Walia, Y. | bioRxiv Science

bioRxiv2021

Arabidopsis inositol polyphosphate kinases IPK1 and ITPK1 modulate crosstalks between SA-dependent immunity and phosphate-starvation responses

The propensity for polyphosphorylation makes myo-inositol derivatives, the inositol polyphosphates (InsPs), especially phytic acid or inositol hexakisphosphate (InsP6) the major form of phosphate storage in plants. Acts of pyrophosphorylation on InsP6 generates InsP7 or InsP8 containing high-energy phosphoanhydride bonds that are harnessed during energy requirements of a cell. Also implicated as co-factors for several phytohormone signaling networks, InsP7/InsP8 modulate key developmental processes. With recent identification as the common moeity for transducing both jasmonic acid (JA) and phosphate-starvation responses (PSR), InsP8 is the classic example of a metabolite that may moonlight crosstalks to different cellular pathways during diverse stress adaptations. We show here that Arabidopsis thaliana INOSITOL PENTAKISPHOSPHATE 2-KINASE (IPK1), INOSITOL 1,3,4-TRISPHOSPHATE 5/6-KINASE 1 (ITPK1), and DIPHOSPHOINOSITOL PENTAKISPHOSPHATE KINASE 2 (VIH2), but not other InsP-kinases, suppress basal salicylic acid (SA)-dependent immunity. In ipk1, itpk1 or vih2 mutants, elevated endogenous SA levels and constitutive activation of defense signaling lead to enhanced resistance against the virulent Pseudomonas syringae pv tomato DC3000 (PstDC3000) strain. Our data reveal that activated SA-signaling sectors in these mutants modulate expression amplitudes of phosphate-starvation inducible (PSI)-genes, reported earlier. In turn, via mutualism the heightened basal defenses in these mutants require upregulated PSI-gene expressions likely highlighting the increased demand of phosphates required to support immunity. We demonstrate that SA is induced in phosphate-deprived plants, however its defense-promoting functions are likely diverted to PSR-supportive roles. Overall, our investigations reveal selective InsPs as crosstalk mediators among diverse signaling networks programming stress-appropriate adaptations.

plant biology↗

bioRxiv2020

Arabidopsis inositol polyphosphate kinases regulate COP9 signalosome functions in phosphate-homeostasis

Plant Cullin RING Ubiquitin E3 ligases (CRLs) facilitate targeted protein degradation during physiological development and adaptation to stress. The deneddylase activity of COP9 signalosome (CSN) regulates cellular ratios of neddylated cullins available for the continuum of CRL functions. While selective inositol polyphosphates (InsPs) function as co-factors in plant responses involving the ubiquitylation of negative regulators, a relationship to CSN-CRL activities has not yet been established. Here, we show that the two Arabidopsis thaliana InsP-kinases IPK1 and ITPK1 physically interact and metabolically connect with the CSN holo-complex to modulate cullin deneddylation efficiency. Specifically, functional deficiency of ITPK1 lowers cullin deneddylation rates and disrupts the dissociation equilibrium of CSN5, the deneddylase catalytic subunit, and CUL1 with the holo-complex. Our results identify a novel auto-regulatory switch of CSN functions, defined by deneddylation activity. We further demonstrate that phosphate starvation response (PSR), which is induced in Pi-starved wild-type plants and constitutive in the above InsP-kinase mutants, is orchestrated in part by reduced deneddylation rates that, in turn, affect the stability of SPX4, a key negative regulator of PSR. Pharmacological inhibition of cullin neddylation stabilizes SPX4 and impairs PSR, thus linking CSN-CRL dynamics to phosphate (Pi)-sensing. Conversely, when exposed to compounds that inhibit CSN5 deneddylase activity, wild-type plants display phenotypes similar to the above InsP-kinase mutants. Overall, our data reveal that the regulation of plant Pi-starvation responses by specific InsP-kinases is caused by a direct role of these kinases in balancing coordination between CRL-CSN activities.

plant biology↗