NEW FINDINGS ON DEVELOPMENTAL, BIOCHEMICAL AND MOLECULAR RESPONSES TO ENVIRONMENT
2.2. A biological system is a complex network of triptychs and not only a complex association of the basic elements of the triptychs
The basic organization is the individual triptych; the complex is a network of triptychs, which is organized in the same manner so that it could be described as a "super triptych".
This concept is applicable since the functioning of the plant can in fact be modeled upon the same principle of the "gasoline engine". Relationships ar~ assured by the various and very specialized substances produced by each triptych, thus forming a cohesive internal exchange universe. This cohesiveness is reinforced when the substance in question acts as a signal among several triptychs. This is known as a "pleiotropic" or a superimposed effect.
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It would seem almost certain that natural selection led to the development of relatively few pleiotropic substances for organizing biological systems, if we bear in mind that energy efficiency or the laws of entropy tend to favor the formation of the simplest systems.
However, some phenomena act independently or are at least disconnected from the environment. For instance, ampelographic variables are expressed by the genome of Vi- tis in the same manner, regardless of the physiological status and the environmental con- ditions. That being true, it is difficult to place the genes involved in a basic triptych con- struction. In fact, if the environmental conditions vary widely, tending towards unusual conditions, such as a wide variations in water supply leading for instance to an extreme weakness of the plant or in day-night temperatures during leaf or berry development (which are sometimes observed when accidents occur under "controlled" environmental CUltivation), the variety cannot be identified using classical ampelographic criteria. An- other example is bud dormancy, which is the absence of activity independently of envi- ronmental conditions. However, in reality deep dormancy can be broken by using the combined effects of anoxia and high temperature (35°C).
From these examples, it is possible to conclude that in each case, it is necessary to con- sider each phenomenon or active gene, not independently, but as connected to one another when the couple is functioning. Such connections can be classified into several categories.
2.3. Three modalities of connections between triptychs exist, which reveal the biological concepts of nntrition or "source-sink" relationships, growth and development
Triptych 1 and triptych 2 can be connected in three possible ways:
~ Modality 1: substance 1 ~ source 2: defmes alimentation or nutrition or classical
"source-sink" functions;
~ Modality 2: substance 1 ~ structure 2: defmes growth or plant building;
~ Modality 3: substance 1 ~ signal 2: defines induction or development.
This series of definitions is a new way to present classical phenomena. It is most im- portant to point out the necessity, for example, to study not only the effect of a given signal on a given structure, but also the conditions - which are part of the regulation of the evolution of the structure - that led to the development of that signal. Consequently, if one manipulates a given gene, which exerts a particular effect on plant development, one must consider at the same time, the origin of the signal and its compatibility within the context of the whole plant.
From this point of view, in the future, it is not impossible that some new fundamental biological laws will be established to explain the regulation of a particular phenomenon within a biological system. This could be realized on the basis of the "general relativity law", insofar as certain universal biological limitations exist. For example, maximum pho- tosynthetic efficiency is relatively constant; therefore could it be considered in biology to be analogous to the speed of light in physics? That being the case, one could then speak in terms of "relative biology".
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2.4. Water constraint does not equate precisely to water "limitation"
The former exists because of the enormous hydric potential gradient that exists between the air and the soil, even ifreal transpiration is equal to its maximum value of unity. Wa- ter constraint appears from many experiments, to be the main regulating factor within the plant and it is superimposed to most of plant functions (Carbonneau, 1996). The coher- ence of plant physiology is mainly due to the necessity to adapt to that gradient. This predominance of water regime involves development or growth or nutrition.
In this context, it is a new challenge to study plant physiology in response to the tran- spiration requirement by the microclimate, simultaneously with the water supply to the soil and the root system: this would be the only method to adequately defme the limiting factor (solar energy absorption, root absorption, deep root system, water reservoirs in the plant etc.). The activity of the corresponding genes should be analyzed concurrently.
Referring back to point 2, if some pleiotropic effects exist, they must be related to water constraint which may be thought of in terms of the circulatory system of the plant.
2.5. Feed back mechanism
Optimal plant activity also supposes the existence of a feed back mechanism, for equili- brating the general regulatory system, which in turn is particularly influenced by the water constraint. This could be affected by the last formed substances retro-acting on the engines of the biological system, a role that could be fulfilled by ABA synthesised in rootlet tips.
Another example is the flux of root exudates, which could interact with the regulating sys- tem via the biological activity of the soil, which consumes those exudates. Enhancing bio- logical activity of the soil could consequently stimulate the physiological activity of the plant, whereas inhibiting the biological activity of the soil, could reduce biological activity by enhancing the feedback effect, due to exudate accumulation at root-soil interface.
This general consideration also explains the modification of the micro-environment of the root by the plant (Le. pH which can differ greatly from the pH of the soil itself).
Therefore, plant adaptation is organized in respect to the micro-environment, which may or may not be modified by the prior activities of the plant. This dynamic concept of ad- aptation fits well with the triptych idea. Similar observations involve the aerial parts of the plant, which furthermore, may be considered either as an input or an output port of the biological system (i.e. CO2).
