Plants have the ability to recognize and respond to a multitude of microorganisms. Recognition of pathogens results in a massive reprogramming of the plant cell to activate and deploy defense responses to halt pathogen growth. Such responses are associated with increased demands for energy, reducing equivalents, and carbon skeletons that are provided by primary metabolic pathways. Although pathogen recognition and downstream resistance responses have been the focus of major study, an intriguing and comparatively understudied phenomenon is how plants are able to recruit energy for the defense response. To that end, this review will summarize current research on energy-producing primary metabolism pathways and their role in fueling the resistance response.

https://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI-22-5-0487

Primary metabolism and plant defense--fuel for the fire.

A number of different types of induced resistance have been defined based on differences in signalling pathways and spectra of effectiveness, including systemic acquired resistance and induced systemic resistance. Such resistance can be induced in plants by application of a variety of biotic and abiotic agents. The resulting resistance tends to be broad-spectrum and can be long-lasting, but is rarely complete, with most inducing agents reducing disease by between 20 and 85%. Since induced resistance is a host response, its expression under field conditions is likely to be influenced by a number of factors, including the environment, genotype, crop nutrition and the extent to which plants are already induced. Although research in this area has increased over the last few years, our understanding of the impact of these influences on the expression of induced resistance is still poor. There have also been a number of studies in recent years aimed at understanding of how best to use induced resistance in practical crop protection. However, such studies are relatively rare and further research geared towards incorporating induced resistance into disease management programmes, if appropriate, is required.

Controlling crop diseases using induced resistance: challenges for the future

Plants can be induced to develop enhanced resistance to pathogen infection by treatment with a variety of abiotic and biotic inducers. Biotic inducers include infection by necrotizing pathogens and plant-growth-promoting rhizobacteria, and treatment with nonpathogens or cell wall fragments. Abiotic inducers include chemicals which act at various points in the signaling pathways involved in disease resistance, as well as water stress, heat shock, and pH stress. Resistance induced by these agents (resistance elicitors) is broad spectrum and long lasting, but rarely provides complete control of infection, with many resistance elicitors providing between 20 and 85% disease control. There also are many reports of resistance elicitors providing no significant disease control. In the field, expression of induced resistance is likely to be influenced by the environment, genotype, and crop nutrition. Unfortunately, little information is available on the influence of these factors on expression of induced resistance. In order to maximize the efficacy of resistance elicitors, a greater understanding of these interactions is required. It also will be important to determine how induced resistance can best fit into disease control strategies because they are not, and should not be, deployed simply as "safe fungicides". This, in turn, will require information on the interaction of resistance elicitors with crop management practices such as appropriate-dose fungicide use.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-95-1368

Induced Resistance for Plant Disease Control: Maximizing the Efficacy of Resistance Elicitors

Costs and trade-offs associated with induced resistance

652 ( I ) Direct effects of non-vector herbivores 653 (2) Indirect effects, mediated through the host 654 IV. Root-shoot interactions 657 ( I ) Above-ground insects and below-ground fungi 657 (2) Above-ground fungi and below-ground insects 658 . . . . . . 11. The effect of plant pathogenic fungi on herbivorous insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (c) Effects on insects . . . . . . . . . . . . 648 . . . . . . 111. The effect of herbivorous insects on plant pathogenic fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Induced resistance a common defence against pest and pathogen ? . . . . 660 ( I ) The hypersensitive response and lignification . . . . . . . . 661 ( 2 ) Phytoalexins . . . . . . . . . . . . . . 662 (3) Proteinase inhibitors 663 (4) Lipoxygenase 664

Three‐way interactions between plant pathogenic fungi, herbivorous insects and their host plants

SUMMARY
Photosynthesis and resistance to rust infection were studied in upper, uninfected leaves of broad bean, Vicia faba L., where the lower two leaves were infected with rust, Uromyces viciae-fabae (Pers.) Shroet. Following inoculation of the lower leaves, rates of net photosynthesis were significantly increased in the upper, uninfected, fully-developed leaves and the young developing leaves, compared to controls. In contrast, photosynthesis was substantially reduced in the lower, rusted leaves. When 14CO2 was fed to the upper, uninfected leaves of rusted plants, there was a considerable increase in labelled assimilate in those leaves, compared to controls. In addition, there was substantial movement of labelled assimilate into lower, rusted leaves, into young, developing leaves and into roots. On the other hand, there was a substantial reduction in labelled assimilates moving into shoot apices.

Upper, uninfected leaves of rusted beans exhibited Increased resistance to rust infection. Thus, the percentage leaf area covered with rust and the number of pustules per cm2 were reduced in these leaves compared to controls. This resistance to rust infection was greatest when the upper leaves were challenged 1 d after inoculation of the lower leaves with rust and decreased with increasing time after inoculation of the lower leaves. Young, developing leaves on rusted beans also exhibited increased resistance to rust infection. When photosynthesis in the upper leaves was reduced to near control values or well below control values by shading, resistance to rust infection in those leaves was also reduced, although not in proportion to the reduction in photosynthesis. It is suggested that the increased photosynthesis in upper, uninfected bean leaves probably facilitates maximum expression of resistance to infection in those leaves.

D. C. Murray

Papers

Increased photosynthesis and resistance to rust infection in upper, uninfected leaves of rusted broad bean (Vicia faba L.)