Abstract
The hormone auxin is critical for many plant developmental processes. Unlike the model eudicot plant Arabidopsis (Arabidopsis thaliana), auxin distribution and signaling in rice tissues has not been systematically investigated due to the absence of suitable auxin response reporters. In this study we observed the conservation of auxin signaling components between Arabidopsis and model monocot crop rice (Oryza sativa), and generated complementary types of auxin biosensor constructs, one derived from the Aux/IAA-based biosensor DII-VENUS but constitutively driven by maize ubiquitin-1 promoter, and the other termed DR5-VENUS in which a synthetic auxin-responsive promoter (DR5rev) was used to drive expression of the yellow fluorescent protein (YFP). Using the obtained transgenic lines, we observed that during the vegetative development, accumulation of DR5-VENUS signal was at young and mature leaves, tiller buds and stem base. Notably, abundant DR5-VENUS signals were observed in the cytoplasm of cortex cells surrounding lateral root primordia (LRP) in rice. In addition, auxin maxima and dynamic re-localization were seen at the initiation sites of inflorescence and spikelet primordia including branch meristems (BMs), female and male organs. The comparison of these observations among Arabidopsis, rice and maize suggests the unique role of auxin in regulating rice lateral root emergence and reproduction. Moreover, protein localization of auxin transporters PIN1 homologs and GFP tagged OsAUX1 overlapped with DR5-VENUS during spikelet development, helping validate these auxin response reporters are reliable markers in rice. This work firstly reveals the direct correspondence between auxin distribution and rice reproductive and root development at tissue and cellular level, and provides high-resolution auxin tools to probe fundamental developmental processes in rice and to establish links between auxin, development and agronomical traits like yield or root architecture.
Highlights
The phytohormone auxin regulates many critical growth and developmental processes in plants
To reveal whether the auxin responsive element AuxRE or ARF transcription binding sites located at promoter regions of primary auxin responsive gene families in Arabidopsis, such as GH3, AUX/IAA, and SAUR (Abel and Theologis, 1996; Ulmasov et al, 1999; Chen et al, 2014) genes are conserved in rice, we searched for multiple AuxRE sites by scanning the 3,000bp promoter regions upstream of translation start sites of 11 OsGH3 genes
We observed that auxin responsive sequences (ARS, TGTCTC) were highly enriched in rice promoter regions of OsGH3.3, OsGH3.5, OsGH3.12 (Supplementary Table S2), while no ARS was present within the OsGH3.10 promoter, which are well in line with the responses of increased expression of OsGH3.3, OsGH3.5, OsGH3.12, and no detectable change in transcriptional level of OsGH3.10 induced by auxin treatment (Jain et al, 2006b; Terol et al, 2006)
Summary
The phytohormone auxin (indole-3-acetic acid, IAA) regulates many critical growth and developmental processes in plants. Subtle differences in auxin abundance can be visualized through changes in fluorescence, allowing high-resolution spatio-temporal changes in auxin distribution and response during plant growth and development (Brunoud et al, 2012). These two systems have been extensively used to characterize functions of genes associated with auxin signaling (Steenackers et al, 2016), gravitropic response (Band et al, 2012; Zou et al, 2016) and stomatal patterning (Le et al, 2014). DR5v2 is composed of the DR5 promoter and a novel binding site for ARF transcription factors designed to increase sensitivity and precision of auxin response visualization in Arabidopsis (Liao et al, 2015)
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