Impact of UV-B radiation on primary root elongation: effect on meristem cell proliferation and cell elongation

MARÍA DEL LUJAN SHERIDAN; PAULA CASATI

Congreso; Joint LV Reunión SAIB and XIV PABMB Congress; 2019

SAIB

Two common phenotypes of plants after UV-B exposure are the inhibition of leaf growth and ofprimary root elongation . In proliferating leaves, the reduction in leaf area is in part a result of the inhibition of cell proliferation mediated by miR396, a microRNA that downregulates the expression of Growth Regulating Factors (GRFs) transcription factors (FTs). These FTs regulate numerous developmental processes acting redundantly. In leaf primordia, miR396 is expressed at low levels and increases during organ development, whereas GRFs are expressed in an opposite way. MicroRNAs (miRNAs) are short RNA molecules that control gene expression at the post-transcriptional level by targeting the cleavage of complementary mRNAs or by inhibiting their translation. In plants, miRNAs are produced from RNA polymerase II transcripts in a multi-step process. At least four proteins are involved in the processing of primary miRNA precursors (pri-miRNAs) in Arabidopsis thaliana: DCL1, HYL1, SE and HEN1. DCL1 participates in early stages of pri-miRNA maturation by consecutive trimming at the 5′ and 3′ ends leading to the so-called pre-miRNAs. HYL1 protein forms a nuclear complex with DCL1 and is important for precise and efficient cleavage of at least several pri-miRNAs, SE is crucial for the accumulation of multiple miRNAs and trans-acting small interfering RNAs (ta-siRNA) and is found in nuclear bodies together with DCL1 and HYL1. Finally, the nuclear protein HEN1 specifically methylates miRNA:miRNA* and siRNA:siRNA* duplexes. In this work, we analyzed the participation of miR396 in particular, and the microRNA processing pathway in general, in the response ofArabidopsis thaliana roots to UV-B exposure. For this, plants were irradiated with a single UV-B treatment during 1 h at an intensity of 2w/m2 5 days after stratification, and then they allowed continuing their growth in the absence of UV-B. The studies were carried out using WT lines, and using lines deficient in microRNA processing pathway (hyl1-2, se-1 and hen1-8), and also transgenic lines with and altered miR396 pathway (MIM396 line, which expresses a target mimic that competes with GRFs for the binding of miR396 and rGRF3 a transgenic plant resistant to the regulation by miR396). Root growth rate was analyzed by monitoring primary root elongation, and by means of confocal microscopy, analyzing the number of meristematic dead cells after exposure, and also the length and number of cells in the meristematic and elongation zones. The results show that all lines show differences in least one of the parameters analyzed, indicating that proteins involved in the biosynthetic pathways of miRNAs, and in particular of miR396, modulate the response of Arabidopsis roots to UV-B radiation, acting in the development of the meristematic zone.