BOTANY VOLUME 2 - PLANT PHYSIOLOGY - 2007

9. ALLELOPHYSIOLOGY

Plants not only respond to physical or chemical stimuli from their inanimate (abiotic) environment (see 7.7), but also engage in diverse interactions with other living organisms. Phytochrome-controlled responses to shading or to light reflected by neighboring plants (see 7.7.2.1) have already been mentioned as Examples. The Study of the molecular processes involved in plant-Organism interactions is now an independent branch of physiology, collectively referred to here as allelophysiology (from Greek allelos, mutual, reciprocal).

Allelophysiology is closely linked to plant ecology (see 13.8) and plant pathology, although these connections can only be outlined in general terms within The Scope of this book. Various Interactions Between Plants and their pollinators are discussed in section 11.2 in the context of the respective taxa. Since floral organ movements are often related to pollinators, this topic was covered in the section on movement physiology (see 8.3.2).

The closest form of organismal interaction occurs in Symbiosis (see 9.2). This refers to the intimate coexistence of two different species of organisms from which both derive at least temporary benefit. Symbiosis is thus distinct from commensalism (benefit to one partner without noticeable effect on the other) and parasitism (see 9.1.1, benefit to one at the expense of the other's suppression). Symbiotic coexistence also clearly demonstrates that it often originates from reciprocal parasitism (alleloparasitism), in which a balance between the partners in terms of infection and defense is established, and they mutually exchange nutrients and Hormones. If one partner becomes dominant during symbiosis, this equilibrium can break down and revert to parasitism, as seen in the Digestion of ROOT nodule Bacteria by their host Cells (see 9.2.1).

There is also no sharp boundary between parasites and pathogens. Diseases are typically caused by microbial parasites that inflict such severe damage on the host organism that characteristic symptoms appear and the parasites themselves multiply (often significantly) (pathogens, see 9.3). This damage can lead to the death of the host or a specific tissue. If a pathogen derives nutrients from dead tissue areas, it feeds saprophytically (see 9.1.1).

Heterotrophs also include plant-eating animals (herbivores, see 9.4), which satisfy their organic matter requirements exclusively or predominantly at the expense of autotrophic plants—the primary producers standing at the Base of the food chain.

All of these interactions exhibit a varying—sometimes very high—degree of Specificity toward the interacting organisms. Plants are resistant to most potential pathogens and susceptible to only a few; they are defended against most herbivores and attacked by only a small number of them; similarly, parasitic relationships and symbioses generally form only between specific partners. The basis of host specificity can be considered "recognition processes," during which a mutual exchange of signaling molecules frequently takes place between the participating organisms (these molecules determine, for example, whether a plant is resistant or susceptible to a pathogen). Chemical interactions between plants, whether between individuals of the same species or (more commonly) of different species, are termed allelopathy (see 9.5). In such cases, the phenomenon often involves inhibitors produced by one individual that are released into the environment and exert an inhibitory effect on the growth of competing plants.

9.1. Characteristics of Heterotrophic Nutrition

Plant interactions with other organisms are directly or indirectly linked to Nutrition. Allelopathy underlies intra- or interspecific competition among autotrophic plants for limited nutrients; all other interactions involve aspects of heterotrophic nutrition, either by the plant itself and/or by the organism interacting with it.

Unlike autotrophic organisms, which assimilate inorganic nutrients (see 6.4–6.9), heterotrophs feed on organic matter. If a predominantly autotrophic organism requires certain simple Organic compounds for growth, the condition is termed mixotrophy or prototrophy. Mutants that have lost The ability to synthesize a specific organic substance required for growth (e.g., an amino acid or a cofactor) are designated as auxotrophs: they must obtain such substances from an external source.

Among heterotrophs, a distinction is made between Saprophytes, which obtain organic nutrition from dead substrates, and parasites, which exploit living organisms or cells.

9.1.1. Saprophytes and Parasites

Saprophytes comprise the majority of bacteria and Fungi; notably, they are entirely absent among higher plants. Nutritional substrate requirements, particularly among saprophytes, vary widely. Along with inorganic substances, they require a carbon source, which can include not only CARBOHYDRATES, fats, or Proteins, but also alcohols, organic acids, as well as petroleum, paraffin, benzene, and naphthalene. Saprophytes frequently secrete exoenzymes that extracellularly degrade high-molecular-weight substrates (e.g., Lignin, Cellulose, proteins) into absorption-ready products. The assimilated organic material is then integrated into normal (catabolic or anabolic) METABOLISM. Many saprophytes do not require organically bound nitrogen. For instance, Yeasts can grow on NH⁺₄, and the mold Aspergillus niger can utilize NO^-₃ as its sole nitrogen source (see 6.6).

