AtCRK5 Protein Kinase Exhibits a Regulatory Role in Hypocotyl Hook Development during Skotomorphogenesis

Abstract

Seedling establishment following germination requires the fine tuning of plant hormone levels including that of auxin. Directional movement of auxin has a central role in the associated processes, among others, in hypocotyl hook development. Regulated auxin transport is ensured by several transporters (PINs, AUX1, ABCB) and their tight cooperation. Here we describe the regulatory role of the Arabidopsis thaliana CRK5 protein kinase during hypocotyl hook formation/opening influencing auxin transport and the auxin-ethylene-GA hormonal crosstalk. It was found that the Atcrk5-1 mutant exhibits an impaired hypocotyl hook establishment phenotype resulting only in limited bending in the dark. The Atcrk5-1 mutant proved to be deficient in the maintenance of local auxin accumulation at the concave side of the hypocotyl hook as demonstrated by decreased fluorescence of the auxin sensor DR5::GFP. Abundance of the polar auxin transport (PAT) proteins PIN3, PIN7, and AUX1 were also decreased in the Atcrk5-1 hypocotyl hook. The AtCRK5 protein kinase was reported to regulate PIN2 protein activity by phosphorylation during the root gravitropic response. Here it is shown that AtCRK5 can also phosphorylate in vitro the hydrophilic loops of PIN3. We propose that AtCRK5 may regulate hypocotyl hook formation in Arabidopsis thaliana through the phosphorylation of polar auxin transport (PAT) proteins, the fine tuning of auxin transport, and consequently the coordination of auxin-ethylene-GA levels.

CRK5 Protein Kinase Contributes to the Progression of Embryogenesis of Arabidopsis thaliana

“…Among others, the D6PROTEIN KINASE and PINOID/WAG AGCVIII kinases are types of kinases that are known to activate the PIN-mediated auxin efflux [65]. Previously, we have shown that the AtCRK5 kinase can phosphorylate in vitro the hydrophilic loops of various PIN proteins, like PIN2 and PIN3 [57,59]. Based on in vitro kinase assays and phospho-peptide analysis data, here we also report the possible phosphorylation sites of AtCRK5 in PIN1, PIN4, and PIN7.…”

“…The localization and activation of the various PIN proteins are known to be affected by their phosphorylation status [15,[63][64][65]. We have already published that AtCRK5 phosphorylates the PIN2 and PIN3 auxin efflux transporters and this may have a role in the gravitropic response of the Arabidopsis root upon gravistimulation and the hypocotyl hook bending during skotomorphogenesis [57,59]. The decreased abundance of PIN1-GFP, PIN4-GFP, and PIN7-GFP in Atcrk5-1 mutant embryos might reflect a defect in PIN protein stability and/or localization.…”

“…The Calcium-Dependent Protein Kinase superfamily members (CDPKs) are among the main effectors of Ca 2+ signal transduction in plants [71,72]. Members of the CDPK superfamily-including the Ca-dependent protein kinases (CDPKs), the CDPK-related protein kinases (CRKs), the Ca 2+ and calmodulin-activated kinases (CCaMKs), and the sucrose non-fermenting1-related kinases3 (SnRK3s)-in addition to abiotic/biotic stress responses play important roles in many aspects of the regulation of plant growth and development [57][58][59]67,[72][73][74]. In Arabidopsis, the CRK5 protein kinase is active in most Arabidopsis organs and it is certainly involved in the establishment of the proper auxin gradient necessary for gravitropic response of the root and for proper bending of the hypocotyl hook in Arabidopsis [57,59].…”

“…The AtCRK5 protein kinase is active in most Arabidopsis organs and it is involved in the establishment of the proper auxin gradient that is necessary for the gravitropic response of the root and the bending of the hypocotyl during skotomorphogenesis [57][58][59]. Here, we describe the importance of the CRK5 protein kinase in Arabidopsis embryo development.…”

“…The shifted embryo developmental stages and smaller embryo size could be restored in the mutant embryos by exogenous GA treatment. Previously, we showed that the CRK5 kinase is able to phosphorylate the PIN2 [57,58] and PIN3 [59] auxin transporters and its mutation results in impaired auxin transport. Therefore, we tested whether this protein kinase also regulates the auxin flow during embryogenesis and if the delayed embryogenesis of the mutant can be ascribed to altered auxin transport efficiency.…”