Plant defense inducers

S.N. Lavanya completed her MSc in Botany and MPhil in Seed Technology from the University of Mysore, India and is currently working as a research fellow in the UGC sponsored project. Her research areas include lipid transfer proteins in plant defense, induced resistance in plants, biological control and isolation of biomolecules from plants and microbes. 

Forty-six different specific fungicide and bactericide modes of action are actually classified in the FRAC lists [3], including the unknown modes of action (Table 12.1). In addition, numerous multi-site inhibitors and plant defense inducers are available for the control of plant diseases worldwide. Therefore, a sufficient diversity of modes of action seems to be available for the control of plant diseases and for an effective resistance management. However, many fungicides are only available in a restricted number of regions and crops because they may not be registered everywhere or because the market size may not be big enough. 

Specific modes of action [unknown (U) included except host plant defense inducers (P) and multi-site inhibitors (M)]. 

FRAC Mode of Action Classification and Resistance Risk of Fungicides Table 12.15 Group P Host plant defense inducers. 

Agrawal A (2000), Benefits and costs of induced plant defenses for Lepidium virginicum (Brassicaceae) , Ecology, 87, 804-813. 

Owing to the costs of defenses, natural selection will optimize defenses. There are various aspects in this optimization, including defense allocation with respect to temporal variation in grazing pressure and within-plant defense allocation with respect to grazing risk and the value of different plant parts (reviewed in Stamp 2003). Here we will focus on both the induced defenses and within-plant allocation of defenses, because these aspects have been widely studied in macroalgae. 

Fritig B, Heitz T, Legrand M (1998) Antimicrobial proteins in induced plant defense. Curr Opin Immunol 10 16-22... 

Fig. 11.1 Simplified scheme of the emission and function of herbivore-induced plant volatiles in plant defense. DMNT, 4,8-dimethyl-l,3,7-nonatriene) TMTT, 4,8,12-trimethyltrideca-l,3,7, 11-tetraene...

Hilker M, Meiners T (2006) Early herbivore alert insect eggs induce plant defense. J Chem Ecol 32 1379-1397... 

Both 15 and 16 are involved in plant defense, they may have a role in the flavor spectrum, and their concentration can influence the nutritional safety and quality of food crops. For example, Soledade and colleagues demonstrated the crucial role of indole-3-aldoxime in the biogenesis of cruciferous defense against fungi-induced plant diseases. This is achieved by inserting the indolyl moiety via indole-3-aldoxime, which is a precursor of several secondary chemical defense metabolites of cruciferae. 

A better understanding of the role of phytoalexins in plant defenses and of the mechanisms of induced resistance may potentially open a powerful new approach to the control of insect pests of cultivated plants. If indeed, in light of the hypothesis of optimal defense strategies (3), a post-attack response is a more efficient line of defense than the attack-independent accumulation of allelochemics, the exploitation of phytoalexin-producing mechanisms may represent a fertile field for future investigations. Several uses of induced resistance may be conceived. Four of these approaches are briefly discussed. 

Arimura et al. (2000a) found that several of the induced volatiles themselves can serve as elicitors by triggering gene activation in neighboring leaves that leads to further emissions. In this context, (Z)-jasmone was shown to be a potent plant-derived volatile elicitor that triggers the release of -ocimene in the bean plant, Viciafaba (Birkett et al, 2000). These examples of plant odors inducing plant defense pathways have important implications for plant-plant communication (see below). 

As yet, there is no specific pattern in how induced volatiles affect the attractiveness of plants to herbivores. Obviously, the responses will be correlated with fitness consequences. Insects vulnerable to natural enemies and induced plant toxins are, therefore, expected to avoid induced plants, whereas those that are adapted to plant defenses and/or benefit from aggregating are likely to be attracted. Comparative studies could test such hypotheses. 

Agrawal, A. A., Tuzun, S. and Bent,E. (1999). Induced Plant Defenses Against Pathogens and Herbivores. St Paul, MO APS Press. 

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