In nature, entire groups of different organisms frequently cooperate, with one species utilizing and feeding on the decomposition products or waste of another, while its own metabolic byproducts may in turn serve as a nutritional substrate for other species—acting partly as "raw Materials" for energy-yielding chemosynthetic reactions (H₂S, H₂, NH₃). Processes of this kind occur during putrefaction, where organic material derived from dead plants, plant parts, or animals is converted back into Inorganic Compounds (remineralized) by bacteria and fungi; putrefaction is therefore a crucial link in the biogeochemical cycle. The "biological self-purification" of polluted Water is based on similar processes. In wastewater Treatment plants utilizing activated sludge, saprophytic communities are harnessed to process organic waste. Similar mineralization processes also take place in the soil (e.g., during composting). Taken together, all these processes are of immense importance for the global metabolic balance on Earth.

Products of microbiological decomposition serve as precursors for The formation of humus, coal, and petroleum, although subsequent abiotic chemical transformations—often under high pressure—play a decisive role in these latter processes (coal and oil).

Parasites occur among bacteria, fungi, Lichens, and seed plants. Some heterotrophic red Algae parasitize closely related red algal species (adelphoparasitism). Organisms that can feed both saprophytically and parasitically in nature are termed facultative parasites, whereas those naturally and permanently dependent on living hosts are obligate parasites. However, even obligate parasites can often be cultured saprophytically on a suitable artificial nutrient medium in experiments.

Microbiological parasites (bacteria, fungi) cause numerous diseases in plants, animals, and humans, functioning as disease agents (pathogens). Because of the extreme complexity of their interactions with plants, microbial pathogens are treated in a separate section (see 9.3).

Among gymnosperms, only a single parasitic species is known: Parasitaxus ustus, a member of the Podocarpaceae family, which parasitizes its close relative Falcatifolium taxoides within the same family; this endemic species is native to New Caledonia. The parasite establishes contact with the host's xylem and thereby obtains water and mineral salts.

Among parasitic angiosperms, which are invariably obligate parasites, a distinction is made between hemiparasites and holoparasites (semi- and total parasites). Hemiparasites (e.g., most mistletoes and scrophulariaceous genera such as Rhinanthus, Melampyrum, Pedicularis, Euphrasia) are capable of Photosynthesis and can absorb inorganic nutrients and water, but they do so not through soil roots, but via haustoria connected to the host's xylem. However, they generally grow only on specific hosts (various strains of Viscum album, for instance, on firs, pines, and deciduous trees), suggesting that organic compounds present in low concentrations within the xylem are also important to them (see 6.3.5; 6.8). These hemiparasites must "extract" the fluid from the host's conducting system against the internal turgor pressure; consequently, they typically exhibit an unusually high Transpiration rate per unit leaf area (which explains, for example, the rapid withering of plucked Melampyrum). Remarkably, plants that lack fully developed transpiring leaves either during specific ontogenetic stages (e.g., Tozzia and Bartsia) or throughout their entire life cycle (Lathraea) form hydathodes on their rhizome scale leaves, which secrete water and thereby maintain the necessary water potential gradient between host and parasite. The culminating point of this evolutionary series within the hemiparasitic Scrophulariaceae (Rhinanthoideae) is represented by Lathraea, a scale-leaved rhizomatous plant that parasitizes perennial hosts and evidently obtains sufficient organic material from the host xylem to live as a holoparasite.

Even among mistletoes, a fully parasitic species is known—the leafless Tristerix aphyllus (Loranthaceae), which parasitizes cacti. It remains unclear whether it connects to the host's xylem or phloem. Other fully parasitic angiosperms, such as Striga, Orobanche, and Cuscuta (see Fig. 11.28), connect to the host's sieve tubes, from which they draw assimilates by means of specialized absorbing cells (transfer cells) through mechanisms not yet fully understood.

Higher plants frequently serve as hosts for parasitic animals (zooparasites). Arthropods predominantly parasitize shoots, whereas nematodes (Roundworms) attack roots. Globally, nematodes cause severe agricultural damage every year. Plant conditions associated with parasitic infections and the manifestation of characteristic disease symptoms are termed parasitoses. Parasitic arthropods include, for example, leaf-mining moths whose larvae feed on mesophyll Tissues within the leaves of host plants. More commonly, however, the presence of parasites leads to the formation of plant galls (cecidia). In general, this term refers to any active disruption of Tissue and organ development induced by a foreign, parasitic organism, resulting in limited growth. If the damaged tissues undergo unlimited proliferation, they are called tumors (see 9.3.3, 7.6.2.3, Box 9.2). Symbiotic structures (such as root nodules, see 9.2.1) are no longer classified as galls today, although gall-like formations induced by parasitic bacteria or fungi do exist (for instance, "witches' brooms" resulting from infection by Rhodococcus fascians, see 7.6.2.3, and fungi of the genus Taphrina, see 11.2, Fig. 11.29).

Organoid galls consist of heavily modified yet still clearly recognizable principal Organs of the host plant (e.g., "witches' brooms"). More common histoid galls (Fig. 9.1) obscure the organ architecture, arising instead as neoformations derived from stem, leaf, or root parts; they are typically induced by gall-forming animals—on shoots primarily by gall midges (cecidomyiids), cynipid wasps, psyllids, and gall mites, and on roots by cyst nematodes of the genera Heterodera and Globodera, or root-knot nematodes of the genus Meloidogyne (Fig. 9.2). In these cases, the parasite penetrates deep into the plant tissue, which is why such gall-forming animals are termed endoparasites. Ectoparasites, by contrast, do not fully penetrate the host plant but parasitize its surface (e.g., sap-sucking aphids, plant bugs, leafhoppers, and certain nematodes).

