PLANT DEFENSE by Dale R. Walters

Author:Dale R. Walters
Language: eng
Format: epub
Publisher: A John Wiley & Sons
Published: 2011-06-02T16:00:00+00:00

Chapter 4

Plant Defense in the Real World: Multiple Attackers and Beneficial Interactions

4.1 Introduction

As we have seen in previous chapters, plants are equipped with an array of defense mechanisms to protect themselves against attack by pathogens, herbivores, and even other plants. Some of these defense mechanisms are preexisting or constitutive, while others are only activated upon attack. The primary immune response in plants evolved to recognize common features of organisms that interact with the plant and to convert this recognition into a defense response targeted specifically against the particular attacker (Jones & Dangl, 2006). But plants can also activate another line of defense, referred to as induced resistance (see Chapter 3). Depending on the type of organism attacking the plant, it can activate different types of induced resistance. It might, for example, activate systemic acquired resistance (SAR), induced systemic resistance (ISR), or wound-induced resistance. We saw in the previous chapter that salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are key players in the regulation of the signaling pathways involved in plant defense. When the plant is attacked, it responds by producing a specific blend of SA, JA, and ET—a sort of signal signature—which contributes to its primary induced defense response (De Vos et al., 2005). The signaling pathways subsequently activated regulate different defense responses that are effective against different types of attackers. As indicated in Chapter 3, generally speaking, biotrophic pathogens tend to be sensitive to SA-mediated induced defenses, while necrotrophs and herbivorous insects tend to be more sensitive to defenses mediated by JA and ET (Glazebrook, 2005).

However, plants often need to deal with more than one type of attacker, sometimes simultaneously. Therefore, the regulatory and signaling mechanisms used by the plant in its defense must be able to adapt to this changing, hostile environment. It appears that the plant achieves this through cross-talk between the different signaling pathways. This cross-talk can have outcomes that are positive or negative, depending on whether the signal interactions are synergistic or mutually exclusive (Bostock, 2005). But activating defenses to ward off attackers also runs the risk of affecting interactions with beneficial microorganisms such as mycorrhizal fungi and nitrogen-fixing bacteria. This chapter examines, therefore, not just the mechanisms plants use to deal with multiple attackers, but also whether activation of defense compromises interactions with beneficial microbes.

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