ER bodies in plants of the Brassicales order: biogenesis and association with innate immunity

Abstract

The endoplasmic reticulum (ER) forms highly organized network structures composed of tubules and cisternae. Many plant species develop additional ER-derived structures, most of which are specific for certain groups of species. In particular, a rod-shaped structure designated as the ER body is produced by plants of the Brassicales order, which includes Arabidopsis thaliana. Genetic analyses and characterization of A. thaliana mutants possessing a disorganized ER morphology or lacking ER bodies have provided insights into the highly organized mechanisms responsible for the formation of these unique ER structures. The accumulation of proteins specific for the ER body within the ER plays an important role in the formation of ER bodies. However, a mutant that exhibits morphological defects of both the ER and ER bodies has not been identified. This suggests that plants in the Brassicales order have evolved novel mechanisms for the development of this unique organelle, which are distinct from those used to maintain generic ER structures. In A. thaliana, ER bodies are ubiquitous in seedlings and roots, but rare in rosette leaves. Wounding of rosette leaves induces de novo formation of ER bodies, suggesting that these structures are associated with resistance against pathogens and/or herbivores. ER bodies accumulate a large amount of β-glucosidases, which can produce substances that potentially protect against invading pests. Biochemical studies have determined that the enzymatic activities of these β-glucosidases are enhanced during cell collapse. These results suggest that ER bodies are involved in plant immunity, although there is no direct evidence of this. In this review, we provide recent perspectives of ER and ER body formation in A. thaliana, and discuss clues for the functions of ER bodies. We highlight defense strategies against biotic stress that are unique for the Brassicales order, and discuss how ER structures could contribute to these strategies.

Keywords: ER body; endoplasmic reticulum; glucosinolate; organelle biogenesis; plant defenses; secondary metabolites; β-glucosidase.

Figure 1

ER bodies in Arabidopsis thaliana. A confocal micrograph (A) and an electron micrograph (B) of cotyledon and root epidermal cells, respectively, of A. thaliana. Arrowheads indicate ribosomes on the surface of the ER body membranes. ER-localized GFP (SP-GFP-HDEL) labels ER bodies as well as the typical ER network, and electron microscopy identifies ribosomes at the cytosolic surface of ER bodies, both of which indicate the luminal continuity between ER and ER bodies. Enlarged images of the squared regions are shown below. CW, cell wall; V, vacuole; Mt, mitochondrion; G, Gold body; Bars, 10 μm (A) and 1 μm (B).

Figure 2

Models of ER body formation in A. thaliana seedlings. (A) NAI1 is expressed in epidermal cells to regulate ER body formation by inducing the expression of PYK10, NAI2, MEB1, and MEB2. (B) PYK10 and NAI2 may physically interact to form ER body structures from the ER. (C,D) NAI2 forms a complex with MEB1and MEB2, then recruits them to the ER body to form the ER body-specific membrane.

Figure 3

Tissue localization of ER bodies. ER bodies are present in the epidermis of whole tissues in young seedlings, but are absent in aerial organs of mature plants. Root epidermal, cortical, and endodermal cells continue to develop ER bodies. Epidermal cells of a transgenic A. thaliana expressing ER-localized GFP from several tissues are shown.

Figure 4

Both classical and atypical myrosinases form Brassicales-specific clades. The full-length protein sequences of β-glucosidases in A. thaliana (blue squares), Capusella rubella (light-blue squares), Thellungiela salsuginea (cyan squares), Carica papaya (green diamonds), Vitis vinifera (dark purple diamonds), Glycine max (magenta diamonds), Oryza sativa (red circles), and Physcomitrella patens (dark yellow triangles) were retrieved from the Phytozome database (http://www.phytozome.net/) and aligned by ClustalW (http://clustalw.ddbj.nig.ac.jp/index.php). BGLU numbers of the β-glucosidases in A. thaliana are indicated by blue letters. At3g06510 was used as an out group. Subfamilies proposed by Xu et al. (2004) are indicated accordingly.

Figure 5

Indole glucosinolate structure includes a side chain similar to that of scopolin and esculin, but not similar to that of sinigrin. I3G, indol-3-ylmethylglucosinolate; 1M-I3G, 1-methoxyindol-3-ylmethylglucosino late; 4M-I3G, 4-methoxyindol-3-ylmethylglucosinolate; 4-MU-glucoside, 4-methylumbelliferyl-β-D-glucoside; 4-MU-fucoside, 4-methylumbelliferyl-β-D-fucoside.

Figure 6

PYK10 and ER bodies may have a role in iron uptake via hydrolysing scopolin. ER bodies may fuse with the plasma membrane under iron-deficient conditions, resulting in the relocation of MEB1 and MEB2 to the plasma membrane and secretion of PYK10 to the apoplast. Scopolin, secreted in an ABCG37-dependent manner, is then converted to scopoletin by PYK10, which in turn helps the cells to take up iron via the chelating activity of scopoletin. This uptake may be carried out by putative MEB transporters.

Figure 7

Two distinct myrosinase-glucosinolate systems occurring in Brassicales. (A) In leaves, myrosinases and glucosinolates accumulate in vacuoles of distinct cells. Tissue collapse accompanied by destruction of multiple cells results in the mixing of enzymes and substrates, and the production of toxic products against herbivores. (B) In roots, myrosinases and glucosinolates accumulate in ER bodies and vacuoles, respectively, in the same cells. Enzymatic reactions may be activated by (i) cosecretion of enzymes and substrates, (ii) translocation of enzymes into vacuoles, or (iii) destruction of a single cell.