo hormone action in plant tissues.Abbreviations and symbols FC fusicoccin - IAA indole-3-acetic acid - Mes 2-(N-morpholino)ethanesulfonic acid - pH_c cytosolic pH - Tris 2-amino-2-(hydroxymethyl)-1,3-propanediol - _m membrane potential difference (mV)     

Phaseolus vulgaris RbohB functions in lateral root development

Respiratory burst oxidase homologs (RBOHs) catalyze the reduction of oxygen to generate superoxide anion, a kind of reactive oxygen species (ROS). The ROS produced by RBOHs play essential roles in diverse processes, such as root hair development, stomata closure and signaling mechanisms in response to abiotic stimuli and during plant-pathogen interactions. Recently, we found that PvRbohB silencing in transgenic Phaseolus vulgaris roots had a negative impact on lateral root density. In this work, we show that the downregulation of PvRbohB affects both the growth and ROS levels in recently emerged lateral roots. In addition, we found that the PvRbohB promoter was activated during lateral root primordium initiation in the pericycle, and remained active throughout lateral root development. This study identifies RBOHs as potentially important players in lateral root development in P. vulgaris.     

Form matters: morphological aspects of lateral root development

Background

The crucial role of roots in plant nutrition, and consequently in plant productivity, is a strong motivation to study the growth and functioning of various aspects of the root system. Numerous studies on lateral roots, as a major determinant of the root system architecture, mostly focus on the physiological and molecular bases of developmental processes. Unfortunately, little attention is paid either to the morphological changes accompanying the formation of a lateral root or to morphological defects occurring in lateral root primordia. The latter are observed in some mutants and occasionally in wild-type plants, but may also result from application of external factors.

Scope and Conclusions

In this review various morphological aspects of lateral branching in roots are analysed. Morphological events occurring during the formation of a typical lateral root are described. This process involves dramatic changes in the geometry of the developing organ that at early stages are associated with oblique cell divisions, leading to breaking of the symmetry of the cell pattern. Several types of defects in the morphology of primordia are indicated and described. Computer simulations show that some of these defects may result from an unstable field of growth rates. Significant changes in both primary and lateral root morphology may also be a consequence of various mutations, some of which are auxin-related. Examples reported in the literature are considered. Finally, lateral root formation is discussed in terms of mechanics. In this approach the primordium is considered as a physical object undergoing deformation and is characterized by specific mechanical properties.     

Heterogeneity in G2 duration during lateral root development

Developing lateral roots of V. faba were treated with 5-aminouracil for up to 6 hours using the 5-AU inhibition method discussed in this paper; the duration of G₂+mitosis/2 and the percentages of slow dividing cells were estimated from the fall in MI observed in just emerged meristems, very large primordia and large primordia. The results indicate that during the period of development studied here there are two subpopulations of dividing cells: 1) fast dividing population which makes up about 84 % of the dividing cells and which has a G₂+mitosis/2 duration of about 3.3 hours, and 2) a slow dividing population which constitutes about 16 % of the dividing cells and which has a G₂ duration in excess of 12 hours. This heterogeneity is discussed in relationship to the behaviour of different populations of proliferating cells during root morphogenesis.     

Proportionality of ELF electric field-induced growth inhibition to induced membrane potential inZea mays andVicia faba roots

Summary The postulate that 60-Hz electric field-induced bioeffect severity is proportional to induced transmembrane potential [ ] magnitude was tested and supported using a plant root model cell system. Statistically significant correlations were obtained upon regression of the relative rates of exposedVicia faba andZea mays root segment growth on the average (calculated) arising in those segments under specified 60 Hz field exposure conditions. The associated with the apparent threshold for growth inhibition was similar inZea andVicia roots (2.5 vs 2.4 mV, respectively). At greater than this threshold,Zea root growth declined by about 9% per mV, andVicia root growth by about 19% per mV induced potential.Abbreviations EF electric field - RGR relative growth rate - RSGR relative segmental growth rate - induced membrane potential - segmental-average induced membrane potential - VC _d region of root tip in which a complete, nascent vascular cylinder is first distinguishable in histological sections     

