Dichotomous Root Research Articles

Cellular and gene expression patterns associated with root bifurcation in Selaginella.

The roots of lycophytes branch through dichotomy or bifurcation, during which the root apex splits into two daughter roots. This is morphologically distinct from lateral root (LR) branching in the extant euphyllophytes, with LRs developing along the root axis at different distances from the apex. Although the process of root bifurcation is poorly understood, such knowledge can be important, because it may represent an evolutionarily ancient strategy that roots recruited to form new stem cells or meristems. In this study, we examined root bifurcation in the lycophyte Selaginella moellendorffii. We characterized an in vitro developmental time frame based on repetitive apex bifurcations, allowing us to sample different stages of dichotomous root branching and analyze the root meristem and root branching in S. moellendorffii at the microscopic and transcriptomic level. Our results showed that, in contrast to previous assumptions, initial cells (ICs) in the root meristem are mostly not tetrahedral but rather show an irregular shape. Tracking down the early stages of root branching argues for the occurrence of a symmetric division of the single IC, resulting in two apical stem cells that initiate root meristem bifurcation. Moreover, we generated a S. moellendorffii root branching transcriptome that resulted in the delineation of a subset of core meristem genes. The occurrence of multiple putative orthologs of meristem genes in this dataset suggests the presence of conserved pathways in the control of meristem and root stem cell establishment or maintenance.

Mutation of MEDIATOR 18 and chromate trigger twinning of the primary root meristem in Arabidopsis.

Plants adapt to soil injury and biotic stress via cell regeneration. In Arabidopsis, root tip damage by genotoxic agents, antibiotics, UV light and cutting induces a program that recovers the missing tissues through activation of stem cells and involves ethylene response factor 115 (ERF115), which triggers cell replenishment. Here, we show that mutation of the gene encoding an MED18 subunit of the transcriptional MEDIATOR complex and chromate [Cr(VI)], an environmental pollutant, synergistically trigger a developmental program that enables the splitting of the meristem in vivo to produce twin roots. Expression of the quiescent centre gene marker WOX5, auxin-inducible DR5:GFP reporter and the ERF115 factor traced the changes in cell identity during the conversion of single primary root meristems into twin roots and were induced in an MED18 and chromate-dependent manner during the root twinning events, which also required auxin redistribution and signalling mediated by IAA14/SOLITARY ROOT (SLR1). Splitting of the root meristem allowed dichotomous root branching in Arabidopsis, a poorly understood process in which stem cells may act to enable whole organ regeneration.

Multiple origins of dichotomous and lateral branching during root evolution.

Roots of extant vascular plants proliferate through lateral branching (euphyllophytes) or dichotomy (lycophytes)1-4. The origin of these distinct modes of branching was key for plant evolution because they enabled the development of structurally and functionally different root systems that supported a diversity of shoot systems3-6. It has been unclear when lateral branching originated and how many times it evolved4,7,8. Here, we report that many euphyllophytes that were extant during the Devonian and Carboniferous periods developed dichotomous roots. Our data indicate that dichotomous root branching evolved in both lycophytes and euphyllophytes. Lateral roots then evolved at different times in three major lineages of extant euphyllophytes-the lignophytes, ferns and horsetails. The multiple origins of dichotomous and lateral root branching are extreme cases of convergent evolution that occurred during the Devonian and Carboniferous periods when the land-plant flora underwent a radiation in morphological diversity.

Regional distribution patterns of wetland monocots with different root turnover strategies are associated with local variation in soil temperature.

Herbaceous perennial wetland monocots in Northern Ontario, Canada, show dichotomous root overwintering patterns, either with full senescence in autumn or survival over the winter, comparable to deciduous and evergreen leaf habits in trees. To test if these root strategies differ in their associations with growing season length, soil temperatures were recorded in autumn, winter and spring on 38 wetland sites with their dominant species being either of these two root overwintering strategies, altogether 19 monocot species. Traits associated with these strategies were assessed in garden experiments for a subset of these species. Sites with species with autumn-senescing roots were, on average, warmer than sites with species with overwintering roots. The 13 coldest sites were all sites of species with overwintering roots. Spring shoot growth in the field was delayed in species with autumn-senescing roots, despite their higher shoot relative growth rates in the garden, possibly due to the necessity to produce roots first. We conclude that species with autumn-senescing roots are more constrained by temperature than species with overwintering roots, limiting their occurrence in cool climates to locally warm soils and constraining their distribution towards the north. This is comparable to constraints on distribution of deciduous and evergreen trees.

