Publikationen - Molekulare Signalverarbeitung

SummaryArabidopsis seeds release large capsules of mucilaginous polysaccharides, which are shaped by an intricate network of cellulosic microfibrils. Cellulose synthase complexes is guided by the microtubule cytoskeleton, but it is unclear which proteins mediate this process in the seed coat epidermis (SCE).Using reverse genetics, we identified IQ67 DOMAIN 9 (IQD9) and KINESIN LIGHT CHAIN-RELATED 1 (KLCR1) as two highly expressed genes during seed development and comprehensively characterized their roles for cell wall polysaccharide biosynthesis and cortical microtubule (MT) organization.Mutations in IQD9 as well as in KLCR1 lead to compact mucilage capsules with aberrant cellulose distribution, which can be rescued by transgene complementation. Double mutant analyses revealed that their closest paralogs (IQD10 and KLCR2, respectively) are not required for mucilage biosynthesis. IQD9 physically interacts with KLCR1 and localizes to cortical MTs to maintain their organization in SCE cells. Similar to the previously identified TONNEAU1 (TON1) RECRUITING MOTIF 4 (TRM4) protein, IQD9 is required to maintain the velocity of cellulose synthases.Our results demonstrate that IQD9, KLCR1 and TRM4 are MT-associated proteins that are required for seed mucilage architecture. This study provides the first direct evidence that members of the IQD, KLCR and TRM families have overlapping roles in guiding the distribution of cell wall polysaccharides. Therefore, SCE cells provide an attractive system to further decipher the complex genetic regulation of polarized cellulose deposition.

Plant microtubules form a highly dynamic intracellular network with important roles for regulating cell division, cell proliferation and cell morphology. Its organization and dynamics are coordinated by various microtubule-associated proteins (MAPs) that integrate environmental and developmental stimuli to fine-tune and adjust cytoskeletal arrays. IQ67 DOMAIN (IQD) proteins recently emerged as a class of plant-specific MAPs with largely unknown functions. Here, using a reverse genetics approach, we characterize Arabidopsis IQD5 in terms of its expression domains, subcellular localization and biological functions. We show that IQD5 is expressed mostly in vegetative tissues, where it localizes to cortical microtubule arrays. Our phenotypic analysis of iqd5 loss-of-function lines reveals functions of IQD5 in pavement cell (PC) shape morphogenesis, as indicated by reduced interdigitation of neighboring cells in the leaf epidermis of iqd5 mutants. Histochemical analysis of cell wall composition further suggests reduced rates of cellulose deposition in anticlinal cell walls, which correlate with reduced asymmetric expansion. Lastly, we provide evidence for IQD5-dependent recruitment of calmodulin calcium sensors to cortical microtubule arrays. Our work thus identifies IQD5 as a novel player in PC shape regulation, and, for the first time, links calcium signaling to developmental processes that regulate multi-polar growth in PCs.

Lipoxygenases (LOXs) and other LOX pathway enzymes are potentially able to form a large set of compounds being of commercial interest. Among them are conjugated dienic acids, jasmonates, and volatile aldehydes. Additionally, fatty acid hydroperoxides, formed by LOX, can serve as precursors for further transformation by either enzymes of the so‐called LOX pathway or by chemical reactions. In the case of linoleic acid more than one hundred products generated from its LOX‐derived fatty acid hydroperoxides have been described. Many of these products exhibit biological activity, suggesting a significant biological function of LOXs. This will be described for two different 13‐LOXs. (I) In various oilseeds we found that specific 13‐LOXs are localized at the lipid body membrane. They are capable of oxygenating esterified polyenoic fatty acids, such as triacylglycerols and phospho‐lipids. In addition, they form with arachidonic acid as substrate preferentially either 8‐ or 11‐hydroperoxy eicosatetraenoic acid, which is a very unusual positional specificity for plant LOXs. (II) From barley leaves we isolated another linoleate 13‐LOX form, which is localized within chloroplasts and is induced by jasmonic acid methyl ester. It is suggested, that this LOX form is capable of oxygenating linolenic acid residues of galactolipids. Examples will be presented for barley leaves of oxygenated derivatives of linolenic acid and compounds resulting from the hydroperoxide lyase‐branch of the LOX pathway.

Jasmonic acid (JA) is an ubiquitously occurring plant growth regulator which functions as a signal of developmentally or environmentally regulated expression of various genes thereby contributing to the defense status of plants [1–5]. The formation of jasmonates in a lipid‐based signalling pathway via octadecanoids seems to be a common principle for many plant species to express wound‐ and stressinduced genes [4, 5].There are various octadecanoid‐derived signals [3]. Among them, jasmonic acid and its amino acid conjugates are most active in barley, supporting arguments that β‐oxidation is an essential step in lipid‐based JA mediated responses. Furthermore, among derivatives of 12‐oxophytodienoic acid (PDA) carrying varying length of the carboxylic acid side‐chain, only those with a straight number of carbon atoms are able to induce JA responsive genes in barley leaves after treatment with these compounds. Barley leaves stressed by treatment with sorbitol solutions exhibit mainly an endogenous rise of JA and JA amino acid conjugates suggesting that both of them are stress signals. Data on organ‐ and tissue‐specific JA‐responsive gene expression will be presented and discussed in terms of “JA as a master switch” among various lipid‐derived signals.