Approaches to the identification of ABAR as an abscisic acid receptor

Integrating the multiple functions of CHLH into chloroplast-derived signaling fundamental to plant development and adaptation as well as fruit ripening

Chlorophyll (Chl)-mediated oxygenic photosynthesis sustains life on Earth. Greening leaves play fundamental roles in plant growth and crop yield, correlating with the idea that more Chls lead to better adaptation. However, they face significant challenges from various unfavorable environments. Chl biosynthesis hinges on the first committed step, which involves inserting Mg2+ into protoporphyrin. This step is facilitated by the H subunit of magnesium chelatase (CHLH) and features a conserved mechanism from cyanobacteria to plants. For better adaptation to fluctuating land environments, especially drought, CHLH evolves multiple biological functions, including Chl biosynthesis, retrograde signaling, and abscisic acid (ABA) responses. Additionally, it integrates into various chloroplast-derived signaling pathways, encompassing both retrograde signaling and hormonal signaling. The former comprises ROS (reactive oxygen species), heme, GUN (genomes uncoupled), MEcPP (methylerythritol cyclodiphosphate), β-CC (β-cyclocitral), and PAP (3'-phosphoadenosine-5'-phosphate). The latter involves phytohormones like ABA, ethylene, auxin, cytokinin, gibberellin, strigolactone, brassinolide, salicylic acid, and jasmonic acid. Together, these elements create a coordinated regulatory network tailored to plant development and adaptation. An intriguing example is how drought-mediated improvement of fruit quality provides insights into chloroplast-derived signaling, aiding the shift from vegetative to reproductive growth. In this context, we explore the integration of CHLH's multifaceted roles into chloroplast-derived signaling, which lays the foundation for plant development and adaptation, as well as fruit ripening and quality. In the future, manipulating chloroplast-derived signaling may offer a promising avenue to enhance crop yield and quality through the homeostasis, function, and regulation of Chls.

Arabidopsis pentatricopeptide repeat protein SOAR1 plays a critical role in abscisic acid signalling

A dominant suppressor of the ABAR overexpressor, soar1-1D, from CHLH/ABAR [coding for Mg-chelatase H subunit/putative abscisic acid (ABA) receptor (ABAR)] overexpression lines was screened to explore the mechanism of the ABAR-mediated ABA signalling. The SOAR1 gene encodes a pentatricopeptide repeat (PPR) protein which localizes to both the cytosol and nucleus. Down-regulation of SOAR1 strongly enhances, but up-regulation of SOAR1 almost completely impairs, ABA responses, revealing that SOAR1 is a critical, negative, regulator of ABA signalling. Further genetic evidence supports that SOAR1 functions downstream of ABAR and probably upstream of an ABA-responsive transcription factor ABI5. Changes in the SOAR1 expression alter expression of a subset of ABA-responsive genes including ABI5. These findings provide important information to elucidate further the functional mechanism of PPR proteins and the complicated ABA signalling network.

Cochaperonin CPN20 negatively regulates abscisic acid signaling in Arabidopsis

Previous study showed that the magnesium-protoporphyrin IX chelatase H subunit (CHLH/ABAR) positively regulates abscisic acid (ABA) signaling. Here, we investigated the functions of a CHLH/ABAR interaction protein, the chloroplast co-chaperonin 20 (CPN20) in ABA signaling in Arabidopsis thaliana. We showed that down-expression of the CPN20 gene increases, but overexpression of the CPN20 gene reduces, ABA sensitivity in the major ABA responses including ABA-induced seed germination inhibition, postgermination growth arrest, promotion of stomatal closure and inhibition of stomatal opening. Genetic evidence supports that CPN20 functions downstream or at the same node of CHLH/ABAR, but upstream of the WRKY40 transcription factor. The other CPN20 interaction partners CPN10 and CPN60 are not involved in ABA signaling. Our findings show that CPN20 functions negatively in the ABAR-WRKY40 coupled ABA signaling independently of its co-chaperonin role, and provide a new insight into the role of co-chaperones in the regulation of plant responses to environmental cues.

Roles of the different components of magnesium chelatase in abscisic acid signal transduction

The H subunit of Mg-chelatase (CHLH) was shown to regulate abscisic acid (ABA) signaling and the I subunit (CHLI) was also reported to modulate ABA signaling in guard cells. However, it remains essentially unknown whether and how the Mg-chelatase-catalyzed Mg-protoporphyrin IX-production differs from ABA signaling. Using a newly-developed surface plasmon resonance system, we showed that ABA binds to CHLH, but not to the other Mg-chelatase components/subunits CHLI, CHLD (D subunit) and GUN4. A new rtl1 mutant allele of the CHLH gene in Arabidopsis thaliana showed ABA-insensitive phenotypes in both stomatal movement and seed germination. Upregulation of CHLI1 resulted in ABA hypersensitivity in seed germination, while downregulation of CHLI conferred ABA insensitivity in stomatal response in Arabidopsis. We showed that CHLH and CHLI, but not CHLD, regulate stomatal sensitivity to ABA in tobacco (Nicotiana benthamiana). The overexpression lines of the CHLD gene showed wild-type ABA sensitivity in Arabidopsis. Both the GUN4-RNA interference and overexpression lines of Arabidopsis showed wild-type phenotypes in the major ABA responses. These findings provide clear evidence that the Mg-chelatase-catalyzed Mg-ProtoIX production is distinct from ABA signaling, giving information to understand the mechanism by which the two cellular processes differs at the molecular level.