**5. SL versus KAR signaling**

there is no evidence that MAX2/D3 can interact with SLs, there are reasons to assume that

For a long time, it was not known which proteins are recognized by the SCFMAX2/D3 complex, but recently the SL repressors degraded during SL signal transduction were identified in rice (D53) [51, 52] and in *A. thaliana* (Suppressor of MAX2‐Like 6 to 8, SMXL 6 to 8) [53–55]. Gain‐of‐function mutation in D53 resulted in semi‐dwarf plants with increased tillering, a phenotype which is characteristic for SL mutants. Similar effects were observed after over‐ expression of *D53*, whereas reduced expression of *D53* in a *d53*‐mutant background inhib‐ ited tiller formation [52]. D53 shows the nuclear localization and it was confirmed that in the presence of SL molecules degradation of D53 occurs. This degradation proceeds through the proteasome‐dependent pathway and requires the presence of D14 and D3 proteins [51, 52]. There are evidences that D53 may also interact with D3 in the absence of D14, although this interaction is less efficient. In contrast to rice, where only one SL repressor has been identi‐ fied, *A. thaliana* contains three proteins—SMXL 6 to 8—that may act redundantly. First reports indicated that only a triple mutant *smxl6/7/8* will result in a phenotype with reduced number of tillers [54]. Later on, it was found that the phenotype characteristic for SL mutants could be produced by the expression of the non‐degradable form of SMXL 7 under a native promoter [55]. All three SMXLs interact with D14 and are degraded in an SL‐dependent manner in the presence of MAX2 and D14 [53, 54]. It still remains an open question whether SMXLs 6 to 8 do

The SL repressors of both *A. thaliana* and rice contain the conserved amino acid sequences (F/L‐D‐L‐N‐L) which is known as an ethylene‐responsive element‐binding factor‐associated amphiphilic repression (EAR) motif. This motif plays a key role in interactions with transcrip‐ tional corepressors from Topless (TPL) and Topless‐Related Proteins (TPR) families [52, 53]. TPL and TPR regulate the expression of genes in response to different classes of hormones, such as auxin or jasmonates [56]. The presence of an EAR motif in SL repressors suggests that D53/SMXLs may bind TPL/TPR corepressors. An ensuing degradation of these corepressors may then result in the expression of transcriptional factors, previously suppressed by TPL/ TRP. Interactions between SMXL6 to 8 and proteins from the TPR family were already con‐

Phytohormones induce a change in gene expression. This response is usually mediated by transcription factors. Until now, only one family of transcription factors has been identified as a downstream component in SL signaling, namely the TEOSINTE BRANCHED1/CYCLOIDEA/ PROLIFERATING CELL FACTOR1 family (TCP). In different species, single transcription fac‐ tors from this family, related to SL signal, were already characterized: Branched1 (BRC1) in *A. thaliana* [57], Fine Culm1/Teosinte Branched1 (FC1/OsTB1) in rice [58], TB1 in *Zea mays* L. [59] and PsBRC1 in *Pisum sativum* L. [60]. Expression of these genes is particularly strong in axillary buds, and mutations in these genes lead to an increased branched phenotype, which

MAX2/D3 may act as a receptor for other signaling molecules.

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

act redundantly or they are involved in different responses to SLs.

firmed using a yeast‐two hybrid and co‐immunoprecipitation assays [54].

**4. Transcription response to SL signal**

The unique features of SLs signaling have been discussed elsewhere [7, 63]. Here, the simi‐ larities and differences between SLs and KARs will be summarized. Though SLs and KARs play different roles in plant development [64], there are some striking similarities in the signal transduction mechanisms of these two classes of plant growth regulators [65] what might be crucial in understanding the mechanisms of their actions in plants. As already mentioned, the signal transduction of SL and KAR is mediated by the same F‐box protein MAX2. However, since the signals generated by SLs and KARs are not interchangeable these phytohormones must be recognized by different receptors. Indeed, the D14 receptor has found to be specific for SLs, whereas the KAI2 receptor is specific for KARs and based on the differences in the size of active pocket site they cannot recognize the signal from second group of plant growth regulators [66, 67]. Both D14 and KA12 display a conserved catalytic triad, but only in case of D14 its catalytic activity was confirmed [33, 37]. Not only has a catalytic function of KAI2 never been proven, modeling studies of the KAR‐KAI2 complex indicate that the distance between the KAR molecule and the catalytic Ser from KAI2 prohibits nucleophilic attack [67, 68]. Nevertheless, since mutation in the catalytic triad of KA12 can abolish the function of this receptor [69], the catalytic triad of the KA12 receptor may be essential for ligand binding. Similar observations have been made for D14 [38]. The second similarity between both recep‐ tors is their degradation during perception, though in the case of KAI2 the presence of MAX2 is not required for its degradation [70].

MAX2 is a component of SCF complexes which are involved in the conversion of SL and KAR signals. Therefore, the phenotypic effects caused by a mutation of MAX2 are due to an insen‐ sitivity to both plant growth regulators.

Due to the presence of different receptors, the respective SCF complexes guide the degrada‐ tion of different suppressors: SMXL6 to 8 in the case of D14‐SL‐Max2 and Supressor of Max2 1 (SMAX1) in the case of KAI2‐KAR‐MAX2 [71]. *SMAX1* and *SMXL6/7/8* show similar patterns of expression in *A. thaliana* seedlings and all four proteins may interact with TPL corepres‐ sors *via the* EAR motif [53] which indicates that ultimately SLs and KARs may regulate gene expression by a similar mechanism.