Re-inventing ancient human DNA.

For a long time, the analysis of ancient human DNA represented one of the most controversial disciplines in an already controversial field of research. Scepticism in this field was only matched by the long-lasting controversy over the authenticity of ancient pathogen DNA. This ambiguous view on ancient human DNA had a dichotomous root. On the one hand, the interest in ancient human DNA is great because such studies touch on the history and evolution of our own species. On the other hand, because these studies are dealing with samples from our own species, results are easily compromised by contamination of the experiments with modern human DNA, which is ubiquitous in the environment. Consequently, some of the most disputed studies published - apart maybe from early reports on million year old dinosaur or amber DNA - reported DNA analyses from human subfossil remains. However, the development of so-called next- or second-generation sequencing (SGS) in 2005 and the technological advances associated with it have generated new confidence in the genetic study of ancient human remains. The ability to sequence shorter DNA fragments than with PCR amplification coupled to traditional Sanger sequencing, along with very high sequencing throughput have both reduced the risk of sequencing modern contamination and provided tools to evaluate the authenticity of DNA sequence data. The field is now rapidly developing, providing unprecedented insights into the evolution of our own species and past human population dynamics as well as the evolution and history of human pathogens and epidemics. Here, we review how recent technological improvements have rapidly transformed ancient human DNA research from a highly controversial subject to a central component of modern anthropological research. We also discuss potential future directions of ancient human DNA research.

Conserved transport mechanisms but distinct auxin responses govern shoot patterning in Selaginella kraussiana

To provide a comparative framework to understand the evolution of auxin regulation in vascular plants, the effect of perturbed auxin homeostasis was examined in the lycophyte Selaginella kraussiana. Polar auxin transport was measured by tracing tritiated IAA in excised shoots. Shoots were cultured in the presence of auxin efflux inhibitors and exogenous auxin, and developmental abnormalities were documented. Auxin transport in Selaginella shoots is exclusively basipetal, as in angiosperms. Perturbed auxin transport results in the loss of meristem maintenance and abnormal shoot architecture. Dichotomous root branching in Selaginella appears to be regulated by an antagonistic relationship between auxin and cytokinin. The results suggest that basipetal polar auxin transport occurred in the common ancestor of lycophytes and euphyllophytes. Although the mechanisms of auxin transport appear to be conserved across all vascular plants, distinct auxin responses govern shoot growth and development in lycophytes and euphyllophytes.

Simulating form and function of root systems: efficiency of nitrate uptake is dependent on root system architecture and the spatial and temporal variability of nitrate supply

1 Root-system architecture, and plastic variation in architecture and physiological function, influence the capacity of plants to acquire nutrients from non-uniform soil. Previous theoretical analyses of the relationship between root architecture and nutrient acquisition have largely assumed uniform soils and unresponsive root systems. We extend these studies by considering non-uniform nutrient supply and plasticity in root growth and uptake physiology. 2 Using modelling, we investigated the growth and nitrate uptake of two extreme theoretical architectural types: dichotomous (highly branched) and herringbone (sparsely branched). Root systems with plastic or non-plastic root-growth and nitrate-uptake responses, supplied with non-uniform distributions of soil nitrate, were simulated. 3 The simulated herringbone root system had a higher nitrate-uptake efficiency (NUE, nitrate-N m−3 soil) when supply varied spatially and temporally (compared with variation in space alone), and NUE was affected only by the capacity to elevate uptake kinetics locally. In contrast, the efficiency of the dichotomous root system decreased under spatially and temporally dynamic nitrate supply (compared with static supply), and was influenced only by the capacity to proliferate roots in nitrate patches. 4 These results suggest that root-system NUE is not solely a function of the ion-transport characteristics of soil, but is also influenced by the transitory nature of the nutrient source and the structure of the root system.

