The wheat MAP kinase phosphatase 1 confers higher lithium tolerance in yeast

Differential regulation of the durum wheat MAPK phosphatase 1 by calmodulin, bivalent cations and possibly mitogen activated protein kinase 3

MAPK phosphatases (MKPs) are relevant negative regulators of MAPKs in eukaryotes as they mediate the feedback control of MAPK cascades in multiple cellular processes. Despite their relevance, our knowledge on the role of cereal MKPs in stress tolerance is scarce and TMKP1 remains today the only studied MKP in wheat. TMKP1 was previously reported to be involved in plant salt stress tolerance. Moreover, TMKP1 was shown to interact with calmodulin (CaM), 14-3-3 and TMPK3/TMPK6 proteins, which regulate its catalytic activity. To further understand the functional properties of TMKP1, we investigate here the contribution of its phosphorylation status, and of TMPK3 together with CaM and bivalent cations on the catalytic activity. In-gel kinase assays revealed that TMKP1 can be phosphorylated by similar wheat and Arabidopsis MAPKs, including most likely MPK3 and MPK6. In addition, we provide evidence for the capacity of wheat TMPK3 to bind to TMKP1 via a conserved Kinase Interacting Domain (KID) located on its C-terminal part. This interaction leads to a stimulation of TMKP1 activity in the presence of Mn²⁺ or Mg²⁺ ions, but to its inhibition in the presence of Ca²⁺ ions. However, the phosphorylation status of TMKP1 seems to be dispensable for TMKP1 activation by TMPK3. Remarkably, in assays combining TMPK3 with CaM/Ca²⁺ complex, we registered rather an inhibition of TMKP1 activity which however can be suppressed by Mn²⁺ cations. Our data are in favor of complex differential regulation of TMKP1 by its MPK substrates, metallic cations that might help in fine-tuning the plant cellular responses to various stresses.

Co-overexpression of AVP1 and PP2A-C5 in Arabidopsis makes plants tolerant to multiple abiotic stresses

Abiotic stresses are major threats to agricultural production. Drought and salinity as two of the major abiotic stresses cause billions of losses in agricultural productivity worldwide each year. Thus, it is imperative to make crops more tolerant. Overexpression of AVP1 or PP2A-C5 was previously shown to increase drought and salt stress tolerance, respectively, in transgenic plants. In this study, the hypothesis that co-overexpression of AVP1 and PP2A-C5 would combine their respective benefits and further improve salt tolerance was tested. The two genes were inserted into the same T-DNA region of the binary vector and then introduced into the Arabidopsis genome through Agrobacterium-mediated transformation. Transgenic Arabidopsis plants expressing both AVP1 and PP2A-C5 at relatively high levels were identified and analyzed. These plants displayed enhanced tolerance to NaCl compared to either AVP1 or PP2A-C5 overexpressing plants. They also showed tolerance to other stresses such as KNO₃ and LiCl at harmful concentrations, drought, and phosphorus deficiency at comparable levels with either AVP1 or PP2A-C5 overexpressing plants. This study demonstrates that introducing multiple genes in single T-DNA region is an effective approach to create transgenic plants with enhanced tolerance to multiple stresses.

Regulation of the wheat MAP kinase phosphatase 1 by 14-3-3 proteins

Plant MAP kinase phosphatases (MKPs) are major regulators of MAPK signaling pathways and play crucial roles in controlling growth, development and stress responses. The presence of several functional domains in plant MKPs such as a dual specificity phosphatase catalytic domain, gelsolin, calmodulin-binding and serine-rich domains, suggests that MKPs can interact with distinct cellular partners, others than MAPKs. In this report, we identified a canonical mode I 14-3-3-binding motif (₅₇₄KLPSLP₅₇₉) located at the carboxy-terminal region of the wheat MKP, TMKP1. We found that this motif is well-conserved among other MKPs from monocots including Hordeum vulgare, Brachypodium distachyon and Aegilops taushii. Using co-immunoprecipitation assays, we provide evidence for interaction between TMKP1 and 14-3-3 proteins in wheat. Moreover, the phosphatase activity of TMKP1 is increased in a phospho-dependent manner by either Arabidopsis or yeast 14-3-3 isoforms. TMKP1 activation by 14-3-3 proteins is enhanced by Mn²⁺, whereas in the presence of Ca²⁺ ions, TMKP1 activation was limited to Arabidopsis 14-3-3φ (phi), an isoform harboring an EF-hand motif. Such findings strongly suggest that 14-3-3 proteins, in conjunction with specific divalent cations, may stimulate TMKP1 activity and point-out that 14-3-3 proteins bind and regulate the activity of a MKP in eukaryotes.

The wheat MAP kinase phosphatase 1 alleviates salt stress and increases antioxidant activities in Arabidopsis

Mitogen-activated protein kinase phosphatases (MKPs) are important negative regulators in the MAPK signaling pathways, which play crucial roles in plant growth, development and stress responses. We have previously shown that the heterologous expression of a durum wheat MKP, TMKP1, results in increased tolerance to salt stress in yeast but its particular contribution in salt stress tolerance in plants was not investigated. Here, TMKP1 was overexpressed in Arabidopsis thaliana and physiological changes were assessed in transgenic plants exposed to stress conditions. Under salt stress and especially LiCl, the TMKP1 overexpressors displayed higher germination rates in comparison to wild type plants. The enhancement of salt stress tolerance was accompanied by increased antioxidant enzyme activities, namely superoxide dismutase, catalase and peroxydases. Such increases in antioxidant activities were concomitant with lower malondialdehyde, superoxide anion O2(-) and hydrogen peroxide levels in the TMKP1 transgenic seedlings. Moreover, we provide evidence that, in contrast to the Arabidopsis ortholog AtMKP1, TMKP1 acts as a positive regulator of salt stress tolerance via its ectopic expression in the Arabidopsis mkp1 mutant.

The activity of the wheat MAP kinase phosphatase 1 is regulated by manganese and by calmodulin

MAPK phosphatases (MKPs) are negative regulators of MAPKs in eukaryotes and play key roles in the regulation of different cellular processes. However in plants, little is known about the regulation of these Dual Specific Phosphatases (DSPs) by Ca(2+) and calmodulin (CaM). Here, we showed that the wheat MKP (TMKP1) harboring a calmodulin (CaM) binding domain, binds to CaM in a Ca(2+)-dependent manner. In addition, TMKP1 exhibited a phosphatase activity in vitro that is specifically enhanced by Mn(2+) and to a lesser extent by Mg(2+), but without any synergistic effect between the two bivalent cations. Most interestingly, CaM/Ca(2+) complex inhibits the catalytic activity of TMKP1 in a CaM-dose dependent manner. However, in the presence of Mn(2+) this activity is enhanced by CaM/Ca(2+) complex. These dual regulatory effects seem to be mediated via interaction of CaM/Ca(2+) to the CaM binding domain in the C-terminal part of TMKP1. Such effects were not reported so far, and raise a possible role for CaM and Mn(2+) in the regulation of plant MKPs during cellular response to external signals.