The Role of Arabidopsis Inositol Polyphosphate Kinase AtIPK2ß in Glucose Suppression of Seed Germination and Seedling Development.

Seed germination and subsequent seedling development are critical phases in plants. These processes are regulated by a complex molecular network in which sugar has been reported to play an essential role. However, factors affecting sugar responses remain to be fully elucidated. In this study, we demonstrate that AtIPK2ß, known to participate in the synthesis of myo-inositol 1,2,3,4,5,6-hexakisphosphate (IP6, phytate), affects Arabidopsis responses to glucose during seed germination. The loss-of-function mutant atipk2ß showed increased sensitivity to 6% glucose and paclobutrazol (PAC). Yeast two-hybrid assay showed that AtIPK2ß interacts with sucrose non-fermenting-1-related protein kinase (SnRK1.1), and bimolecular fluorescence complementation (BiFC) and pull-down assay further confirmed this interaction. Moreover, AtIPK2ß was phosphorylated by SnRK1.1 in vitro, and the effect of restoring AtIPK2ß to yeast cells lacking IPK2 (Δipk2) was abolished by catalytically active SnRK1.1. Further analysis indicated that IP6 reduces the suppression of seed germination caused by glucose, accompanied by altered expression levels of glucose-/hormone-responsive genes. Collectively, these findings indicate that AtIPK2ß and IP6 are involved in glucose suppression of seed germination and that AtIPK2ß enzyme activity is likely to be regulated by SnRK1.1.

Rice Inositol Polyphosphate Kinase (OsIPK2) Directly Interacts with OsIAA11 to Regulate Lateral Root Formation.

The plant hormone auxin controls many aspects of plant growth and development by promoting the degradation of Auxin/Indole-3-acetic acid (Aux/IAA) proteins. The domain II (DII) of Aux/IAA proteins is sufficient for eliciting the degradation by directly interacting with the auxin receptor F-box protein TIR1 to form a TIR1/AFBs-Aux/IAA complex in an auxin-dependent manner. However, the underlying mechanisms of fine-tuning Aux/IAA degradation by auxin stimuli remain to be elucidated. Here, we show that OsIPK2, a rice (Oryza sativa) inositol polyphosphate kinase, directly interacts with an Aux/IAA protein OsIAA11 to repress its degradation. In a rice protoplast transient expression system, the auxin-induced degradation of Myc-OsIAA11 fusion was delayed by co-expressed GFP-OsIPK2 proteins. Furthermore, expressing additional OsIPK2 or its N-terminal amino acid sequence enhanced the accumulation of OsIAA11 proteins in transgenic plants, which in turn caused defects in lateral root formation and auxin response. Taken together, we identify a novel co-factor of Aux/IAA in auxin signaling and demonstrate its role in regulating lateral root development.

Arabidopsis inositol polyphosphate kinase AtIpk2ß is phosphorylated by CPK4 and positively modulates ABA signaling.

Arabidopsis inositol polyphosphate kinase 2ß (AtIpk2ß) has multiple functions in plant development and in responding to abiotic stress. Although some related clues suggested a potential role of AtIpk2ß in ABA signaling, the defined evidence was still lack. Here we discovered that a key ABA signaling component calcium-dependent protein kinase 4 (CPK4) can interact with AtIpk2ß under ABA treated conditions through affinity purification and mass spectrometry detection. The interaction between CPK4 and AtIpk2ß were further confirmed by yeast two hybrid and bimolecular fluorescence complementation assays. Expression of AtIpk2ß also can be rapidly induced by ABA. In addition, we found that CPK4 can phosphorylate AtIpk2ß in vitro and identified five novel phosphorylation sites of AtIpk2ß by CPK4 kinase, including Tyr46, Ser48, Ser51, Thr128, Ser147. Overexpression of AtIpk2ß in Arabidopsis was more sensitive to ABA in seed germination, primary root inhibition, ABA-responsive gene expression than wild type plants, whereas knockout mutant atipk2ß exhibited no significant difference. The AtIpk2ß variants containing Tyr46, Thr128, Ser147 mutated to Ala cannot complement the yeast mutant ipk2 growth in high temperature, suggesting that those three amino acid residues are critical for AtIpk2ß. These findings provide insight into the modulation of ABA signaling by AtIpk2ß.