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

The fine tuning of hormone (e.g., auxin and gibberellin) levels and hormone signaling is required for maintaining normal embryogenesis. Embryo polarity, for example, is ensured by the directional movement of auxin that is controlled by various types of auxin transporters. Here, we present pieces of evidence for the auxin-gibberellic acid (GA) hormonal crosstalk during embryo development and the regulatory role of the Arabidopsis thaliana Calcium-Dependent Protein Kinase-Related Kinase 5 (AtCRK5) in this regard. It is pointed out that the embryogenesis of the Atcrk5-1 mutant is delayed in comparison to the wild type. This delay is accompanied with a decrease in the levels of GA and auxin, as well as the abundance of the polar auxin transport (PAT) proteins PIN1, PIN4, and PIN7 in the mutant embryos. We have previously showed that AtCRK5 can regulate the PIN2 and PIN3 proteins either directly by phosphorylation or indirectly affecting the GA level during the root gravitropic and hypocotyl hook bending responses. In this manuscript, we provide evidence that the AtCRK5 protein kinase can in vitro phosphorylate the hydrophilic loops of additional PIN proteins that are important for embryogenesis. We propose that AtCRK5 can govern embryo development in Arabidopsis through the fine tuning of auxin-GA level and the accumulation of certain polar auxin transport proteins

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

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