Class="center">. Fig. 9.1. Various histoid galls on a leaf of Fagus sylvatica. The specific shape of the galls is determined by the inducing animal: 1 — pouch gall caused by the beech Hair gall midge Mikiola fagi (see frontispiece, Part 4); 2 — pubescent pouch gall of the gall midge Hartigiola annulipes; 3 — felt gall along the leaf Veins caused by the mite Eriophyes nervisequus.

Fig. 9.2. Gall-forming endoparasitic nematodes: A — life cycles of cyst-forming and root-knot nematodes; B — adult female of the root-knot nematode (Meloidogyne incognita) in a cucumber root gall; C — Cytology/practical/72.html">Cross section of an uninfected Arabidopsis thaliana root (top) as well as (below) a cross section through the syncytial region of an infected root (syncytium at the bottom, fourth-stage female larva of the cyst nematode Heterodera schachtii). L — larval stages

Histoid galls are primarily adapted to the needs of gall-forming animals. These adaptations are often unusual and complex. Pocket galls, for example, frequently develop on beech leaves (see Fig. 9.1) due to localized surface growth induced by the larvae of the beech hair gall midge. The larvae "mold" a gall casing for themselves using their saliva. Eventually, the saliva-moistened areas arch outward into a pocket-like dome, completely enclosing the pathogens within pockets that feature an exit duct at the bottom. Many galls also undergo subsequent thickening and the formation of sclerenchymatous elements, creating a durable protective case for the developing animal. Abundant hair-like structures and thin-walled, nutrient-rich cells inside the casing typically serve to nourish the gall-forming organisms.

In the aforementioned examples, the Influence of the foreign organism triggers the formation of cellular and organ structures for which the plant likely possesses the genetic potential, but which do not develop under normal conditions. There is no doubt that various galls arise due to the specific¹ influence of gall-forming organisms, in which phytohormones evidently play a crucial role.

¹ This specificity is determined by the temporal and spatial factors of plant tissue development. — Ed. note.

To meet agricultural demands, plant responses induced by endoparasitic nematodes have been extensively studied in recent times (Fig. 9.2). In the second larval stage (L₂), nematodes infect plant roots near the root tip. Cyst-forming nematodes penetrate procambial cells with their stylet, which initially swell significantly due to the secretions of the animal's Salivary Glands. Subsequently, through partial dissolution of The Cell walls and the fusion of protoplasts, large, highly metabolically active syncytia ("cysts") comprising over 200 cells are formed, from which the now immobile parasite extracts nutrients (Fig. 9.2, A, C). Well-nourished animals develop into females that, upon dying, carry numerous embryo-laden eggs capable of surviving in the soil for several years. Root-knot nematodes cause dramatic alterations in the procambial cells at the root tip, forming not syncytia, but multinucleate (via endomitosis) giant cells with high metabolic activity (Fig. 9.2, A, B) containing up to 100 large cell nuclei. Functioning as strong sinks (see 6.8.3), these cellular structures drive a substantial influx of nutrients from the plant's productive organs. The parasite obtains nutrients from the giant cells via the symplastic pathway. It is well established that the plant's responses are triggered by the animal's metabolic products. The Nature of the nematodes' lytic substances remains poorly understood. However, species or varieties—such as within the genera Beta and Solanum—have been found that exhibit significant resistance to nematode infestation. The cloning of genes (see Box 7.3) conferring resistance (resistance genes, R-genes) yielded an unexpected result: some of these genes bear a striking resemblance to genes conferring resistance to pathogenic bacteria or Viruses (see 9.3.4). Plants possess resistance mechanisms directed against a broad spectrum of pests (see 9.3.4; 9.4.1).

9.1.2. Carnivorous Plants

Plants capable of capturing animals (carnivorous plants — see Box 4.4), with the exception of trap-forming fungi (see 11.2; Fig. 11.62), invariably possess chlorophyll, are capable of C₃ photosynthesis, and can be easily cultivated under adequate mineral nutrition without animal prey. Only when nutrients are scarce in their natural habitat, as frequently occurs (e.g., in raised bogs), do they capture animals primarily to secure nitrogen and phosphorus. In Utricularia exoleta, Flower Formation is significantly stimulated by animal nutrition.

The adaptations of carnivorous plants enable them to attract specific animals and retain them using trapping structures (trapping mechanisms, see Box 4.4, as well as section

8.3.2.4). Digestion proceeds with the participation of exoenzymes, primarily proteases, which are secreted by specialized glands upon stimulation by the prey (e.g., in Drosera) or independently thereof (e.g., a Pepsin-like protease with an acidic pH optimum in the pitchers of Nepenthes; see Box 4.4, Fig. A). In Sarracenia, digestive Enzymes must be secreted by bacteria into the trapping fluid. The digestion products are often absorbed by the plant via absorptive hairs and integrated into its metabolism.