lateral-root-inducible-system-in-arabidopsis-and-maize

Lateral Root Inducible System in <em>Arabidopsis</em> and Maize

Lateral root development contributes significantly to the root system, and hence is crucial for plant growth. The study of lateral root initiation is ...

Ghent

Ghent University

In Arabidopsis, lateral root initiation occurs in a subset of pericycle cells at the xylem pole that will divide asymmetrically to give rise to a new lateral root organ. While lateral roots never develop at the phloem pole, it is unclear how the interaction with xylem and phloem poles determines the distinct pericycle identities with different competences. Nevertheless, pericycle cells at these poles are marked by differences in size, by ultrastructural features and by specific proteins and gene expression. Here, we provide transcriptional evidence that pericycle cells are intimately associated with their vascular tissue instead of being a separate concentric layer. This has implications for the identification of cell- and tissue-specific promoters that are necessary to drive and/or alter gene expression locally, avoiding pleiotropic effects. We were able to identify a small set of genes that display specific expression in the phloem or xylem pole pericycle cells, and we were able to identify motifs that are likely to drive expression in either one of those tissues.

In silico analyses of pericycle cell populations reinforce their relation with associated vasculature in Arabidopsis.

In Arabidopsis thaliana, lateral-root-forming competence of pericycle cells is associated with their position at the xylem poles and depends on the establishment of protoxylem-localized auxin response maxima. In maize, our histological analyses revealed an interruption of the pericycle at the xylem poles, and confirmed the earlier reported proto-phloem-specific lateral root initiation. Phloem-pole pericycle cells were larger and had thinner cell walls compared with the other pericycle cells, highlighting the heterogeneous character of the maize root pericycle. A maize DR5::RFP marker line demonstrated the presence of auxin response maxima in differentiating xylem cells at the root tip and in cells surrounding the proto-phloem vessels. Chemical inhibition of auxin transport indicated that the establishment of the phloem-localized auxin response maxima is crucial for lateral root formation in maize, because in their absence, random divisions of pericycle and endodermis cells occurred, not resulting in organogenesis. These data hint at an evolutionarily conserved mechanism, in which the establishment of vascular auxin response maxima is required to trigger cells in the flanking outer tissue layer for lateral root initiation. It further indicates that lateral root initiation is not dependent on cellular specification or differentiation of the type of vascular tissue.

Phloem-associated auxin response maxima determine radial positioning of lateral roots in maize.

The developmental plasticity of the root system represents a key adaptive trait enabling plants to cope with abiotic stresses such as drought and is therefore important in the current context of global changes. Root branching through lateral root formation is an important component of the adaptability of the root system to its environment. Our understanding of the mechanisms controlling lateral root development has progressed tremendously in recent years through research in the model plant Arabidopsis thaliana (Arabidopsis). These studies have revealed that the phytohormone auxin acts as a common integrator to many endogenous and environmental signals regulating lateral root formation. Here, we review what has been learnt about the myriad roles of auxin during lateral root formation in Arabidopsis.

Lateral root development in Arabidopsis: fifty shades of auxin.

The root system is fundamental for plant development, is crucial for overall plant growth and is recently being recognized as the key for future crop productivity improvement. A major determinant of root system architecture is the initiation of lateral roots. While knowledge of the genetic and molecular mechanisms regulating lateral root initiation has mainly been achieved in the dicotyledonous plant Arabidopsis thaliana, only scarce data are available for major crop species, generally monocotyledonous plants. The existence of both similarities and differences at the morphological and anatomical level between plant species from both clades raises the question whether regulation of lateral root initiation may or may not be conserved through evolution. Here, we performed a targeted genome-wide transcriptome analysis during lateral root initiation both in primary and in adventitious roots of Zea mays and found evidence for the existence of common transcriptional regulation. Further, based on a comparative analysis with Arabidopsis transcriptome data, a core of genes putatively conserved across angiosperms could be identified. Therefore, it is plausible that common regulatory mechanisms for lateral root initiation are at play in maize and Arabidopsis, a finding that might encourage the extrapolation of knowledge obtained in Arabidopsis to crop species at the level of root system architecture.

Comparative transcriptomics as a tool for the identification of root branching genes in maize.

In Arabidopsis, more than 1000 putative small signalling peptides have been predicted, but very few have been functionally characterized. One class of small post-translationally modified signalling peptides is the C-TERMINALLY ENCODED PEPTIDE (CEP) family, of which one member has been shown to be involved in regulating root architecture. This work applied a bioinformatics approach to identify more members of the CEP family. It identified 10 additional members and revealed that this family only emerged in flowering plants and was absent from extant members of more primitive plants. The data suggest that the CEP proteins form two subgroups according to the CEP domain. This study further provides an overview of specific CEP expression patterns that offers a comprehensive framework to study the role of the CEP signalling peptides in plant development. For example, expression patterns point to a role in aboveground tissues which was corroborated by the analysis of transgenic lines with perturbed CEP levels. These results form the basis for further exploration of the mechanisms underlying this family of peptides and suggest their putative roles in distinct developmental events of higher plants.

Article	Year
In silico analyses of pericycle cell populations reinforce their relation with associated vasculature in Arabidopsis. Philosophical transactions of the Royal Society of London. Series B, Biological sciences\| PubMed ID: 22527390	2012
Phloem-associated auxin response maxima determine radial positioning of lateral roots in maize. Philosophical transactions of the Royal Society of London. Series B, Biological sciences\| PubMed ID: 22527395	2012
Lateral root development in Arabidopsis: fifty shades of auxin. Trends in plant science\| PubMed ID: 23701908	2013
Comparative transcriptomics as a tool for the identification of root branching genes in maize. Plant biotechnology journal\| PubMed ID: 23941360	2013
The CEP family in land plants: evolutionary analyses, expression studies, and role in Arabidopsis shoot development. Journal of experimental botany\| PubMed ID: 24179095	2013

Ianto Roberts

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