Etude du metabolisme des xyloglucanes incluant leur apparition, leur biosynthese, leur biodegradation, leur organisation et leur role dans la regulation de la croissance

Xyloglucans in the Primary Cell Wall

Process considerations in the enzymatic hydrolysis of biomass

The feeding sites induced by sedentary root-endoparasitic nematodes have long fascinated researchers. Nematode feeding sites are constructed from plant cells, modified by the nematode to feed itself. Powerful new techniques are allowing us to begin to elucidate the molecular mechanisms that produce the ultrastructural features in nematode feeding cells. Many plant genes that are expressed in feeding sites produced by different nematodes have been identified in several plant species. Nematode-responsive plant genes can now be grouped in categories related to plant developmental pathways and their roles in the making of a feeding site can be illuminated. The black box of how nematodes bring about such elaborate cell differentiation in the plant is also starting to open. Although the information is far from complete, the groundwork is set so that the functions of the plant and nematode genes in feeding site development can begin to be assessed.

Gene expression in nematode feeding sites

Plant hormones are important biotic factors to regulate root growth. Among the seven kinds of plant hormones, auxin and gibberellin (GA) are strong accelerators of shoot growth, but these are not always accelerators for root growth. The classical views of root-growth regulation by auxin and gibberellin are summarized and current theory of the regulation mechanism is described in this review. The concentration-dependent deceleration of root growth is a key to understanding the auxin action on roots, since the endogenous concentration of indole-3-acetic acid (IAA) is inversely proportional to the growth rate. As massive IAA is transported from shoots to roots by polar transport, the influx speed of IAA mainly controls IAA levels in root cells. The classical view of IAA transport in roots has been supported by recent discoveries of IAA-carrier proteins such as AUX1, PINs and MDRs. The role of plasma membrane-located H+-ATPase and its regulation by IAA has also been described for the acid growth phenomenon ca...

Regulation of Root Growth by Plant Hormones—Roles for Auxin and Gibberellin

Seed dormancy is considered as an aspect of growth cessation. It is characterized by partial metabolic arrest with its inception and termination under endogenous hormonal control. Although several distinct types of seed dormancy have been recognized traditionally, an examination of the experimental literature suggests that a common regulatory mechanism exists. Dormancy onset, control, and termination are apparently regulated by a balance of growth inhibitors and promotors. At the onset of dormancy this balance is shifted in favor of the inhibitor component; at its termination it is shifted back in favor of the promotor component. The experimental evidence indicates that the levels of growth-promoting hormones decrease markedly during seed maturation, imposing dormancy. A trigger mechanism stimulates hormone activation and/or synthesis, thus terminating dormancy and culminating in resumption of embryo growth (germination). A model is proposed that embodies a hormonal regulation of four phases of dormancy: an inductive mechanism, a maintenance mechanism, a trigger mechanism, and a germination mechanism. This hormonal regulation, in conjunction with specific metabolic inhibitors, is considered to mediate the activation and/or synthesis of hydrolytic enzymes. From this model the dormant seed is viewed as a cybernetic system. Particular emphasis is given to the regulatory role of gibberellins in seed dormancy and germination.

A model of seed dormancy

Measurements were made over a 4-day period of the effect of added indoleacetic acid (IAA), puromycin, actinomycin D and 5-fluorodeoxyuridine (FUdR) on growth and the levels of total DNA, RNA, protein and cellulase in segments of tissue at the apex of decapitated etiolated epicotyls of Pisum sativum, L. var. Alaska.The hormone induced swelling of parenchyma cells and cell division. By 3 days after IAA application, the amounts of DNA and protein were approximately double, RNA triple and cellulase 12 to 16 times the levels in controls. All of these changes were prevented by both puromycin and actinomycin D. FUdR prevented DNA synthesis and cell division but not swelling or synthesis of RNA, protein and cellulase.It is concluded that IAA-induced RNA synthesis is required for cellulase synthesis and lateral cell expansion, whether or not cell division takes place.

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Der-Fong Fan

Papers

Massive Synthesis of Ribonucleic Acid and Cellulase in the Pea Epicotyl in Response to Indoleacetic Acid, With and Without Concurrent Cell Division