An Ethylene-Protected Achilles’ Heel of Etiolated Seedlings for Arthropod Deterrence

Abstract

A small family of Kunitz protease inhibitors exists in Arabidopsis thaliana, a member of which (encoded by At1g72290) accomplishes highly specific roles during plant development. Arabidopsis Kunitz-protease inhibitor 1 (Kunitz-PI;1), as we dubbed this protein here, is operative as cysteine PI. Activity measurements revealed that despite the presence of the conserved Kunitz-motif the bacterially expressed Kunitz-PI;1 was unable to inhibit serine proteases such as trypsin and chymotrypsin, but very efficiently inhibited the cysteine protease RESPONSIVE TO DESICCATION 21. Western blotting and cytolocalization studies using mono-specific antibodies recalled Kunitz-PI;1 protein expression in flowers, young siliques and etiolated seedlings. In dark-grown seedlings, maximum Kunitz-PI;1 promoter activity was detected in the apical hook region and apical parts of the hypocotyls. Immunolocalization confirmed Kunitz-PI;1 expression in these organs and tissues. No transmitting tract (NTT) and HECATE 1 (HEC1), two transcription factors previously implicated in the formation of the female reproductive tract in flowers of Arabidopsis, were identified to regulate Kunitz-PI;1 expression in the dark and during greening, with NTT acting negatively and HEC1 acting positively. Laboratory feeding experiments with isopod crustaceans such as Porcellio scaber (woodlouse) and Armadillidium vulgare (pillbug) pinpointed the apical hook as ethylene-protected Achilles’ heel of etiolated seedlings. Because exogenous application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and mechanical stress (wounding) strongly up-regulated HEC1-dependent Kunitz-PI;1 gene expression, our results identify a new circuit controlling herbivore deterrence of etiolated plants in which Kunitz-PI;1 is involved.

The complex world of plant protease inhibitors: Insights into a Kunitz-type cysteine protease inhibitor ofArabidopsis thaliana

“… 3,43 By contrast, AtSerpin1 expression was constitutive and insensitive to phytohormones ( refs. 3 & 43 , and unpublished results).…”

Jasmonic acid protects etiolated seedlings of Arabidopsis thaliana against herbivorous arthropods

“…We found that also ethylene can trigger Kunitz-PI;1 expression in the apical hook and augment arthropod deterrence of etiolated seedlings, whereas inhibition of ethylene biosynthesis reduced isopod resistance. 57 Thus complex regulatory circuits operate to protect etiolated seedlings against biotic and abiotic foes in their environment.…”

New homologues of Brassicaceae water‐soluble chlorophyll proteins shed light on chlorophyll binding, spectral tuning, and molecular evolution

“…This evolutionary trend suggests that type-II WSCPs may have at least one more function in addition to Chl binding. While the specific nature of this function, and its physiological role remain to be determined, the sequence and structural homology between type-II WSCPs and STI [3] and other plant protease inhibitors [38,39], as well as biochemical and physiological evidence [4][5][6][7][8][9][10][11][12], suggest that they may serve as serine-and/or cysteine protease inhibitors to protect against biotic or abiotic stress [5,[7][8][9][10][11][12][13], or to regulate endogenous protease activities [4,14,15].…”

“…Only a few are water-soluble, including the bacteriochlorophyll-binding Fenna-Matthews-thylakoids and do not participate in photosynthetic reactions. Furthermore, the sequences and structures of type-II WSCPs classify them as part of the Kunitz soybean trypsin inhibitor (STI) superfamily [3], and some were shown to inhibit serine [4] and/or cysteine [5,6] proteases. Their high levels of expression in response to various plant stress conditions such as drought, salinity, and heat [7][8][9][10][11][12], led to suggestions of their possible role in plant defense against herbivores [5,13], or in regulation of endogenous protease activities [4,14,15].…”