Regulation and Function of Defense-Related Callose Deposition in Plants

Abstract

Plants are constantly exposed to a wide range of potential pathogens and to protect themselves, have developed a variety of chemical and physical defense mechanisms. Callose is a β-(1,3)-D-glucan that is widely distributed in higher plants. In addition to its role in normal growth and development, callose plays an important role in plant defense. Callose is deposited between the plasma membrane and the cell wall at the site of pathogen attack, at the plasmodesmata, and on other plant tissues to slow pathogen invasion and spread. Since it was first reported more than a century ago, defense-related callose deposition has been extensively studied in a wide-spectrum of plant-pathogen systems. Over the past 20 years or so, a large number of studies have been published that address the dynamic nature of pathogen-induced callose deposition, the complex regulation of synthesis and transport of defense-related callose and associated callose synthases, and its important roles in plant defense responses. In this review, we summarize our current understanding of the regulation and function of defense-related callose deposition in plants and discuss both the progresses and future challenges in addressing this complex defense mechanism as a critical component of a plant immune system.

Keywords: PMR4; callose; papillae; plant cell wall defense; plant immunity; plasmodesmata.

Figure 1

Signaling of pathogen-induced callose deposition in plants. (A) pathogen-associated molecular pattern (PAMP)-induced callose deposition is mediated by pattern-recognition receptors (PRRs) and promoted by indole glucosinolates (IGSs), reactive oxygen species (ROS), and RNAi regulatory protein Argonaute1 (AGO1). ABA stimulates but represses PAMP-induced callose deposition in the presence of high sucrose concentrations. (B) pathogeninduced callose deposition in plasmodesmata (PD) through the SA-mediated signaling pathway.

Figure 2

Synthesis, transport, activation, and recruitment of Arabidopsis PMR4 callose synthase during plant defense responses. Some of the important factors identified to be involved in these processes are indicated.

Figure 3

Induction and roles of defense-related callose deposition in plants. Pathogen elicitor-activated signaling of plant innate immune responses leads to increased callose deposition at the sites of pathogen attack, at plasmodesmata and in the vascular tissues. Formation of callose-rich papilla at the infection sites helps restrict penetration and colonization by invading pathogens. Increased callose deposition at plasmodesmata leads to plasmodesmata closure, which helps restrict pathogen spread. Increased callose deposition in the vascular tissues such as phloem sieve tubes could also functions as a defense mechanism for reducing the colonization and transport of vascular pathogens. In Arabidopsis, pathogen-induced SA signaling is negative regulated by PMR4-dependent callose deposition. Pathogens contain effector proteins that inhibit or block defense-related callose deposition as counter-defense mechanisms.