Circuitries of signaling pathways integrate distinct hormonal and environmental signals, and influence development in plants. While a crosstalk between brassinosteroid (BR) and gibberellin (GA) signaling pathways has recently been established, little is known about other components engaged in the integration of the two pathways. Here, we provide supporting evidence for the role of HSP90 (HEAT SHOCK PROTEIN 90) in regulating the interplay of the GA and BR signaling pathways to control hypocotyl elongation of etiolated seedlings in Arabidopsis. Both pharmacological and genetic depletion of HSP90 alter the expression of GA biosynthesis and catabolism genes. Major components of the GA pathway, like RGA (REPRESSOR of ga1–3) and GAI (GA-INSENSITIVE) DELLA proteins, have been identified as physically interacting with HSP90. Interestingly, GA-promoted DELLA degradation depends on the ATPase activity of HSP90, and inhibition of HSP90 function stabilizes the DELLA/BZR1 (BRASSINAZOLE-RESISTANT 1) complex, modifying the expression of downstream transcriptional targets. Our results collectively reveal that HSP90, through physical interactions with DELLA proteins and BZR1, modulates DELLA abundance and regulates the expression of BZR1-dependent transcriptional targets to promote plant growth.

Closely related FKBP orthologs, FKBP42/TWISTED DWARF1 (TWD1) and FKBP38, have been shown to control the biogenesis of plant and mammalian ATP-binding cassette (ABC) transporters, respectively. However, the mechanistic role of the described FKBP-ABCB interaction is widely unknown. Here, we verify cytosolic HEAT-SHOCK PROTEIN90 (HSP90) isoforms as valid interactors of TWD1 and map HSP90 binding to an amphiphilic alpha-helix preceding its TPR domain. We provide pharmacological and genetic evidence that a subset of TWD1-interacting ABCBs, in contrast to mammalian ABCBs, are constitutive HSP90 clients in plants. This effect and its specificity are presumably provided by TWD1. Our data strongly correlate the impact of HSP90 inhibition on ABCB-mediated development and ABCB plasma membrane stability on the one hand and ABCB cycling rate on the other. In summary, we uncover a dynamic mechanism of HSP90 for differential stabilization of the plasma membrane ABCB isoforms to regulate polar auxin transport and to confer developmental plasticity.

In plants, the transition from heterotrophic to autotrophic growth is a critical developmental shift, tightly coupled to the establishment of photosynthesis. This process demands a precise interplay between the nucleus and chloroplasts, with communication schemes providing essential checkpoints to synchronize gene expression during seedling greening and establishment. While light response and photomorphogenesis are known to rely on transcriptional networks, recent evidence highlights a key role for alternative pre-mRNA splicing in facilitating plant adaptation to new light regimes, thereby enhancing transcriptome diversity. LEFKOTHEA, a dual-localized nuclear and chloroplast protein, has emerged as a potential integrator of these processes; it mediates the splicing of both chloroplast group II introns and nuclear introns via interactions with spliceosomal proteins. Here, we demonstrate that LEFKOTHEA is an active component of early light response signaling, regulating gene expression both transcriptionally and post-transcriptionally. Transcriptomic analysis reveals that LEFKOTHEA shapes the transcriptome both quantitatively and qualitatively by modulating alternative splicing, a mechanism essential for plant plasticity and adaptation. Furthermore, we show that the dynamic subcellular localization of LEFKOTHEA underpins its role in establishing a nucleus–chloroplast communication network that guides plant development.