Functional Characterization of transcription factor AtHB40

CHAN, RAQUEL LÍA; MORA CATIA

Santa Fe

Encuentro; Arabidopsis thaliana root meeting; 2019

Instituto de agrobiotecnologia del litroral

Homeodomain-leucine zipper I (HD-Zip I) transcription factors (TFs) are unique to plants and have been mainly associated with developmental processes and also related to abiotic stress responses in several species. It was previously reported that AtHB40, an Arabidopsis member of this TF subfamily, is expressed in roots, particularly in the tip of the main and lateral roots. Such expression circumscribed to the quiescent center, columella cells and the vascular system. The obtaining and characterization of AtHB40 mutants (athb40) and overespressors (OE40) as well as plants transformed with AtHB40 promoter driving GUS expression (PrAtHB40:GUS), allowed us to determine that AtHB40 is a repressor of main root elongation in an ABA dependent manner. Regarding the inhibition of root elongation produced by AtHB40, we stated several hypotheses: a) AtHB40 regulates cyclins; b) AtHB40 regulates auxin transporters; c) AtHB40 inhibits root elongation when seedlings are subjected to abiotic stress such as high salinity; d) the gravitropic response is altered in athb40 and OE40 plants. To investigate our hypotheses, we obtained crossed plants in which the promoters of the putative target genes fused to the reporter GUS were expressed in AtHB40 mutant or overexpressor backgrounds. The analyses of these crossed lines indicated that LAX2 is downregulated by AtHB40 in 3-day-old seedlings. Moreover, CYCB1 (cyclin) was repressed by AtHB40 in the root tip of 7-day-old plants. AtHB40 mutant plants exhibited longer main roots than controls in the presence of ABA, Fluridone (an inhibitor of ABA synthesis), auxin (IAA) or NaCl indicating certain insensitivity to the growing media. Moreover, athb40 mutants showed a larger survival percentage when seedlings were grown in high salinity medium. The response to gravitropism was also investigated indicating that athb40 lines exhibited an enhanced positive gravitropism whereas OE40 a reduced response compared to WT plants. Altogether, our results suggest that AtHB40 is a repressor of main root elongation and has a functional role in the gravitropoic response as well as in cell division and auxin transport in the root tip.