Within the angiosperms, major differences in root architecture between dicotyledonous and monocotyledonous plants exist. We are working on the genetic and molecular mechanisms involved in the development of the root system in major crop species.
Because of the different roles the root system plays in overall plant growth, root architecture is a fundamental aspect of plant growth and development. The root system especially acquires water and nutrients from the soil, anchors the plant in the substrate, synthesizes hormones and metabolites, interacts with symbiotic microorganisms, and insures storage functions. In light of these characteristics, more and more breeders focus their attention on this underground organ in order to increase yield. This requires a better understanding of the relation of this part of the plant with the environment and of its highly adaptive behavior (Lynch, 2007; Gewin, 2010; Den Herder et al., 2010). Within the angiosperms, major differences in root architecture between dicotyledonous and monocotyledonous plants exist. Dicots develop a tap root system composed of a main primary root, already formed during embryogenesis, which grows vertically into the soil and gives rise to the emergence of numerous lateral roots extending the surface area. Monocots have a fibrous root system in which the embryonic primary root is only important for the early development of the plant (Feix et al., 2002) and in which an extensive postembryonic shoot-born root system is formed later on. Very little is known about the genetic and molecular mechanisms involved in the development and architecture of the root system in major crop species, generally monocotyledonous plants. Lack of insight is certainly a consequence of the difficulty to access and observe this organ in its natural habitat, namely the soil. Moreover, and probably because of this hidden character, the root has been neglected for a long time in crop improvement and in agricultural approaches aiming at increasing shoot biomass. Nevertheless, while most of the work has been done on Arabidopsis thaliana, the awareness of the importance of the root system in modulating plant growth, together with progress in sequencing and new molecular techniques, has caused renewed interest in understanding molecular mechanisms in crop species (Hochholdinger and Zimmermann, 2008; Coudert et al., 2010; Parizot et al., 2012).
Auxin and Pericycle
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. Phloempole 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.
Lateral root formation in maize. Transversal section through a developing lateral root primordium, visualized with Feulgen stain and toluidine blue dye. Pc, pericycle; X, xylem; asterisk, phloem. Scale bar, 50 um. (Jansen et al., 2012).
Auxin response in the maize root visualized by DR5::RFP. (a) Longitudinal section through the root tip. (g) An auxin response maximum is formed at the tip of the growing primordium. Scale bars: (a) 200 mm; (g) 100 mm. (Jansen et al., 2012).
Genomics of root development.Root Genomics and Soil Interactions.
Parizot B, Beeckman T (2012) John Wiley & Sons, pp 3–28. [free full text]
Inducible System for Lateral Roots in Arabidopsis thaliana and Maize,
Jansen L, Parizot B, Beeckman T (2013) in Plant Organogenesis: Methods and Protocols, Ive De Smet (ed.), Methods in Molecular Biology, vol. 959, pp. 149-158.
Phloem-associated auxin response maxima determine radial positioning of lateral roots in maize.
Jansen L, Roberts I, Rycke RD, Beeckman T (2012) Phil Trans R Soc B 367: 1525–1533. [free full text]
Analyzing Lateral Root Development: How to Move Forward.
De Smet I, White PJ, Bengough AG, Dupuy L, Parizot B, Casimiro I, Heidstra R, Laskowski M, Lepetit M, Hochholdinger F, et al (2012) Plant Cell 24: 15–20. [full text]
Developmental Biology of Roots: One Common Pathway for All Angiosperms?
Grunewald W, Parizot B, Inzé D, Gheysen G, Beeckman T (2007) Int J Plant Dev Biol 1: 212–225. [free full text]