To be most effective anticancer drugs must penetrate tissue efficiently, reaching all the cancer cells that comprise the target population in a concentration sufficient to exert a therapeutic effect. Most research into the resistance of cancers to chemotherapy has concentrated on molecular mechanisms of resistance, whereas the role of limited drug distribution within tumours has been neglected. We summarize the evidence that indicates that the distribution of many anticancer drugs in tumour tissue is incomplete, and we suggest strategies that might be used either to improve drug penetration through tumour tissue or to select compounds based on their abilities to penetrate tissue, thereby increasing the therapeutic index.

Drug penetration in solid tumours

Phytoalexins: enzymology and molecular biology.

Phytoalexins are low-molecular-weight antimicrobial compounds that accu­ mulate in plants as a result of infection or stress. The rapidity and extent of their accumulation is determined by their release or the release of immediate precursors from conjugates andlor de novo synthesis, as well as detoxification as a result of plant or microbial enzymes. The rapidity of phytoalexin accu­ mulation is associated with resistance in plants to diseases caused by fungi and bacteria, although the genetic information for phytoalexin synthesis is found in susceptible and resistant plants. Phytoalexins are only one component of the complex mechanisms for disease resistance in plants. This review considers the contribution of phytoalexins to disease resistance and evaluates molecular and nonmolecular strategies for their biosynthesis as potential tech­ nologies for disease control.

Phytoalexins, stress metabolism, and disease resistance in plants

Summary

1. Induced resistance of plants to incompatible races of their microbial pathogens is often characterized by rapid cell death (the hypersensitive response) and the accumulation of low-Mr antimicrobial compounds termed phytoalexins. There is much indirect evidence to support a major role for phytoalexin induction as an event determining host resistance, and genetical analysis of race-specific pathogen-host interactions predicts that induced resistance occurs via interaction between a pathogen avirulence gene product and a host resistance gene product. Elicitors (inducing agents) of phytoalexin accumulation are possible candidates for the active products directly or indirectly resulting from the expression of microbial avirulence genes. The present review considers the process of phytoalexin induction, at the molecular level, from the fungal elicitor to the early changes in host gene expression associated with its action.



2. A number of microbial polysaccharides, glycoproteins, pectic enzymes, peptides and fatty acids have potent elicitor activity.



3. The results of studies on the purification and characterization of microbial elicitors depend on the nature of the bioassay used. Methods for elicitor extraction may lead to artifacts or loss of race specificity.



4. Elicitors isolated from plant pathogens may be race-specific or race-non-specific. In cases where only race-non-specific elicitors can be shown, race-specific induction of phytoalexins may result from the action of enhancer or suppressor molecules. Fungal glucans have been proposed as candidates to act in both these roles.



5. Molecular genetic approaches to the identification of phytopathogenic bacterial avirulence genes may help to prove or disprove the role of elicitors of the phytoalexin response as agents responsible for the induction of host resistance. Similar analyses, involving genetic transformation, should soon be possible for phytopathogenic fungi.



6. Studies on the nature of host receptors for microbial elicitors are still in their infancy. Such receptors are probably localized in the plant plasma membrane, and elicitation results in often striking changes in host membrane properties.



7. Cyclic 3′,5′-adenosine monophosphate and polyamines do not appear to act in plants as intracellular transducers of the phytoalexin response. Interest is now being shown in a possible role for calcium in intracellular signalling.



8. Plant cells contain endogenous elicitor molecules whose synthesis or release may play a role in the intercellular transmission of the phytoalexin response. The main candidates for endogenous elicitors are pectic fragments of the host cell wall, although no direct evidence for their involvement in plant-pathogen interactions is available. Pectic fragments may act as synergists in parallel with, rather than as couplers in series with, microbial elicitors. The role of ethylene as a response coupler for induced resistance phenomena is difficult to assess.



9. Some elicitors may themselves move to the host cell nucleus, although whether this is their effective site of action remains unclear.



10. Induction of the phytoalexin response is associated with specific changes in host gene expression related to the selective induction of new mRNA species and enzyme activities.



11. Progress is now being made in the characterization of enzymes specific for phytoalexin biosynthesis. Enzyme induction and/or infection appears to involve rapid modulation of gene transcription, although some post-translational events may also be involved in determining induction patterns.



12. Work is now commencing on the sequencing of the genes encoding elicitor-inducible enzymes. Multigene families have been identified for phenylalanine ammonia-lyase and chalcone synthase.



13. Phytoalexin accumulation may be accompanied by the rapid induction of ethylene biosynthesis and the expression of genes encoding activities involved in the synthesis of hydroxyproline-rich glycoproteins.



