Developmental role of the tomato Mediator complex subunit MED18 in pollen ontogeny

[EN] Pollen development is a crucial step in higher plants, which not only makes possible plant fertilization and seed formation, but also determines fruit quality and yield in crop species. Here, we reported a tomato T-DNA mutant, pollen deficient1 (pod1), characterized by an abnormal anther development and the lack of viable pollen formation, which led to the production of parthenocarpic fruits. Genomic analyses and the characterization of silencing lines proved that pod1 mutant phenotype relies on the tomato SlMED18 gene encoding the subunit 18 of Mediator multi-protein complex involved in RNA polymerase II transcription machinery. The loss of SlMED18 function delayed tapetum degeneration, which resulted in deficient microspore development and scarce production of viable pollen. A detailed histological characterization of anther development proved that changes during microgametogenesis and a significant delay in tapetum degeneration are associated with a high proportion of degenerated cells and, hence, should be responsible for the low production of functional pollen grains. Expression of pollen marker genes indicated that SlMED18 is essential for the proper transcription of a subset of genes specifically required to pollen formation and fruit development, revealing a key role of SlMED18 in male gametogenesis of tomato. Additionally, SlMED18 is able to rescue developmental abnormalities of the Arabidopsis med18 mutant, indicating that most biological functions have been conserved in both species. Significance Statement Pollination is a key development process in the life cycle of flowering plants. Genetic and molecular characterization of a tomato mutant have led to the identification of POD1 gene encoding the Mediator complex subunit MED18 whose function is required for tapetum tissue degeneration, a crucial step for pollen development. Furthermore, we show that MED18 fulfils an essential role in tomato, ensuring proper gene regulation during pollen ontogeny.This research was supported by the Spanish Ministry of Economy and Competitiveness (grants AGL2015-64991-C3-1-R, AGL2015-64991-C3-2-R, AGL2015-64991-C3-3-R, BIO2013-43098-R, BFU2016-77243-P and BIO2016-77559-R) and Junta de Andalucia (grant P12-AGR-1482).Pérez Martín, F.; Juan Yuste-Lisbona, F.; Pineda, B.; García Sogo, B.; Del Olmo, I.; Alché, JDD.; Egea, I.... (2018). Developmental role of the tomato Mediator complex subunit MED18 in pollen ontogeny. The Plant Journal. 96(2):300-315. https://doi.org/10.1111/tpj.14031S300315962Allen, B. L., & Taatjes, D. J. (2015). 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Toward plant defense mechanisms against root pathogens

As we consider the importance of the plant microbiome for sustainable agriculture,we note that some of these organisms are beneficial to plants, others arecommensal, and the third category includes pathogens. Root pathogens, such asFusarium oxysporum, are of particular interest as these can be controlled byeffective plant defense mechanisms or by biocontrol microbes. This chapterreviews recent findings of plant defense mechanisms against root pathogens.This process can be greatly assisted by microbes that prime the plant, leading toinduced systemic resistance. These microbes prepare the plant to mount a fasterand stronger response against root pathogens at a very low energetic cost to theplant. New strategies to develop resistance come from the knowledge that agriculturallyimportant root pathogens often hijack the wrong defense pathway inplants. An interesting emerging area is the plant defense mechanism that leads tothe production of root exudates which can result in the recruitment of beneficialmicrobes that assist the plant in developing resistance. Future work may focus onthe selection of plants whose defense responses against pathogens may be betterassisted by agriculturally important beneficial microbes

MEDIATOR18 and MEDIATOR20 confer susceptibility to Fusarium oxysporum in Arabidopsis thaliana

The conserved protein complex known as Mediator conveys transcriptional signals by acting as an intermediary between transcription factors and RNA polymerase II. As a result, Mediator subunits play multiple roles in regulating developmental as well as abiotic and biotic stress pathways. In this report we identify the head domain subunits MEDIATOR18 and MEDIATOR20 as important susceptibility factors for Fusarium oxysporum infection in Arabidopsis thaliana. Mutants of MED18 and MED20 display down-regulation of genes associated with jasmonate signaling and biosynthesis while up-regulation of salicylic acid associated pathogenesis related genes and reactive oxygen producing and scavenging genes. We propose that MED18 and MED20 form a sub-domain within Mediator that controls the balance of salicylic acid and jasmonate associated defense pathways

The resistance of <i>med18</i> is mediated by the roots.

<p>Reciprocal grafts made between the WT (Col-0) and <i>med18</i> seedlings revealed that roots containing the <i>med18</i> root genotype were highly resistant to infection, whereas <i>med18/</i>WT or WT/WT self-grafts were highly susceptible. (A) shows the average percentage of leaf chlorosis from eighteen plants per graft combination. Error bars represent standard error. (B) shows typical disease symptoms at 12 days after infection. *** represents significance (p<0.001) using Student’s T-test of each mutant compared to the WT/WT self-graft.</p

Real time qRT-PCR data of SA-associated genes in WT, <i>med18</i> and <i>med20</i> roots and leaves.

<p>(A, C, E) <i>PR1</i>, <i>PR2</i> and <i>PR5</i> expression in roots of WT, <i>med18</i> and <i>med20</i> with and without <i>F</i>. <i>oxysporum</i> infection. (B, D, F) <i>PR1</i>, <i>PR2</i> and <i>PR5</i> expression in leaves of WT, <i>med18</i> and <i>med20</i> in response to 24 hours of mock or SA treatment. Results were obtained from three independent biological replicates of twenty plants per replicate. (a & b) indicates <i>p</i>-value < 0.05 using a one-way ANOVA, least significant difference test. Error bars represent standard deviation of the biological replicates.</p

<i>MED18</i> and <i>MED20</i> co-regulate a similar subset of genes.

<p>(A) Venn diagram of the genes differentially expressed between WT and <i>med18</i> or WT and <i>med20</i>. (B) Heat map of the 1105 co-regulated genes. Heat map displays the Log2 fold change of <i>med18</i> or <i>med20</i> compared to the WT, with red representing higher and green representing lower expression in the Mediator mutants, respectively. Scale bar shows the colour change according to the Log2 fold change.</p