Antifungal compounds from the root and root exudate of Zea mays

A maize plant (Zea mays) planted in a test tube was found to inhibit the growth of the soil-borne plant pathogen, Fusarium oxysporum f. sp. melongenae. The antifungal compounds, 6-methoxybenzoxazolinone and 6,7-dimethoxybenzoxazolinone, were isolated from an ethanol extract of Zea mays roots, and (6R)-7,8-dihydro-3-oxo-alpha-ionone and (6R,9R)-7,8-dihydro-3-oxo-alpha-ionol were isolated from the root exudate.     

Comparative proteome analyses of maize (Zea mays L.) primary roots prior to lateral root initiation reveal differential protein expression in the lateral root initiation mutant rum1

The embryonically preformed primary root is the first root type of maize that emerges after germination. In this study the abundant soluble proteins of 2.5-day-old primary roots of wild-type and lateral root mutant rum1 seedlings were compared before the initiation of lateral roots. In CBB-stained 2-D gels, among 350 detected proteins 14 were identified as differentially accumulated (>twofold change; t-test: 95% significance) in wild-type versus rum1 primary roots. These proteins which were identified via ESI MS/MS are encoded by 12 different genes. Functionally, these proteins are involved in lignin biosynthesis, defense, and the citrate cycle. Nine of these genes were further analyzed at the RNA expression level. This study represents the first comparative proteomic analysis of maize primary roots prior to lateral root initiation and will contribute to a better understanding of the molecular basis of root development in cereals.     

Manifestation of heterosis during early maize (Zea mays L.) root development

Heterosis is typically detected in adult hybrid plants as increased yield or vigor compared to their parental inbred lines. Only little is known about the manifestation of heterosis during early postembryonic development. Objective of this study was to identify heterotic traits during early maize root development. Four German inbred lines of the flint (UH002 and UH005) and dent (UH250 and UH301) pool and the 12 reciprocal hybrids generated from these inbred lines were subjected to a morphological and histological analysis during early root development. Primary root length and width were measured daily in a time course between 3 and 7 days after germination (DAG) and displayed average midparent heterosis (MPH) of 17–25% and 1–7%, respectively. Longitudinal size of cortical cells in primary roots was determined 5 DAG and displayed on average 24% MPH thus demonstrating that enlarged primary roots of hybrids can mainly be attributed to elongated cortical cells. The number of seminal roots determined 14 DAG showed on average 18% MPH. Lateral root density of all tested hybrids was determined 5 DAG. This root trait showed the highest degree of heterosis with an average MPH value of 51%. This study demonstrated that heterosis is already manifesting during the very early stages of root development a few days after germination. The young root system is therefore a suitable model for subsequent molecular studies of the early stages of heterosis manifestation during seedling development.     

Genome-wide association analysis of seedling root development in maize (Zea mays L.)

Background

Plants rely on the root system for anchorage to the ground and the acquisition and absorption of nutrients critical to sustaining productivity. A genome wide association analysis enables one to analyze allelic diversity of complex traits and identify superior alleles. 384 inbred lines from the Ames panel were genotyped with 681,257 single nucleotide polymorphism markers using Genotyping-by-Sequencing technology and 22 seedling root architecture traits were phenotyped.

Results

Utilizing both a general linear model and mixed linear model, a GWAS study was conducted identifying 268 marker trait associations (p ≤ 5.3×10^-7). Analysis of significant SNP markers for multiple traits showed that several were located within gene models with some SNP markers localized within regions of previously identified root quantitative trait loci. Gene model GRMZM2G153722 located on chromosome 4 contained nine significant markers. This predicted gene is expressed in roots and shoots.

Conclusion

This study identifies putatively associated SNP markers associated with root traits at the seedling stage. Some SNPs were located within or near (<1 kb) gene models. These gene models identify possible candidate genes involved in root development at the seedling stage. These and respective linked or functional markers could be targets for breeders for marker assisted selection of seedling root traits.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1226-9) contains supplementary material, which is available to authorized users.