Strigolactone and karrikin signal perception: receptors, enzymes, or both?

Abstract

The signaling molecules strigolactone (SL) and karrikin are involved in seed germination, development of axillary meristems, senescence of leaves, and interactions with arbuscular mycorrhizal fungi. The signal transduction pathways for both SLs and karrikins require the same F-box protein (MAX2) and closely related α/β hydrolase fold proteins (DAD2 and KAI2). The crystal structure of DAD2 has been solved revealing an α/β hydrolase fold protein with an internal cavity capable of accommodating SLs. DAD2 responds to the SL analog GR24 by changing conformation and binding to MAX2 in a GR24 concentration-dependent manner. DAD2 can also catalyze hydrolysis of GR24. Structure activity relationships of analogs indicate that the butenolide ring common to both SLs and karrikins is essential for biological activity, but the remainder of the molecules can be significantly modified without loss of activity. The combination of data from the study of DAD2, KAI2, and chemical analogs of SLs and karrikins suggests a model for binding that requires nucleophilic attack by the active site serine of the hydrolase at the carbonyl atom of the butenolide ring. A conformational change occurs in the hydrolase that results in interaction with the F-box protein MAX2. Downstream signal transduction is then likely to occur via SCF (Skp-Cullin-F-box) complex-mediated ubiquitination of target proteins and their subsequent degradation. The role of the catalytic activity of the hydrolase is unclear but it may be integral in binding as well as possibly allowing the signal to be cleared from the receptor. The α/β hydrolase fold family consists mostly of active enzymes, with a few notable exceptions. We suggest that DAD2 and KAI2 represent an intermediate stage where some catalytic activity is retained at the same time as a receptor role has evolved.

Effector-Triggered Immunity: From Pathogen Perception to Robust Defense

“…Although the EDS1 N-terminal domain is remarkably similar to a lipase enzyme structure, EDS1 does not use a catalytic mechanism for TNL or basal resistance signaling (141). In this respect, EDS1 resembles an emerging class of plant lipase-like molecular switches (65,141). Significantly, EDS1 was found to reside in complexes with several Arabidopsis TNL receptors, including RPS4 (10,59,75).…”

The interaction between strigolactones and other plant hormones in the regulation of plant development

“…Both the F-box protein MAX2/RMS4/D3 (Stirnberg et al, 2007; Yoshida et al, 2012) and the α/β-fold hydrolase D14/D88/HTD2/DAD2 (Arite et al, 2009; Liu et al, 2009; Gaiji et al, 2012; Hamiaux et al, 2012) have been shown to be involved in SL downstream signaling. More aspects about SLs biosynthesis, perception, and signaling as well as structure-function relationships have been nicely addressed and updated in several recent reviews (Janssen and Snowden, 2012; Ruyter-Spira et al, 2013; Zwanenburg and Pospisil, 2013). …”

SUPPRESSOR OF MORE AXILLARY GROWTH2 1 Controls Seed Germination and Seedling Development in Arabidopsis

“…Genetic studies have demonstrated that MORE AXILLARY GROWTH2 (MAX2) has a central role in mediating both KAR and SL signaling and controls a broad range of developmental processes (for review, see Janssen and Snowden, 2012). MAX2 has been identified in genetic screens for Arabidopsis mutants with increased lateral shoot branching (max2; Stirnberg et al, 2002), delayed leaf senescence (oresara9 [ore9]; Woo et al, 2001), decreased light responses (pleiotropic photosignaling; Shen et al, 2007), and 3-methyl-2H-furo[2,3-c]pyran-2-one (KAR 1 )-insensitive germination (karrikin-insensitive1 [kai1]; Nelson et al, 2011).…”