Publications - Cell and Metabolic Biology

Plants have developed a robust transcription machinery to combat potential pathogenic organisms. One of the hallmarks of early immune responses is the activation of the WRKY transcription factors post infection. Specific WRKYs proteins from Arabidopsis are known substrates of MAPK pathway to mediate the flg22 elicited early immunity. In the current study, using the Golden Gate cloning strategy, we aim to clone the entire WRKY transcription factor family from Oryza sativa ssp. indica consisting of more than 100 members and study their MAPK interaction and subsequent role in PTI. Using a reporter LUC assay in protoplasts we investigated the early defense responses in a few interesting OsWRKY candidates. Interestingly, we observed stringent regulation of WRKY expression in cells and their transcriptional expression only under specific stress responses. The phenomenon of gene expression regulation by intron retention (IR) was prevalently observed in rice WRKY transcripts. We could show the role of WRKY8, 24, and 77 in early defense responses. It was observed that WRKY24 enhanced the expression of early defense response marker genes like NHL10 while WRKY8 and WRKY77 supressed their expression. This study highlights the complicated mechanism by which OsWRKYs expression is possibly regulated and the distinctive roles of some individual members in plant immunity. At the same time this study serves as a cautionary warning for plant researchers to be mindful of the intron retention mechanism while cloning OsWRKYs.

Expression takes place for most of the jasmonic acid (JA)-induced genes in a COI1-dependent manner via perception of its conjugate JA-Ile in the SCFCOI1-JAZ co-receptor complex. There are, however, numerous genes and processes, which are preferentially induced COI1-independently by the precursor of JA, 12-oxo-phytodienoic acid (OPDA). After recent identification of the Ile-conjugate of OPDA, OPDA-Ile, biological activity of this compound could be unequivocally proven in terms of gene expression. Any interference of OPDA, JA, or JA-Ile in OPDA-Ile-induced gene expression could be excluded by using different genetic background. The data suggest individual signaling properties of OPDA-Ile. Future studies for analysis of an SCFCOI1-JAZ co-receptor-independent route of signaling are proposed.

Jasmonates (JAs) are ubiquitously occurring signaling compounds in plants formed in response to biotic and abiotic stress as well as in development. (+)-7-iso-jasmonoyl isoleucine, the bioactive JA, is involved in most JA-dependent processes mediated by the F-box protein COI1 in a proteasome-dependent manner. However, there is an increasing number of examples, where the precursor of JA biosynthesis, cis-(+)-12-oxophytodienoic acid (OPDA) is active in a JA/COI1-independent manner. Here, we discuss those OPDA-dependent processes, thereby giving emphasis on tomato embryo development. Recent data on seed coat-generated OPDA and its role in embryo development is discussed based on biochemical and genetic evidences.

Oxidative tailoring of C40 carotenoids by double bond-specific cleavage enzymes (carotenoid cleavage dioxygenases, CCDs) gives rise to various apocarotenoids. AtCCD1 generating C13 and C14 apocarotenoids and orthologous enzymes in other plants are the only CCDs acting in the cytosol, while the hitherto presumed C40 substrate is localized in the plastid. A new model for CCD1 action arising from a RNAi-mediated CCD1 gene silencing study in mycorrhizal hairy roots of Medicago truncatula may solve this contradiction. This approach unexpectedly resulted in the accumulation of C27 apocarotenoids but not C40 carotenoids suggesting C27 as the main substrates for CCD1 in planta. It further implies a consecutive two-step cleavage process, in which another CCD performs the primary cleavage of C40 to C27 in the plastid followed by C27 export and further cleavage by CCD1 in the cytosol. We compare the specificities and subcellular locations of the various CCDs and propose the plastidial CCD7 to be the first player in mycorrhizal apocarotenoid biogenesis.

The mutualistic interaction of plants with arbuscular mycorrhizal (AM) fungi is characterized by an exchange of nutrients. The plant provides sugars in the form of hexoses to the heterotrophic fungus in return for phosphate as well as nitrogen, water, and micronutrients. Plant sucrose-cleaving enzymes are predicted to play a crucial role in hexose mobilization as these enzymes appear to be absent in the fungal partner. Here, recent findings concerning the function of plant apoplastic invertases in the AM symbiosis are discussed. Plants with modulated enzyme activity in roots and leaves provide additional insight on the complexity of the regulation of the AM interaction by apoplastic invertases as mycorrhization could be reduced or stimulated depending on the level of invertase activity and its tissue-specific expression.

The induction of chitinase (CAChi2) mRNA started as early as 6 h after inoculation and gradually increased in the incompatible interaction of pepper stems with Phytophthora capsici. In the compatible interaction, the induction of the chitinase transcripts was detected later than that in the incompatible interaction. The CAChi2 mRNA was usually localized in the vascular tissues and their expression was constricted in the phloem-related cells. These results showed that the spatial pattern of CAChi2 mRNA expression was similar in both compatible and incompatible interactions but the temporal patterns were different from each other. In addition, the early induction ofCAChi2 mRNA was quite distinct in the incompatible interaction. Immunogold labelling data showed specific labelling of chitinase on the cell wall of the oomycete in both compatible and incompatible interactions at 24 h after inoculation. In particular, numerous gold particles were deposited on the cell wall of P. capsici with a predominant accumulation over areas showing signs of degradation in the incompatible interaction. Chitinase labelling was also detected in the intercellular space and the host cytoplasm. However, healthy pepper stem tissue was nearly free of labelling.