DETERMINISM OF THE MYCORRHIZOGENIC ROOT FORMATION IN PINUS PINASTER SOL.

Abstract

SUMMARYSeedlings, axillary buds and apical root meristems of Pine (Pinus pinaster Sol.) were used in different controlled environments. A kinetic study of root morphogenesis was performed, supported by light and electron microscopic analysis.Without any symbiotic or parasitic fungus, short roots (histologically similar to the ectomycorrhizae except for the absence of the fungus), able to produce dichotomous branches, were observed on all the kinds of explant and in all the environmental conditions used. Hence the formation of these mycorrhizogenic short roots is 'genome dependent'.In clones of young plantlets (produced by vegetative propagation by means of axillary budding in vitro), the root systems exhibit a similar intensity, strong or weak, to dichotomize; this emphasizes the relationships between the formation of mycorrhizogenic short roots and the pinus genome.This study seems to be the first attempt to demonstrate, under controlled conditions, a direct relationship between plant genome and root branching in a woody species.

Auxin transport inhibitors act through ethylene to regulate dichotomous branching of lateral root meristems in pine

“…The cortical cells in mycorrhizal structures were somewhat larger than cortical cells in P. sylvestris root tips treated with TIBA and ACC. This observation is consistent with observations of Faye et al (1980) and Rupp & Mudge (1985), who compared the structure of ectomycorrhizas with that of di-chotomous root tips formed in the absence of fungal symbionts and noted that the histologies were similar in the two cases except that the cortical cells of ectomycorrhizas were swollen. This might reflect the differences in the physiology of the cortical cells of functional ectomycorrhizas from those of nonfunctional chemically induced coralloid structures.…”

“…By contrast, lateral root tips of P. taeda, P. muricata and P. halepensis showed extensive spontaneous dichotomous branching, leading to the formation of coralloid structures. This spontaneous appearance of ' mycorrhizae-like formations ' was previously reported only in P. pinaster (Faye et al, 1980). Although the present study is by no means exhaustive, the observation that three of six species spontaneously formed coralloid structures indicates that this phenomenon is not uncommon, and supports the conclusion of Faye et al (1980) that the host plant genome contains all of the genetic information required to form an ectomycorrhiza-like organ.…”

“…This spontaneous appearance of ' mycorrhizae-like formations ' was previously reported only in P. pinaster (Faye et al, 1980). Although the present study is by no means exhaustive, the observation that three of six species spontaneously formed coralloid structures indicates that this phenomenon is not uncommon, and supports the conclusion of Faye et al (1980) that the host plant genome contains all of the genetic information required to form an ectomycorrhiza-like organ. These observations, however, do not address the questions of what initiates and regulates the development of this morphology in pine roots.…”

Dichotomous branching: the plant form and integrity upon the apical meristem bifurcation

“…The distinctiveness of superficial layers is maintained during dichotomy progression, as well as the continuity of procambium and vascular tissues in the parental and dichotomous axes. Differentiation of the meristematic cells located between dichotomizing apices ceases the growth of the central part of the original meristem starting the separation of both branches (Härtel, 1937; Nolan, 1969; Tomlinson, 1970, 1971; Boke, 1976; Tomlinson and Posluszny, 1977; Faye et al, 1981; Piche et al, 1982; Laajanen et al, 2007; Raudaskoski and Salo, 2008; Gola and Jernstedt, 2011). …”

Dichotomization of mycorrhizal and NPA-treated short roots inPinus sylvestris

“…8 The formation of dichotomous roots may also occur without contact with a fungus as a response to stress, and thus it is an endogenous property of the short roots enhanced by some environmental factors. 9 The factors regulating the limited growth of the short roots, their ability to host fungal mycelium and the morphological changes at ectomycorrhizal symbiosis have interested the researchers for years. 8,[10][11][12][13][14][15] The formation of dichotomous roots has been suggested to result from changes in either auxin or ethylene concentrations due to the fungal growth in the root.…”