SUMMARYMitogen-activated protein kinase MPK3 and its novel phosphorylation target AZI1 mediate salt-stress signaling in Arabidopsis. AZI1 interacts with the membrane-associated pool of MPK3. MPK3 acts as positive regulator of AZI1 abundance and is required for stress tolerance conferred by AZI1 overexpression.

Abiotic stress poses a huge, ever-increasing problem to plants and agriculture. The dissection of signalling pathways mediating stress tolerance is a prerequisite to develop more resistant plant species. Mitogen-activated protein kinase (MAPK) cascades are universal signalling modules. In Arabidopsis, the MAPK MPK3 and its upstream regulator MAPK kinase MKK4 initiate the adaptation response to numerous abiotic and biotic stresses. Yet, molecular steps directly linked with MKK4 – MPK3 activation are largely unknown. Starting with a yeast-two-hybrid screen for interacting partners of MKK4, we identified a transcription factor, MYB44. MYB44 is controlled at multiple levels by and strongly inter-connected with MAPK signalling. As we had shown earlier, stress-induced expression of the MYB44 gene is regulated by a MPK3-targeted bZIP transcription factor VIP1. At the protein level, MYB44 interacts with MPK3 in vivo. MYB44 is phosphorylated by MPK3 in vitro at a single residue, Ser145. Although replacement of Ser145 by a non-phosphorylatable (S145A) or phosphomimetic (S145D) residue did not alter MYB44 subcellular localisation, dimerization behaviour nor DNA-binding characteristics, abiotic stress tolerance tests in stable transgenic Arabidopsis plants clearly related S145 phosphorylation to MYB44 function: Compared to Arabidopsis wild type plants, MYB44 overexpressing lines exhibit an enhanced tolerance to osmotic stress and are slightly more sensitive to abscisic acid. Interestingly, overexpression of the S145A variant revealed that impaired phosphorylation does not render the MYB44 protein non-functional. Instead, S145A lines are highly sensitive to abiotic stress, and thereby remarkably similar to mpk3-deficient plants. Its in vivo interaction with the nuclear sub-pools of both MPK3 and MKK4 renders MYB44 the first plant transcription factor to have a second function as putative MAPK cascade scaffolding protein.

In any living species, stress adaptation is closely linked with major changes of the gene expression profile. As a substrate protein of the rapidly stress-induced mitogen-activated protein kinase MPK3, Arabidopsis transcription factor MYB44 likely acts at the front line of stress-induced re-programming. We recently characterized MYB44 as phosphorylation-dependent positive regulator of salt stress signaling. Molecular events downstream of MYB44 are largely unknown. Although MYB44 binds to the MBSII element in vitro, it has no discernible effect on MBSII-driven reporter gene expression in plant co-transfection assays. This may suggest limited abundance of a synergistic co-regulator. MYB44 carries a putative transcriptional repression (Ethylene responsive element binding factor-associated Amphiphilic Repression, EAR) motif. We employed a dominant repressor strategy to gain insights into MYB44-conferred stress resistance. Overexpression of a MYB44-REP fusion markedly compromised salt and drought stress tolerance—the opposite was seen in MYB44 overexpression lines. MYB44-mediated resistance likely results from induction of tolerance-enhancing, rather than from repression of tolerance-diminishing factors. Salt stress-induced accumulation of destructive reactive oxygen species is efficiently prevented in transgenic MYB44, but accelerated in MYB44-REP lines. Furthermore, heterologous overexpression of MYB44-REP caused tissue collapse in Nicotiana. A mechanistic model of MAPK-MYB-mediated enhancement in the antioxidative capacity and stress tolerance is proposed. Genetic engineering of MYB44 variants with higher trans-activating capacity may be a means to further raise stress resistance in crops.

BackgroundTransient gene expression systems are indispensable tools in molecular biology. Yet, their routine application is limited to few plant species often requiring substantial equipment and facilities. High chloroplast and chlorophyll content may further impede downstream applications of transformed cells from green plant tissue.ResultsHere, we describe a fast and simple technique for the high-yield isolation and efficient transformation (>70%) of mesophyll-derived protoplasts from red leaves of the perennial plant Poinsettia (Euphorbia pulccherrima). In this method no particular growth facilities or expensive equipments are needed. Poinsettia protoplasts display an astonishing robustness and can be employed in a variety of commonly-used downstream applications, such as subcellular localisation (multi-colour fluorescence) or promoter activity studies. Due to low abundance of chloroplasts or chromoplasts, problems encountered in other mesophyll-derived protoplast systems (particularly autofluorescence) are alleviated. Furthermore, the transgene expression is detectable within 90 minutes of transformation and lasts for several days.ConclusionsThe simplicity of the isolation and transformation procedure renders Poinsettia protoplasts an attractive system for transient gene expression experiments, including multi-colour fluorescence, subcellular localisation and promoter activity studies. In addition, they offer hitherto unknown possibilities for anthocyan research and industrial applications.

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