SUMMARYFew regulators of phenylpropanoids have been identified in monocots having potential as biofuel crops. Here we demonstrate the role of the maize (Zea mays) R2R3-MYB factor ZmMYB31 in the control of the phenylpropanoid pathway. We determined its in vitro consensus DNA-binding sequence as ACC T / A ACC, and chromatin immunoprecipitation (ChIP) established that it interacts with two lignin gene promoters in vivo. To explore the potential of ZmMYB31 as a regulator of phenylpropanoids in other plants, its role in the regulation of the phenylpropanoid pathway was further investigated in Arabidopsis thaliana. ZmMYB31 downregulates several genes involved in the synthesis of monolignols and transgenic plants are dwarf and show a significantly reduced lignin content with unaltered polymer composition. We demonstrate that these changes increase cell wall degradability of the transgenic plants. In addition, ZmMYB31 represses the synthesis of sinapoylmalate, resulting in plants that are more sensitive to UV irradiation, and induces several stress-related proteins. Our results suggest that, as an indirect effect of repression of lignin biosynthesis, transgenic plants redirect carbon flux towards the biosynthesis of anthocyanins. Thus, ZmMYB31 can be considered a good candidate for the manipulation of lignin biosynthesis in biotechnological applications.
Summary
Mitogen‐activated protein kinase (MAPK) cascades have important functions in plant stress responses and development and are key players in reactive oxygen species (ROS) signalling and in innate immunity. In Arabidopsis, the transmission of ROS and pathogen signalling by MAPKs involves the coordinated activation of MPK6 and MPK3; however, the specificity of their negative regulation by phosphatases is not fully known. Here, we present genetic analyses showing that MAPK phosphatase 2 (MKP2) regulates oxidative stress and pathogen defence responses and functionally interacts with MPK3 and MPK6. We show that plants lacking a functional MKP2 gene exhibit delayed wilting symptoms in response to Ralstonia solanacearum and, by contrast, acceleration of disease progression during Botrytis cinerea infection, suggesting that this phosphatase plays differential functions in biotrophic versus necrotrophic pathogen‐induced responses. MKP2 function appears to be linked to MPK3 and MPK6 regulation, as indicated by BiFC experiments showing that MKP2 associates with MPK3 and MPK6 in vivo and that in response to fungal elicitors MKP2 exerts differential affinity versus both kinases. We also found that MKP2 interacts with MPK6 in HR‐like responses triggered by fungal elicitors, suggesting that MPK3 and MPK6 are subject to differential regulation by MKP2 in this process. We propose that MKP2 is a key regulator of MPK3 and MPK6 networks controlling both abiotic and specific pathogen responses in plants.
Cinnamyl alcohol dehydrogenase (CAD) is a key enzyme involved in the last step of monolignol biosynthesis. The effect of CAD down-regulation on lignin production was investigated through a transgenic approach in maize. Transgenic CAD-RNAi plants show a different degree of enzymatic reduction depending on the analyzed tissue and show alterations in cell wall composition. Cell walls of CAD-RNAi stems contain a lignin polymer with a slight reduction in the S-to-G ratio without affecting the total lignin content. In addition, these cell walls accumulate higher levels of cellulose and arabinoxylans. In contrast, cell walls of CAD-RNAi midribs present a reduction in the total lignin content and of cell wall polysaccharides. In vitro degradability assays showed that, although to a different extent, the changes induced by the repression of CAD activity produced midribs and stems more degradable than wild-type plants. CAD-RNAi plants grown in the field presented a wild-type phenotype and produced higher amounts of dry biomass. Cellulosic bioethanol assays revealed that CAD-RNAi biomass produced higher levels of ethanol compared to wild-type, making CAD a good target to improve both the nutritional and energetic values of maize lignocellulosic biomass.
Late-embryogenesis-abundant (LEA) proteins accumulate as plant seeds desiccate and also in vegetative organs during periods of stress. They are predicted to play a role in plant stress tolerance. In the present study, we have initiated the characterization of phosphorylated LEA proteins present in the Arabidopsis seed, using a strategy that combines the thermostability (solubility upon heating) of many LEA-type proteins with the use of phosphoaffinity chromatography to obtain an enriched subpopulation of phosphoproteins. The specificity and efficiency of the procedure was assessed by alkaline phosphatase treatment and by a specific stain for phosphoproteins, in addition to the immunodetection of AtRab18, a phosphorylated LEA protein present in the mature dry seed. The phosphoproteins were identified by MS either by PMF using MALDI-TOF MS after 2-DE separation, or by peptide sequencing using both capillary LC MS/MS (LC muESI-ITMS/MS) and nanoLC coupled to nanoESI-MS/MS (LC-nanoESI-Q-TOF-MS/MS). Several LEA-type and storage-like proteins were identified as components of the phosphoproteome of the Arabidopsis seed.
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