Roles of AGCVIII Kinases in the Hypocotyl Phototropism of Arabidopsis Seedlings

Abstract

Regulation of protein function by phosphorylation and dephosphorylation is an important mechanism in many cellular events. The phototropin blue-light photoreceptors, plant-specific AGCVIII kinases, are essential for phototropic responses. Members of the D6 PROTEIN KINASE (D6PK) family, representing a subfamily of the AGCVIII kinases, also contribute to phototropic responses, suggesting that possibly further AGCVIII kinases may potentially control phototropism. The present study investigates the functional roles of Arabidopsis (Arabidopsis thaliana) AGCVIII kinases in hypocotyl phototropism. We demonstrate that D6PK family kinases are not only required for the second but also for the first positive phototropism. In addition, we find that a previously uncharacterized AGCVIII protein, AGC1-12, is involved in the first positive phototropism and gravitropism. AGC1-12 phosphorylates serine residues in the cytoplasmic loop of PIN-FORMED 1 (PIN1) and shares phosphosite preferences with D6PK. Our work strongly suggests that the D6PK family and AGC1-12 are critical components for both hypocotyl phototropism and gravitropism, and that these kinases control tropic responses mainly through regulation of PIN-mediated auxin transport by protein phosphorylation.

Photoreceptors Regulate Plant Developmental Plasticity through Auxin

“…Regulation of PIN localisation, as well as activation via phosphorylation, occurs through different components such as ARF-GEF GNOM and three families of protein kinases, AGC kinases, Mitogen Activated Protein (MAP) Kinases (MPKs) and Ca 2+ /calmodulin-dependent protein kinase-related kinases (CRKs) [80,92]. Several members of the AGCVIII kinase family, to which phototropin also belongs, have been implied to regulate auxin transport and PIN phosphorylation during phototropism [43,46,47]. Transcript levels of one of these AGCVIII kinases, PINOID (PID), were found to be reduced by light [43].…”

“…Other AGCVIII kinases that are required for hypocotyl bending include D6 Protein Kinase (D6PK), D6PK Like 1 (D6PKL1), D6PKL2 and D6PKL3 as well as AGC1-12 [46,47]. Similar to PID, both D6PKs and AGC1-12 are able to phosphorylate the PIN1 hydrophilic loop in vitro [46].…”

“…Other AGCVIII kinases that are required for hypocotyl bending include D6 Protein Kinase (D6PK), D6PK Like 1 (D6PKL1), D6PKL2 and D6PKL3 as well as AGC1-12 [46,47]. Similar to PID, both D6PKs and AGC1-12 are able to phosphorylate the PIN1 hydrophilic loop in vitro [46]. However, D6PKs and AGC1-12 phosphorylate at least one unique PIN1 serine residue that is not phosphorylated by PID, which might distinguish them from PID in regulating hypocotyl bending [46].…”

“…Similar to PID, both D6PKs and AGC1-12 are able to phosphorylate the PIN1 hydrophilic loop in vitro [46]. However, D6PKs and AGC1-12 phosphorylate at least one unique PIN1 serine residue that is not phosphorylated by PID, which might distinguish them from PID in regulating hypocotyl bending [46]. The function of D6PKs and AGC1-12 in regulating hypocotyl bending appears to extend beyond phototropism as their mutants also show decreased gravitropism [46].…”

The RAF‐like mitogen‐activated protein kinase kinase kinases RAF22 and RAF28 are required for the regulation of embryogenesis in Arabidopsis

“…Phosphorylation and dephosphorylation play important roles in the regulation of auxin polar transport and distribution (Christensen et al, 2000;Benjamins et al, 2001;Sukumar et al, 2009;Zourelidou et al, 2009;Zhang et al, 2010). The phosphorylation of PINs, including PIN1-PIN4 and PIN7 in Arabidopsis, has been revealed (Michniewicz et al, 2007;Huang et al, 2010;Zhang et al, 2010;Zourelidou et al, 2014;Dhonukshe et al, 2015;Daeeun et al, 2016;Jia et al, 2016;Weller et al, 2017;Haga et al, 2018). To date, six phosphosites in PINs and their targeting kinases have been experimentally analyzed.…”