Jean-François Hausman

Concepts in plant stress physiology. Application to plant tissue cultures

Because the term stress is used, most often subjectively, with variousmeanings, this paper first attempts to clarify the physiological definition,andthe appropriate terms as responses in different situations. The flexibility ofnormal metabolism allows the development of responses to environmental changeswhich fluctuate regularly and predictably over daily and seasonal cycles. Thusevery deviation of a factor from its optimum does not necessarily result instress. Stress begins with a constraint or with highly unpredictablefluctuations imposed on regular metabolic patterns that cause bodily injury,disease, or aberrant physiology. Stress is the altered physiological conditioncaused by factors that tend to alter an equilibrium. Strain is any physicaland/or chemical change produced by a stress, i.e. every established condition,which forces a system away from its thermodynamic optimal state. The papersecondly summarises the Strassers state-change concept which is preciselythat suboptimality is the driving force for acclimation (genotype level) oradaptation (population level) to stress. The paper continues with the actualknowledge on the mechanisms of stress recognition and cell signalling. Briefly:plasma membranes are the sensors of environmental changes; phytohormones andsecond messengers are the transducers of information from membranes tometabolism; carbon balance is the master integrator of plant response; betwixtand between, some genes are expressed more strongly, whereas others arerepressed. Reactive oxygen species play key roles in up- and down-regulation ofmetabolism and structure. The paper shows finally that the above concepts canbeapplied to plant tissue cultures where the accumulating physiological andgenetical deviations (from a normal plant behaviour) are related to thestressing conditions of the in vitro culture media and ofthe confined environment. The hyperhydrated state of shoots and the cancerousstate of cells, both induced under conditions of stress in invitro cultures, are identified and detailed, because they perfectlyillustrate the stress-induced state-change concept. It is concluded that stressresponses include either pathologies or adaptive advantages. Stress may thuscontain both destructive and constructive elements : it is a selection factoraswell as a driving force for improved resistance and adaptive evolution.

Indissociable Chief Factors in the Inductive Phase of Adventitious Rooting

It happened that the first discovered and identified phytohormone, indolyl-3-acetic acid (IAA), was early shown to promote or favour adventitious rooting (Thimann and Went, 1934). Later identified natural auxins and synthetic compounds of this category had the same effects (Jackson, 1986). With the years, the rooting property of auxins appeared to be specific to this class of growth regulators since no such clear-cut effect could be apparently obtained by exogenous application of other known phytohormones. Some of them, such as cytokinins and gibberellins, for instance, were even classified as rooting inhibitors (Jackson, 1986; Davis et al, 1988; Davis and Haissig, 1994), although there were papers indicating the necessity of cytokinins for rooting (Letham, 1978) and others showing rooting effects of gibberellins under certain circumstances (Gaspar et al., 1977). On the basis of the effects of exogenous application of auxins, a series of wrong concepts as to their roles had arisen: that auxin is the major triggering agent in rooting, that the application of exogenous auxin is needed to augment the endogenous bulk of auxin, that rooting necessitates the maintenance of a “high” amount of endogenous auxin for a certain (unprecise) time, etc. Because there are inductive/adaptative enzymes to regulate the exo-genously fed hormones (this is well known for auxins and cytokinins) and because application of a hormone may induce modifications in the metabolism of other hormones, such simplistic conclusions may not be drawn. Another associated error was to consider rooting as a single developmental process.

Conversion of putrescine into -aminobutyric acid, an essential pathway for root formation by poplar shoots in vitro

Polyamines have been involved in the control of the inductive phase of rooting in Nicotiana tabacum (Malfatti et al. 1983, Kaur-Sawhney et al. 1988, Altamura et al. 1991, Altamura 1994), Vigna radiata (Friedman et al. 1985), Prunus avium (Biondi et al. 1990) and Beta vulgaris (Biondi et al. 1993). We have come to the same conclusions for poplar shoots raised in vitro (Hausman et al. 1994, 1995a, b) and also for walnut shoots (Heloir et al. 1996). On the contrary, spermidine and spermine inhibited the induction of the rooting process in the same material. Such specific effects of the most prevalent polyamines in plants, i.e. putrescine, spermidine and spermine, were also found by other laboratories (Tiburcio et al. 1989, Altamura et al. 1991, Rey et al. 1994). In poplar shoots, putrescine was suggested to act through its catabolic pathway (Hausman et al. 1994).

