PLANT EPIDERMIS · The role of the DEK1 pathway in plant epidermis cell fate specification

The role of the DEK1 pathway in plant epidermis cell fate specification

Executive Summary: The Dek1 project, supported by this grant and grants to Prof. Olsen from the Norwegian Research Council (2010-2013) has involved partners from the host institution, the Norwegian University of life sciences (NMBU), Hedmark University college, University of Oslo, University of Tromsø, Washington State University, St.Louis, USA and University of South-western Louisiana at Lafayette, Lafayette Louisiana, USA and University of Maryland, USA. Our working hypothesis is that DEK1, consisting of 21 transmembrane segments interrupted by a Loop and a calpain proteinase effector linked to the membrane segments by an Arm, played an essential role in land plant evolution. During the transition from single celled green algae to land plants 3D growth patterns emerged and our hypothesis is that DEK1 assumed a novel function to provide the positional information required to direct 3D growth. Based on the result from our research, the following picture of the evolutionary history of DEK1 has emerged. DEK1 from land plants form a separate clade of TML-calpains, calpains with large trans-membrane domains that appeared approximately 1,5 billion years ago. Algae that are considered ancestors of land plants, Charophyceae, retained only TML-calpains in the form of DEK1, and lost all cytosolic calpains. In land plants, ranging from mosses (Physcomitrella p.) to flowering plants (Arabidopsis t.), a span of 450 million years of evolutionary time, the proteolytic subunit CysPc is structurally and functionally conserved. This is demonstrated by the ability of the CysPc-C2L unit of Physomitrella p. to complement the Arabidopisis dek1-1 mutant. Evidence for an essential role in 3D growth patterning of DEK1 includes that Dek1 deletion mutants of Physomitrella p. is unaffected in two dimensional growth (protonema), but is unable to carry out the growth of its first 3D tissue (gametophore). Using homologous recombination we created Physcomitrella p. dek1 mutants that lack the Loop, displaying severely reduced 3D growth, but not as severe as in the dek1 deletion mutant. In contrast to the CysPc domain, the LOOP is less conserved, phylogenetic analysis identifying three groups, suggesting that the mechanism of DEK1 activation change as land plants evolved. Functional divergence is confirmed by the observation that the Loop of bryophytes, but not monocots or dicots complement the Physocmitrella p. dek1 LOOP deletion mutant. Immunohistochemical analysis detects DEK1 in concentrated foci at or near the plasma membrane similar to Arabidopsis. Similar deletion and substitution analysis is under way for the DEK1-ARM. An additional activity supported by this grant was to aid the applicant in establishing strategic initiatives in Norwegian Plant Molecular biology. Strategic initiatives that were aided by this grant included a subproject to sequences bread wheat chromosome 7B as part of the international consortium for wheat genome sequencing (IWGSC) and BIOKLIMA, a national project to establish growth facilities to study the effect of climate change in plants.

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