The G-Protein β Subunit AGB1 Promotes Hypocotyl Elongation through Inhibiting Transcription Activation Function of BBX21 in Arabidopsis

Abstract

Hypocotyl development in Arabidopsis thaliana is regulated by light and endogenous hormonal cues, making it an ideal model to study the interplay between light and endogenous growth regulators. BBX21, a B-box (BBX)-like zinc-finger transcription factor, integrates light and abscisic acid signals to regulate hypocotyl elongation in Arabidopsis. Heterotrimeric G-proteins are pivotal regulators of plant development. The short hypocotyl phenotype of the G-protein β-subunit (AGB1) mutant (agb1-2) has been previously identified, but the precise role of AGB1 in hypocotyl elongation remains enigmatic. Here, we show that AGB1 directly interacts with BBX21, and the short hypocotyl phenotype of agb1-2 is partially suppressed in agb1-2bbx21-1 double mutant. BBX21 functions in the downstream of AGB1 and overexpression of BBX21 in agb1-2 causes a more pronounced reduction in hypocotyl length, indicating that AGB1 plays an oppositional role in relation to BBX21 during hypocotyl development. Furthermore, we demonstrate that the C-terminal region of BBX21 is important for both its intracellular localization and its transcriptional activation activity that is inhibited by interaction with AGB1. ChIP assays showed that BBX21 specifically associates with its own promoter and with those of BBX22, HY5, and GA2ox1. which is not altered in agb1-2. These data suggest that the AGB1-BBX21 interaction only affects the transcriptional activation activity of BBX21 but has no effect on its DNA binding ability. Taken together, our data demonstrate that AGB1 positively promotes hypocotyl elongation through repressing BBX21 activity.

Photoexcited CRYPTOCHROME 1 Interacts Directly with G-Protein β Subunit AGB1 to Regulate the DNA-Binding Activity of HY5 and Photomorphogenesis in Arabidopsis

“…We then generated the agb1 hy5 hyh triple mutant and found that its hypocotyls were significantly longer than those of the agb1 hy5 double mutant, but were almost as long as those of the hy5 hyh double mutant ( Figure 3A-3H). Given that AGB1 functions through other factors such as BBX21 and BES1 (Xu et al, 2017;Zhang et al, 2018), these results suggest that AGB1 acts partially through HY5/HYH to repress photomorphogenesis.…”

“…Likewise, the Arabidopsis and rice G-protein b subunits, AGB1 and RGB1, have been shown to localize to the plasma membrane, cytosol, and nucleus (Anderson and Botella, 2007;Yadav et al, 2014). It has been demonstrated recently that AGB1 promotes hypocotyl elongation by interacting with BBX21, a B-box-like zinc-finger transcriptional factor, and inhibiting its transcriptional activation activity rather than its DNA-binding activity (Xu et al, 2017). AGB1 also promotes hypocotyl elongation by interacting with BES1, a master transcription factor mediating BR signaling, and promoting its transcriptional activity (Zhang et al, 2018).…”

“…This proposition explains how COP1 and AGB1 act additively to repress photomorphogenesis. Given that HY5 is involved in both CRY and phytochrome signaling (Osterlund et al, 2000;Wang et al, 2001;Yang et al, 2001;Jang et al, 2010;Lian et al, 2011;Liu et al, 2011;Lu et al, 2015;Sheerin et al, 2015), and that CRY and phytochrome share the same downstream components including the COP1, SPA, PIF, and Aux/IAA proteins (Lian et al, 2011;Liu et al, 2011;Lu et al, 2015;Sheerin et al, 2015;Ma et al, 2016;Pedmale et al, 2016;Xu et al, 2017), it is possible that phytochromes may regulate photomorphogenesis by interacting with AGB1 and regulating the DNA-binding activity of HY5.…”

Arabidopsis PEAPODs function with LIKE HETEROCHROMATIN PROTEIN1 to regulate lateral organ growth

“…In Arabidopsis , loss‐of‐function mutations in Gβ ( AGB1 ) lead to shorter hypocotyl, shorter petiole, and round lamina phenotypes caused by reduced cell division (Perfus‐Barbeoch et al 2004). Recent reports showed that AGB1 promotes hypocotyl elongation partially through directly inhibiting a B‐box (BBX)‐like zinc‐finger transcription factor (BBX21) (Xu et al 2017), and the shorter hypocotyl phenotype of agb1 can be partially suppressed by mutations in photoreceptors, cryptochrome 1 ( cry1 ) and phytochrome B ( phyB ) (Lian et al 2018; Xu et al 2018). Interestingly, AGB1 directly interacts with CRY1 or PHYB in a light‐dependent manner (Lian et al 2018; Xu et al 2018).…”

Two B-Box Proteins Regulate Photomorphogenesis by Oppositely Modulating HY5 through their Diverse C-Terminal Domains

“…BBX21 and BBX24 have a putative nuclear localization signal (NLS) sequence at their C-terminus, which 179 has been shown to be required for nuclear localization (Yan et al, 2011;Xu et al, 2017). In order to verify …”