**2. Perception of SLs**

**Figure 3.** Scheme of SLs biosynthesis and a list of enzymes involved in this process. Descriptions are given in the text.

**Figure 2.** Structures of SL precursor carlactone and SLs represent two main stereochemical groups: strigol‐type SL with a β‐oriented C ring – 5‐deoxystrigol and orobanchol type with an α‐oriented C ring – orobanchol. Differences are present

104 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

at the 8b and 3a positions between B and C rings.

The Dwarf14 (D14) protein is the only known receptor of SLs. This protein was originally identified in rice [31] and later on found in other species, such as *A. thaliana* (AtD14) [32], petunia (DAD2) [33], *Hordeum vulgare* L. (HvD14) [34] or *Populus trichocarpa* Torr. & A. Gray (PtD14) [35]. All D14 proteins belong to the α/β‐hydrolase family and exhibit enzy‐ matic activity, which is unusual for hormone receptors. D14 proteins can not only bind SL molecule but also hydrolyze the ligand, which is crucial for the next steps of the signaling cascade [36]. The entry to the active site pocket of D14 is surrounded by four helicases, and mutation in which the size of this aperture is reduced causes an insensitivity to SLs (**Figure 4**) [34]. When a ligand is docked to the receptor, a nucleophilic attack separates the ABC part of the SL molecule from the D ring [37]. The hydrolyze activity of D14 depends on the presence of a highly conserved catalytic Ser/His/Asp triad. Replacement of these amino acids results in a loss of D14 activity and sensitivity to SLs [33]. It was also shown

**Figure 4.** Visualization of SL receptor HvD14. (A) Structure of HvD14. Circle indicates entry to the active site pocket, surrounded by four helices. (B) 3D structure visualization of HvD14. (C) Detailed view of the entry to the active site pocket of the wild‐type protein and (D) mutated protein with smaller aperture, resulting in insensitivity to SLs.

that serine from the catalytic triad is involved in docking of SLs into D14 [38]. With an aver‐ age rate of 0.3 molecule/min, D14‐mediated hydrolysis of SLs into non‐active derivatives is very slow, indicating that this is not the main function of D14 [33, 36]. Crystallographic analysis indicates that the degradation of SL molecules by D14 brings about a change in receptor conformation, which is necessary for the interaction between D14 and other com‐ ponents from SL‐signaling pathway [39]. After nucleophilic attack and release of the ABC part, the D ring remains within the receptor that now assumes a "closed" state unable to bind further molecules, reviewed by Waters [7]. This change in conformation destabilizes the D14 receptor, thus initiating its own degradation [40] (**Figure 5**). This is the first known case where hormone hydrolysis by a receptor causes the degradation receptor as well.

Since SLs are involved in the regulation of the development of different organs, it was expected that D14 will be located in almost all plant tissues. Expression analysis of *Atd14* in *A. thaliana*, however, showed markedly higher levels in vascular tissues of roots and shoots [41]. The discrepancy between expression and distribution pattern of the D14 protein was explained, when the intercellular transport of D14 *via* the phloem was uncovered. This transport is SL‐independent and also in the SL biosynthesis mutants it was observed that D14 was delivered into axillary buds [42] leaving the question by what mechanism D14 is transported and how it is been used by plants to regulate the development and adaptation to different stresses.

**Figure 5.** Overview of SL‐signaling cascade including hydrolysis of SL molecules by receptor and change of the receptor conformation, which allows the interaction with the SCF complex and repressor. Ubiquitination of the repressor, mediated by the SCF complex, results in the expression of genes from the TCP family.

It has to be highlighted that D14 protein is specific receptor only for SLs. There are close D14 homologs, such as receptor for Karrikins (KARs): Karrikin‐Insensitive2 (KAI2) that also belongs to the α/β‐hydrolase. However, it was experimentally confirmed that D14 and KAI2 exhibit the different ligand specification [32].