14. Pectic endogenous elicitors of phytoalexin accumulation are also active as inducers of systemic proteinase inhibitor synthesis in some solanaceous species.



15. Results of studies on the mechanisms of signal transmission and differential gene expression in relation to active defence reactions should be of relevance to many other aspects of the plant's response to environmental stimuli at the physiological, biochemical and molecular genetical levels.

The phytoalexin response: elicitation, signalling and control of host gene expression

Plant secondary metabolites carry out numerous functions in interactions between plants and a broad range of other organisms. Experimental evidence strongly supports the indispensable contribution of many constitutive and pathogen-inducible phytochemicals to plant innate immunity. Extensive studies on model plant species, particularly Arabidopsis thaliana, have brought significant advances in our understanding of the molecular mechanisms underpinning pathogen-triggered biosynthesis and activation of defensive secondary metabolites. However, despite the proven significance of secondary metabolites in plant response to pathogenic microorganisms, little is known about the precise mechanisms underlying their contribution to plant immunity. This insufficiency concerns information on the dynamics of cellular and subcellular localization of defensive phytochemicals during the encounters with microbial pathogens and precise knowledge on their mode of action. As many secondary metabolites are characterized by their in vitro antimicrobial activity, these compounds were commonly considered to function in plant defense as in planta antibiotics. Strikingly, recent experimental evidence suggests that at least some of these compounds alternatively may be involved in controlling several immune responses that are evolutionarily conserved in the plant kingdom, including callose deposition and programmed cell death.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/nph.13325

Secondary metabolites in plant innate immunity: conserved function of divergent chemicals

Studies on Phytoalexins. V. The Structure of Pisatin from Pisum sativum L.

Two challenging questions in developmental biology are the causes of the induction of cell division, and of the cessation of cell division. In plant tissues cell division may be aroused in response to treatment with diffusible substances collectively referred to as kinins1. One might enquire whether the meristems are self-perpetuating, through an ability to produce or accumulate such a kinin2,3. In asking this question our aim has been twofold : to learn whether the activation of the meristem is under the control of a diffusible substance, and to attempt to isolate and identify such a substance with the view of providing a means of chemically, and possibly commercially, controlling meristematic activity.

A kinin in apple fruitlets.

IT is obvious that plants are resistant to attack by fungi that are non-pathogenic to them. However, until recently, the mechanism of this resistance has been obscure. Pisatin, a substance believed to be responsible for such resistance in the garden pea (Pisum sativum L.), has been recently isolated from pea pods inoculated with fungal spores1.

Pisatin: an antifungal substance from Pisum sativum L.

COCONUT milk is recognized as a rich source of plant cell division inducers, which have become known as kinins1–5. Recently, apple fruitlets were also shown to be a rich source6, and the kinins have been extracted from this tissue and concentrated7. The results showed that the extract contains at least four and probably five independently active kinins as revealed by the tobacco stem pith test8. In this test, blocks of tobacco pith are cultured singly in small tubes on a sucrose–mineral salt medium which also contains the auxin, indolyl-3-acetic acid. An ‘active’ extract, added as a supplement to this medium, induces cell division and the formation of microscopically visible clusters of new cells after 14 days. In control cultures the cells enlarge but do not divide. Reduced nitrogen compounds and hexitols are not required in this assay, nor do they enhance the effect of active extracts as they do in the carrot root phloem test9, which has been used in many studies on coconut milk.

A Comparison of Plant Cell Division Inducers from Coconut Milk and Apple Fruitlets

THE genus Duboisia comprises three species: D. hopwoodii F. Muell. is confined to the interior of the Australian continent, D. leichhardtii F. Muell. has a restricted occurrence in the highlands of southeastern Queensland, and D. myoporoides R. Br. occurs along the eastern coast of Australia and in New Caledonia. The leaves of all three species contain major quantities of alkaloids; but the genus is unique in that one species, D. hopwoodii, contains the pyridine alkaloids nicotine and nor-nicotine, whereas the other two contain various alkaloids of the tropane group. Efforts to detect tropane alkaloids among the minor alkaloids of D. hopwoodii have failed. Similarly, although the other two Australian species contain volatile bases, the occurrence of nicotine or nor-nicotine has not been demonstrated.

W. Bottomley

Papers

Studies on Phytoalexins. V. The Structure of Pisatin from Pisum sativum L.

A kinin in apple fruitlets.

Pisatin: an antifungal substance from Pisum sativum L.

A Comparison of Plant Cell Division Inducers from Coconut Milk and Apple Fruitlets

Occurrence of nicotine together with hyoscine in Duboisia myoporoides R. Br.