JXB Volume 72, Issue 15 – Editor’s choice

30 Jul 2021 - By: Mareike Jezek

JXB Volume 72, Issue 15 – Editor’s choice

This article highlights the following publication:

Open stomata 1 exhibits dual serine/threonine and tyrosine kinase activity in regulating ABA signaling

Yun Shang, Dami Yang, Yunmi Ha, Ju Yeon Lee, Jin Young Kim, Man-Ho Oh, Kyoung Hee Nam

Journal of Experimental Botany, erab225, https://doi.org/10.1093/jxb/erab225

Unexpected new insights into plant responses to drought stress

Gas exchange between plants and the environment is vital for life. Oxygen released from leaves enables aerobic metabolism and uptake of CO2 fuels photosynthesis to produce organic material. Furthermore, by mediating water transfer from the soil to the atmosphere, plant transpiration plays a key role in the terrestrial water cycle and the global climate. Plants facilitate gas exchange through leaf pores called stomata. Each stoma is flanked by a pair of guard cells that open and close the pore dynamically in response to internal and external cues. For example, stomata of most species open upon light perception to enable CO2 uptake for photosynthesis, whereas darkness leads to stomatal closure to limit transpirational water loss. What looks like relatively simple mechanistic opening and closing processes from the outside is in fact driven by a sophisticated cellular system of membrane transporters that adjust the guard cells’ osmotic potential and turgor pressure. All transporter activities are constantly fine-tuned to balance photosynthetic carbon fixation and dehydration avoidance under dynamic environmental conditions. Post-translational modifications such as protein phosphorylation is instrumental in these regulatory processes.

A major player in the stomatal closing response to dehydration is Open Stomata 1 (OST1), a member of the sucrose nonfermenting 1 (SNF1)-related protein kinase family (SnRK2). Stomatal closure under drought is triggered by the phytohormone abscisic acid (ABA) and OST1 is a central mediator between ABA perception and downstream processes such as membrane transporter regulation and production of reactive oxygen species. Under non-stress conditions OST1 is dephosphorylated and thereby inactivated by PP2C phosphatases such as ABA-Insensitive 1 (ABI1). When ABA is present in the cell OST1 is released from ABI1 and the drought stress response is initiated.

OST1 is integrated into a vast phosphorylation-based signalling network and manifold interactions with other kinases and phosphatases have been confirmed, yet the entirety of all phosphorylation and dephosphorylation processes is far from understood. Unravelling interactions between participating enzymes is complicated by the fact that they cannot only transfer phosphate groups onto other proteins but also perform autophosphorylation.

In this issue of the Journal of Experimental Botany Shang et al. focus on the previously described ABA-induced complexation of OST1 and the BRI1-associated receptor kinase 1 (BAK1), which is known to be involved, for example, in plant innate immunity. What had started out as a simple experiment to identify additional autophosphorylation and BAK1-mediated transphosphorylation sites on OST1 resulted in unexpected new discoveries.

Using mass spectrometry and site-directed mutagenesis, the authors found four new amino acid residues within OST1’s activation loop that serve as both auto- and transphosphorylation sites and that are essential for OST1 activation and ABA-induced stomatal closure. Different phosphorylation statuses in this region of OST1 may specify the diversity of downstream signalling responses. Interestingly, the identified phosphosite cluster did not only comprise serine and threonine, which are most often targets of kinases and phosphatases, but also phosphorylation of specific tyrosine residues was pivotal for OST1 activation in the presence of ABA. Transgenic ost1 plants transformed with a phospho-dead tyrosine mutated Y182F variant of OST1 failed to close their stomata in response to ABA application and transcription of ABA-responsive genes was significantly decreased compared to wild-type plants. Furthermore, the membrane transporter SLAC1, which is activated by OST1-mediated phosphorylation, had a reduced phosphorylation status in these mutants indicating that the mutated OST1 failed to phosphorylate SLAC1. By contrast, phospho-mimetic Y182D mutagenized OST1 rescued the ost1 phenotype.

These results show for the first time that tyrosine phosphorylation is functionally important in the OST1-mediated drought response, a surprising novelty since OST1 has thus far only been described as a serine/threonine kinase. Even more surprising is the finding that this characteristic of OST1 is obviously not that exceptional but conserved in close homologues of the SnRK2 family. Even a subset of PP2C phosphatases including ABI1 was shown to exert tyrosine phosphatase activity on phosphorylated Y182 of OST1. Since the PP2Cs lack the conserved amino acid sequence motif CX5R that is characteristic for other bona-fide tyrosine phosphatases the authors suspect that these enzymes with dual specificity for serine/threonine and tyrosine may adopt different catalytic mechanisms. Uncovering these mechanisms and finding more non-homologue but functionally similar proteins will be an exciting task for the future.

JXB Volume 72, Issue 15 – Editor’s choice

Tiny pores with massive impact: most gas exchange between leaves of terrestrial plants and the environment occurs through stomata.  Fundamental understanding of molecular mechanisms regulating their opening and closing is essential to breed climate-resilient crops and assess global water and carbon cycles.  

 

 

Author: Mareike Jezek
Category: JXB
Share
Mareike Jezek

Mareike Jezek

Mareike Jezek is the publication Advisor (Institute of Molecular Cell & Systems Biology) of the University of Glasgow and also the Assistant Editor of the Journal of Experimental Botany 

see all

Events

Check out our upcoming events

SEB Annual Conference Montpellier 2022
Montpellier, France
5 - 8 July 2022