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.
Highlights
Seedlings of dicotyledonous plants develop an apical hook in the dark following germination in order to avoid the mechanical damage of the apical meristem when emerging from under the soil [1,2,3,4]
Our initial observation was that 3-days-old Atcrk5-1 seedlings had an altered phenotype as compared to the wild type ones (Col-0): the Atcrk5-1 mutant exhibited a decreased capacity in the closure of the apical hook during skotomorphogenesis
Germinating the wild type seeds in the dark, normal hypocotyl hook formation was obtained with an angle of approximately 180◦ at 3 days, but the Atcrk5-1 mutant seedling hook angles were smaller (145◦–160◦) under the same conditions (Figure 1A)
Summary
Seedlings of dicotyledonous plants develop an apical hook in the dark following germination in order to avoid the mechanical damage of the apical meristem when emerging from under the soil [1,2,3,4]. The first biological function discovered for AtCRK5 was that this protein kinase had a direct role in the regulation of root gravitropic response [42]. Immunolocalization pattern of the auxin efflux protein PIN2—which is a key member of basipetal auxin transport in Arabidopsis roots—exhibited a considerable alteration in the Atcrk mutant in comparison to the wild type [37,42]. Based on the above observations, the potential regulatory role of the AtCRK5 kinase in the formation and maintenance of the differential auxin gradient in the apical hook of the hypocotyl had been investigated during skotomorphogenesis. We further demonstrate the importance of the AtCRK5 protein kinase in Arabidopsis growth and development It seems that this protein kinase - to its role in regulating root gravitropic response – participates in the regulation of hypocotyl hook development during skotomorphogenesis. The AtCRK5-mediated phosphorylation of the PIN3 auxin efflux transporter influencing auxin accumulation and the effect of limited auxin accumulation on ethylene and/or GAs action is discussed
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