Root system topology and diameter distribution of species from habitats differing in inundation frequency

Summary We compared the root systems of seven halophytic species that occur at different elevations on a salt marsh, in order to (i) test the hypothesis that variations in root system architecture reflect adaptation to inundation frequency or nitrogen limitation, and (ii) verify the theoretically predicted relationships between root diameter, link magnitude and root topology. Diameters and lengths of individual laterals were determined along root axes, and branching patterns were quantified by calculating a topological index (TI). Chenopodiaceae (annual dicots) showed that with increasing elevation, the branch density and length of individual first‐order laterals tended to increase, so that the relative length of the main axes decreased. Root branching of the Chenopodiaceae at lower elevations was herringbone‐like, whereas species from higher elevations had smaller TIs because their branching patterns were more complex. The Gramineae, too, showed a tendency to increased length of individual laterals with increasing elevation. However, TI was not related to elevation, did not indicate a herringbone structure for all species, and was within the same range of that of the Chenopodiaceae. As root topology of the Chenopodiaceae is related to elevation, but that of the grasses is not, topology is not necessarily an important adaptive trait in all plant families that inhabit the salt marsh. Short first‐ and second‐order laterals may represent a more general architectural adaptation to frequent inundation, with longer first‐order laterals being favourable to competition for nutrients. Diameters at the root base tended to decrease if root branching was herringbone‐like (TI close to 1). Roots of first‐order laterals were approximately one‐third of the diameter of the main axes; second‐order laterals were approximately half the diameter of the first‐order laterals. These ratios illustrate the value of using the developmental segment‐ordering system in describing roots. The theoretically predicted relationship between root diameter and link magnitude was not present within individual orders of roots, whereas diameter did slowly increase with magnitude when combining different root orders. In the absence of a clear relationship between root diameter and link magnitude, the predicted high carbon costs associated with herringbone root systems disappear, whereas the advantage of minimized inter‐root competition remains. Consequently, herringbone root systems will be most efficient in terms of nutrients gained per carbon invested. However, dichotomous root systems offer a greater potential for exploring the soil, which contributes to the potential competitiveness of plants growing in nutrient limited habitats.

Root system analysis of seedlings of seven tree species from a tropical dry forest in Mexico

Root attributes of tree seedlings of seven species from the tropical deciduous forest along the Pacific Coast of Mexico are described using morphometirc root system analysis. Mean relative growth rate, root/shoot ratios, specific root length, root density, mean number of roots tips and root length/leaf area ratio were determined in seedlings grown for 35 days inside growth chambers. All the species had low relative growth rates, low root/shoot ratios and low root densities (<0.5 cm/cm3). The species associated with disturbed habitats, in contrast to the species characteristic of undisturbed areas, presented small seeds, a dichotomous root branching pattern and large specific root length. No relationship was found between seed size and mean relative growth rate among the species studied.

Involvement of ethylene in ectomycorrhiza formation and dichotomous branching of roots of mugo pine seedlings

The role of ethylene in mycorrhiza formation and root development on axenically grown seedlings of Pinus mugo Turra var. mugo was examined. Mycorrhizal formation by Laccaria laccata and Pisolithus tinctorius in a defined liquid medium was associated with increased ethylene production. Ethephon (100 μM) stimulated dichotomous branching of roots inoculated with P. tinctorius, but had no effect on those inoculated with L. laccata, or on uninoculated roots. Ethephon had no effect on the percentage of susceptible roots that became mycorrhizal with either fungus. The inhibitor of ethylene action, silver thiosulfate, had no significant effect on mycorrhiza formation by P. tinctorius, but it did show a trend toward decreased mycorrhiza formation by L. laccata when applied at concentrations of 10 μM or higher. Silver thiosulfate at 100 or 500 μM slightly increased dichotomous root branching of seedlings inoculated with either fungus, but these concentrations also caused blackening of root meristems and inhibition of root growth. These results are consistent with the interpretation that endogenous ethylene may influence mycorrhiza formation and associated changes in root morphology.

The influence of 2,4‐D on ectomycorrhizal symbiosis in pine and spruce seedlings

Abstract In vitro and in vivo investigations with 2,4‐D concentrations showed that mycorrhizal fungi were suppressed particularly by higher concentrations while treated seedlings exhibited quantitative and qualitative deficiency in mycorrhizal development. Short roots became elongated or sometimes remained uninfected. Dichotomous root branching was absent in Pinus sylvestris L. The difference in terms of mycorrhizal impairment was highly significant (P = 0.01). Practical consequences are discussed.