2.6. Plant aging
A plant operating at its optimal physiological level, simultaneously assures its nutrition, growth and development, while the connections between triptychs are diversified to the maximum extent. Plant aging may be defmed as the reduction in the number of connec- tions between triptychs, which is not in contradiction with the fact that new triptychs can emerge as the plant ages. However, the aging process can order the suppression of cer- tain triptychs, particularly some pleiotropic triptychs. That could explain, why drought
268 A. CARBONNEAU and A. DELOIRE
produces a similar effect to aging, as do most of environmental constraints. Conversely, biological age which differs from chronological age, is determined by the accumulation of temperature effects (plastochrone index) which is the standard reference for aging in all plant systems.
2.7. Strategies of adaptation
The strategies of adaptation to environmental constraints tend to polarize the biological system, in a manner analogous to the aging process. This polarization tends to prioritize some triptych organizations.
The strategies of adaptation can be manifested in 3 different ways:
~ Priority of growth and activity of the plant which is accelerated, promoting growth under the favorable conditions when sources as not limited: avoidance.
~ Priority of development or modification of existing structures which later on enables the plant to survive under unfavorable conditions: strict adaptation.
~ Priority of nutritional regulation or of "source-sink" relationships, which optimizes the system, without greatly modifying growth or development: tolerance.
Tolerance (i.e. optimized biochemical pathways) generally involves reversible phe- nomena; the plant can therefore efficiently return to an optimum physiological activity, and later can possibly tolerate certain environmental limitations. This reversibility could be due to some pleiotropic genes. The general evolution of the basic elements of carbon balance in function of an increasing water limitation and in relation to a tolerance strat- egy is presented in Figure 11.2.
Adaptation (i.e. root depth or leaf hardening) and avoidance (i.e. enhanced growth and activity) are generally irreversible, which give better results in terms of general ad- aptation to environmental constraints, but may prevent the plant from eventually benefit- ing from more favorable environmental conditions.
2.8. Polyvalence
Some substances can intervene as a structure, a source, or as a signal, thus giving the plant the capacity and plasticity to adapt itself. This is the case of response proteins which play an important role in the development of cell structures in response to bio- aggression, in signaling the occurrence of bio-aggressors, and they can also act as a source of defence substances. (Van Loon, 1997). Also, an increasing number of substances are recognized to have pleiotropic effects, which is the case for some phenolic compounds (Dixon and Paiva, 1995).
2.9. The role of genes
The genome or genotype of an organism is the basis of all biological functioning, though it is in fact more accurate to consider the first "genotype-environment" interactions as the basic determinant in plant physiology.
PLANT ORGANIZATION BASED ON SOURCE-SINK RELATIONSHIPS 269
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PHOTO Y THESIS (1)
MAXIMUM TRANSPIRATIO
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LIMITATION SEVERE
LIMITATION
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Figure 11.2. General responses of photosynthesis (1), growth of different organs (2), net carbon availability (3), carbon accumulation in grape bemes (4), to an increasing water limitation (5). ( 1 ) acts as the main source, (2) as a sink or as a source (leaf area), via its structure components in each case, (3) as a temporary sink or an intermediate source, (4) as the final sink via its structure com- ponents, (5) as a signal or a source which is superimposed over (1), (2), (3), (4).
Natural selection has given rise to organisms that are stable or sustainable, as a con- sequence of which, the genome structure must primarily be preserved and protected. At the same time, that same structure must provide sources and signals, which represents the basis of the life. The general functions of DNA are fundamental triptychs, which can be summarized as follows.
The principal function of DNA is to duplicate or reproduce its own structure. That structure can occasionally evolve during its stable phase as a result of mutations, or dur- ing duplication due to errors in the copying process. DNA is also recombined during meiosis in sexual reproduction. Besides such structural characteristics, DNA acts as a signal during transcription of genetic code into messenger RNA; that transfer being the generator of the functioning. The substances (peptides) induced by the genes assure dif- ferent parts of the general functioning of the system, with some of these peptides them acting as regulators of the genome itself.
On that basis, it is clearly necessary to study structural genomics, not only in itself, but for the functions which it governs (functional genomics), since gene manipulation by
270 A. CARBONNEAU and A. DELOlRE
conventional or non-conventional means is used to modify a function and ultimately the phenotype. This requires an integrated approach involving the study of the genome in interaction with the environment, even if initially only a few genes governing a precise mechanism are the object of the study. The questions that arise are: are the active genes the same whatever the environment? what are the regulating genes of the basic mecha- nism? what are the links with directly-connected triptychs of genes under different con- ditions of development or in different environments? what, among them, are the com- mon active genes under those different conditions?
Applying the previous considerations of molecular responses to environment, it is important to focus on:
>- Connections between structural genomics and functional genomics.
>- Research into pleiotropic substances and their corresponding genes which can have
significant effects on plant development, regulation and adaptation.
>- "Genotype - environment" interactions which are necessary to reveal gene activity
and biological triptychs.
Such a methodology can be described as "molecular physiology", which represents something similar to the application of ecophysiology to molecular biology. At the mo- ment, few results in relation to the grapevine have been published in that field. Those results that have appeared are summarized in the following part 3, indicating the re- sponses to environmental constraints and particularities of Vitis species.