Rice A Model Plant to Decipher the Hidden Origin of Adventitious Roots

Authored by: Yoan Coudert , Van Anh Le Thi , Pascal Gantet

Plant Roots

Print publication date:  April  2013
Online publication date:  April  2013

Print ISBN: 9781439846483
eBook ISBN: 9781439846490
Adobe ISBN:

10.1201/b14550-12

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Abstract

In flowering plants, the primary root system is generated by the root apical meristem (RAM), a meristem that is formed during embryogenesis. The RAM will generate the seminal root (SR), which can branch to generate postembryonic lateral roots (LRs). These LRs can branch to generate other LRs, which result in additional branching levels. Some plant species can also generate other postembryonic roots, called adventitious roots (ARs), from the stem. Because ARs generally develop from aerial axes, they are easily observable until they reach the ground. Paradoxically, their cellular origin and the mechanisms controlling their formation remain hidden and are much less known than those governing the genesis of underground LR. In most monocot species, ARs develop constitutively and constitute the main part of the fibrous root system. In dicots, ARs can also develop constitutively or in response to abiotic or biotic stresses conferring the plant some adaptive advantage (Barlow 1986). The induction of AR formation in response to biotic stress is illustrated by the induction of AR formation from any part of the plant by the pathogenic bacterium Agrobacterium rhizogenes through the transfer of genes from the bacterial genome that modify hormonal balance of the plant (Figure 9.1A). These ARs are agravitropic, highly branched, and fast growing and synthesize compounds that sustain the bacteria. These roots can be isolated and easily cultured in vitro because they do not require any hormonal supply to maintain their growth. For this reason they are used as an artificial source of valuable secondary metabolites that are naturally synthesized by roots of medicinal plants (Guillon et al. 2006a,b). Some phytopathogenic fungi or viruses are also known to promote the formation of AR after infection of their host; however, the biological significance of this morphological response is not always clear (Dimond and Waggoner 1953; Rasa and Esau 1961). The AR of Sesbania sp. can develop nitrogen-fixing nodules when infected by symbiotic Rhizobium sp. bacteria (Duboux 1969). AR formation is also stimulated by wounding or mechanical stresses and often allows the plant to proliferate by vegetative propagation. This is the case for plants that are propagated by layering. The formation of AR after stem cutting or in vitro culture is the basis of clonal propagation of valuable genotypes. This can be a rate-limiting step for the multiplication and propagation of interesting varieties and thus stimulates research on the genetic and physiological determinants of AR formation (Geiss et al. 2009; see also Chapter 11).

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