Major components of the KARRIKIN INSENSITIVE2-dependent signaling pathway are conserved in the liverwortMarchantia polymorpha

Abstract

KARRIKIN INSENSITIVE2 (KAI2) was first identified as a receptor of karrikins, smoke-derived germination stimulants. KAI2 is also considered a receptor of an unidentified endogenous molecule called the KAI2-ligand (KL). Upon KAI2 activation, signals are transmitted through degradation of D53/SMXL proteins via MAX2-dependent ubiquitination. Although components in the KAI2-dependent signaling pathway, namely MpKAI2A and MpKAI2B, MpMAX2, and MpSMXL, exist in the genome of the liverwort Marchantia polymorpha, their functions remain unknown. Here, we show that early thallus growth is retarded and gemma dormancy in the dark is suppressed in Mpkai2a and Mpmax2 loss-of-function mutants. These defects are counteracted in Mpkai2a Mpsmxl and Mpmax2 Mpsmxl double mutants indicating that MpKAI2A, MpMAX2 and MpSMXL act in the same genetic pathway. Introduction of MpSMXLd53, in which a domain required for degradation is mutated, into wild-type plants mimicks Mpkai2a and Mpmax2 plants. In addition, detection of citrine fluorescence in Nicotiana benthamiana cells transiently expressing a SMXL-Citrine fusion protein requires treatment with MG132, a proteasome inhibitor. These findings imply that MpSMXL is subjected to degradation, and that degradation of MpSMXL is crucial for MpKAI2A-dependent signaling in M. polymorpha. Therefore, we claim that the basic mechanisms in the KAI2-dependent signaling pathway are conserved in M. polymorpha.

The Physcomitrium (Physcomitrella) patens PpKAI2L receptors for strigolactones and related compounds function via MAX2-dependent and -independent pathways

“…Still, these results are consistent with the view that the KL pathway is ancestral relative to the SL pathway, and that the ancestral role of MAX2 in the land plants lineage is the transduction of the KL signal (Bythell-Douglas et al 2017;Walker et al 2019). Such a KL pathway has been recently reported in M. polymorpha, further supporting this view (Mizuno et al 2021).…”

“…Consequently, the search for proteins that interact with P. patens KL and PpCCD8-derived compound receptors should be a priority in the near future. In particular, since SMXL proteins are key members of both the SL and KL pathways in angiosperms and of the KL pathway in M. polymorpha (Soundappan et al 2015;Wang et al 2015;Khosla et al 2020;Mizuno et al 2021), the specific involvement of PpSMXL homologs (four genes) is currently under investigation by our group.…”

Masks Start to Drop: Suppressor of MAX2 1-Like Proteins Reveal Their Many Faces

“…Initially, the N domain of SMXLs was thought to enable nuclear localization, because nuclear localization signals (NLSs) are present in AtSMXL7 and OsSMAX1 ( Liang et al, 2016 ; Choi et al, 2020a ). Since SMXLs so far appear universally localized to the nucleus, this would fit the conservation of the N domain in these proteins ( Zhou et al, 2013 ; Soundappan et al, 2015 ; Liang et al, 2016 ; Wallner et al, 2017 ; Khosla et al, 2020 ; Zheng et al, 2020 ; Mizuno et al, 2021 ). However, whereas the N domain has been demonstrated to be indeed responsible for the nuclear localization of AtSMXL7, the D2a domain seems to be necessary for the nuclear localization of AtSMAX1 ( Liang et al, 2016 ; Khosla et al, 2020 ).…”

“…This ancestral SMXL is the most similar to the aSMAX1 clade and thought to be involved in the ancient KAI2-MAX2-dependent responses to KL ( Bythell-Douglas et al, 2017 ; Walker et al, 2019 ). Recently, this hypothesis was supported by the discovery that in the liverwort Marchantia polymorpha , KAI2 and MAX2 homologs regulate thallus growth and gemma elongation through the degradation of the only SMXL homolog found in this species ( Mizuno et al, 2021 ).…”

“…The involvement of the D2 domain in the MAX2-dependent degradation of SMXLs can be attributed to the presence of the Walker A motif. D2-Walker A is required in several species for the degradation of SMAX1 ( Khosla et al, 2020 ; Wang et al, 2020b ; Zheng et al, 2020 ; Mizuno et al, 2021 ) and SMXL6/7/8 ( Jiang et al, 2013 ; Zhou et al, 2013 ; Soundappan et al, 2015 ; Wang et al, 2015 ; Liang et al, 2016 ; Struk et al, 2018 ; Kerr et al, 2021 ). Several publications term this motif P-loop or (F)RGKT, according to its structure or its amino-acid sequence, respectively ( Zhou et al, 2013 ; Soundappan et al, 2015 ; Wang et al, 2015 ; Liang et al, 2016 ; Struk et al, 2018 ).…”