PYL8 mediates ABA perception in the root through non-cell-autonomous and ligand-stabilization–based mechanisms

Abstract

The phytohormone abscisic acid (ABA) plays a key role regulating root growth, root system architecture, and root adaptive responses, such as hydrotropism. The molecular and cellular mechanisms that regulate the action of core ABA signaling components in roots are not fully understood. ABA is perceived through receptors from the PYR/PYL/RCAR family and PP2C coreceptors. PYL8/RCAR3 plays a nonredundant role in regulating primary and lateral root growth. Here we demonstrate that ABA specifically stabilizes PYL8 compared with other ABA receptors and induces accumulation of PYL8 in root nuclei. This requires ABA perception by PYL8 and leads to diminished ubiquitination of PYL8 in roots. The ABA agonist quinabactin, which promotes root ABA signaling through dimeric receptors, fails to stabilize the monomeric receptor PYL8. Moreover, a PYL8 mutant unable to bind ABA and inhibit PP2C is not stabilized by the ligand, whereas a PYL85KR mutant is more stable than PYL8 at endogenous ABA concentrations. The PYL8 transcript was detected in the epidermis and stele of the root meristem; however, the PYL8 protein was also detected in adjacent tissues. Expression of PYL8 driven by tissue-specific promoters revealed movement to adjacent tissues. Hence both inter- and intracellular trafficking of PYL8 appears to occur in the root apical meristem. Our findings reveal a non-cell-autonomous mechanism for hormone receptors and help explain the nonredundant role of PYL8-mediated root ABA signaling.

ABA inhibits myristoylation and induces shuttling of the RGLG1 E3 ligase to promote nuclear degradation of PP2CA

“…In contrast, PYR1 did not co‐immunoprecipitate with RGLG1‐GFP and PP2CA (Figure a). Therefore, it seems that ABA facilitates the formation of the PYL8–RGLG1–PP2CA complex by the upregulated transcription of PP2CA , which leads to increased PP2CA levels, the stabilization of PYL8 protein levels and high‐affinity binding of PP2CA to PYL8 (Santiago et al ., ; Antoni et al ., ; Irigoyen et al ., ; Wu et al ., ; Belda‐Palazon et al ., ). Indeed, using transient expression in tobacco cells, we demonstrated that increased PP2CA levels lead to an enhanced recruitment of RGLG1 in the nucleus (Figure b–d).…”

“…Given that PYL8–ABA–PP2CA complexes have been previously identified (Antoni et al ., ), the most likely explanation is that ternary complexes are formed in the presence of ABA. A lower input of PYL8 and PP2CA is observed in the absence of ABA treatment because of the enhanced ubiquitination of PYL8 (Belda‐Palazon et al ., ) and reduced PP2CA transcript levels (Wu et al ., ). In contrast, PYR1 did not co‐immunoprecipitate with RGLG1‐GFP and PP2CA (Figure a).…”

“…We examined the subcellular localization of RGLG1-GFP in hypocotyl and root cells of Arabidopsis, where ABA signaling plays important roles, and which allows the easy identification of the cell nucleus by bright-field microscopy Belda-Palazon et al, 2018). In hypocotyl cells, we found that RGLG1-GFP was localized both in the plasma membrane and in the small vesicles found close to or associated with the plasma membrane ( Figure 3a).…”

“…We focused additional work on the PYL8-RGLG1 interaction because PYL8 is an efficient inhibitor of PP2CA and is predominantly localized in the nucleus as PP2CA (Antoni et al, 2012Belda-Palazon et al, 2018). To this end, we generated double transgenic lines in Arabidopsis coexpressing RGLG1-GFP and either HA-PYL8 or HA-PYR1 (as a putative negative control), and tested the interaction of RGLG1 with the HA-tagged receptors and endogenous PP2CA by co-immunoprecipitation assays for either mock or ABA-treated plants (Figure 5a).…”

Brassinosteroids: Multidimensional Regulators of Plant Growth, Development, and Stress Responses

“…Most phenotypic defects of BR biosynthetic mutants can be rescued by exogenous BR treatment. As with other hormones, plant responses to exogenous BR application are dosedependent, with growth-promoting effects observed for lower concentrations and growth retardation when higher doses are used (Chaiwanon and Wang, 2015;Belda-Palazon et al, 2018). Initial dissection of BR signaling at the tissue level has provided clues that the epidermal layer is the site of the most intensive BR signaling events, as the dwarf phenotypes of BR receptor or BR biosynthetic enzyme mutants could be rescued by epidermisspecific expression of the mutated genes (Savaldi-Goldstein et al, 2007).…”