Edouard Boex-Fontvieille, Sachin Rustgi, Diter von Wettstein, Steffen Reinbothe, and Christiane Reinbothe

PLANT BIOLOGY

Significance

Herbivory is one of the most important processes in the biosphere. When plants germinated underneath the soil or fallen leaves undergo skotomorphogenesis, they are especially prone to a vast range of seed predators and herbivorous arthropods. How greening plants protect themselves against these foes was thus far largely unknown. Here, we describe a mechanism how etiolated seedlings deter arthropod devourers. Our article thus contributes to the understanding of plant survival strategies in the natural environment.

Abstract

Water-soluble chlorophyll proteins (WSCPs) constitute a small family of unusual chlorophyll (Chl)-binding proteins that possess a Kunitz-type protease inhibitor domain. In Arabidopsis thaliana, a WSCP has been identified, named AtWSCP, that forms complexes with Chl and the Chl precursor chlorophyllide (Chlide) in vitro. AtWSCP exhibits a quite unexpected expression pattern for a Chl binding protein and accumulated to high levels in the apical hook of etiolated plants. AtWSCP expression was negatively light-regulated. Transgenic expression of AtWSCP fused to green fluorescent protein (GFP) revealed that AtWSCP is localized to cell walls/apoplastic spaces. Biochemical assays identified AtWSCP as interacting with RD21 (RESPONSIVE TO DESICCATION 21), a granulin domain-containing cysteine protease implicated in stress responses and defense. Reconstitution experiments showed tight interactions between RD21 and WSCP that were relieved upon Chlide binding. Laboratory feeding experiments with two herbivorous isopod crustaceans, Porcellio scaber (woodlouse) and Armadillidium vulgare (pillbug), identified the apical hook as Achilles’ heel of etiolated plants and that this was protected by RD21 during greening. Because Chlide is formed in the apical hook during seedling emergence from the soil, our data suggest an unprecedented mechanism of herbivore resistance activation that is triggered by light and involves AtWSCP.

See: http://www.pnas.org/content/112/23/7303.abstract.html?etoc

PNAS June 9, 2015 vol. 112 no. 23 7303-7308  

Fig. 7.

Structural model for the AtWSCP–RD21 interaction, predicted by using ClusPro. (A) Ribbon diagram of the AtWSCP–RD21 complex (front view). The fifth or reactive site loop (RSL) that spans between the fifth and sixth β-strands of AtWSCP is shown in purple, and it’s beginning at Lys84 and end at Ser95 are marked by arrows. The Trp88 and Pro89 residue at the RSL of AtWSCP that intrudes between the catalytic triad, i.e., Cys161, His297, and Asn317 at the active site cleft of RD21 are shown by lines. The β-strands and α-helices of RD21 are shown respectively in yellow and red, whereas the β-strands of AtWSCP are depicted in magenta, an α-turn in cyan, and connecting loops are shown in deep salmon. (B) An enlarged view of the reactive site loop showing the intruding amino acid residues and their respective locations in AtWSCP (a). Specific amino acid residues from AtWSCP and RD21 that form hydrogen bonds are shown in purple or orange and green, respectively. These interactions involve the following: AtWSCP:Lys92-RD21:Asp154 (b), and AtWSCP:Leu41 and Pro42-RD21:Lys227 (c). Amino acid numbering for RD21 is based on their locations in the full-length preproprotein, and for AtWSCP is based on the mature protein after removal of the signal peptide.