Are hyperhydric shoots of Prunus avium L. energy deficient

The content of oxidized and reduced pyridine nucleotides and some enzymatic activities of the oxidative pentose phosphate and glycolytic pathways were compared in normal (NS, growing on agar) and hyperhydric (HS, growing on gelrite) shoots of Prunus avium L. after 4 weeks of in vitro culture. The chlorophyll fluorescence from leaves and the redox capacity of the plasma membrane (reduction of exogenously added ferricyanide) of both types of shoots were recorded. The pool of oxidized and reduced pyridine nucleotides was lower in HS than in NS. These results suggested a reduced metabolism of HS in comparison to normal ones. This hypothesis was also supported by other observations. First, chlorophyll fluorescence measurements showed a lower chlorophyll content and a slight reduction of the photosynthetic capacity in HS. Second, the low activity of some enzymes of oxidative pentose phosphate pathway (OPP) and glycolysis indicated a decline of these biochemical pathways in HS with the consequence of a reduced production of chemical energy in the form of NAD(P)H and ATP. Finally, the lower reduction of ferricyanide by HS suggested a lower rate of redox reactions at the level of the plasma membrane of these shoots in comparison to NS.

Impact of Silicon in Plant Biomass Production: Focus on Bast Fibres, Hypotheses, and Perspectives

Silicon (Si) is an abundant element which, when supplied to plants, confers increased vigor and resistance to exogenous stresses, as well as enhanced stem mechanical strength. Plant species vary in their ability to take Si up and to accumulate it under the form of silicon dioxide (SiO2) in their tissues: emblematic of this is the example of Poales, among which there is rice, a high Si accumulator. Monocots usually accumulate more Si than dicots; however, the impact that Si has on dicots, notably on economically important dicots, is a subject requiring further study and scientific efforts. In this review, we discuss the impact that Si has on bast fibre-producing plants, because of the potential importance that this element has in sustainable agriculture practices and in light of the great economic value of fibre crops in fostering a bio-economy. We discuss the data already available in the literature, as well as our own research on textile hemp. In particular, we demonstrate the beneficial effect of Si under heavy metal stress, by showing an increase in the leaf fresh weight under growth on Cd 20 µM. Additionally, we propose an effect of Si on bast fibre growth, by suggesting an action on the endogenous phytohormone levels and a mechanical role involved in the resistance to the turgor pressure during elongation. We conclude our survey with a description of the industrial and agricultural uses of Si-enriched plant biomass, where woody fibres are included in the survey.

Proteomics as a Toolbox to Study the Metabolic Adjustment of Trees During Exposure to Metal Trace Elements

Although trees form an important part of the global environment, the use of proteomics in the study of environmental adaptation of trees, and specifically in the study of heavy metal exposure, is currently limited. This lack of proteomics studies is mainly due to the characteristics of trees making them difficult species to work with but can also partially be attributed to the limited genetic information that is available for most tree species. In this chapter, the tools and their potential, but also their limits, that are currently used in the proteome studies of trees will be discussed.

Involvement of Putrescine and of its Catabolic Pathway in the Induction of Rooting of Walnut Shoots In Vitro

Walnut (Juglans regia) shoots raised in vitro did not root on a single agar rooting medium, with or without auxin, in the light or under darkness. About 100% rooting was achieved through the use of two successive rooting media: a gelrite gelified MS “inductive” medium with auxin (3 mgT-1 IBA for 5 days in the dark), followed by a gelrite-ver- miculite “expressive” medium (1/4 DKW nutrients) without auxin (in the light). Using peroxidase as marker, it was determined that the time spent on the “inductive” medium corresponded to the physiological inductive and initiative phases of rooting. The level of endogenous IAA underwent a typical transient increase at inductive phase before decreasing at the initiative one.