The Muday Laboratory, Wake Forest University
Protein Phosphorylation and
Reversible modification of proteins by covalent phosphorylation is an important regulatory mechanism controlling the activity of cellular proteins. Several lines of evidence suggest that reversible protein phosphorylation regulates auxin transport and gravity response in plants. We are examining the role that phosphorylation plays in these processes using seedlings of the Arabidopsis mutant, rcn1 (roots curl in NPA1), in collaboration with Dr. Alison DeLong (Brown University: http://biomed.brown.edu/Faculty/D/delonga.html). The rcn1 mutant has a defect in a gene encoding a regulatory subunit of protein phosphatase 2A (PP2A) and has reduced PP2A activity (Garbers et al., 1986; Deruere et al., 1999). This rcn1 mutant was isolated due to a defect in differential cell elongation in the presence of an auxin transport inhibitor and the mutant phenotype can be mimicked by treatment of wild-type plants with the phosphatase inhibitor cantharidin. We have found that reduced phosphatase activity alters auxin transport and dependent physiological processes in the seedling root (Rashotte et al, 2001). Gravitropic bending of rcn1 roots is dramatically reduced, as compared to wild-type, as shown in the accompanying figure. Our results support the role of protein phosphorylation in regulating auxin transport, and suggest that the acropetal and basipetal auxin transport streams are differentially regulated.
We have also examined
auxin transport and gravity response in hypocotyls of rcn1
and found that the rcn1 mutation enhances auxin transport,
just as we observed in roots (Muday et al., 2006). Yet, the enhanced
auxin transport has the opposite effect on gravitropism, in that it
is enhanced in the rcn1 mutant. We also examined whether
the altered hypocotyl phenotypes might be do to interactions between
auxin and ethylene. We found that the hypocotyl elongation phenotypes
of rcn1 were likely tied to enhanced ethylene synthesis, that the
gravity phenotype of rcn1 appeared to be ethylene independent
(Muday et al. 2006). We have now begun to examine mutants with
defects in protein kinases, such as the PINOID kinase, that may act
to positively regulate the auxin transport proteins that are inhibited
(graduate students in bold-face type)
Sukumar, P, Edwards, KS, Rahman, A, and DeLong, A, and Muday, GK (2009) PINOID kinase regulates root gravitropism through modulation of PIN2-dependent basipetal auxin transport in Arabidopsis thaliana. Plant Physiol: 150: 722-735.
Deruère, J., Jackson, K., Garbers, C., Soll, D., and DeLong, A. (1999). The RCN1-encoded A subunit of protein phosphatase 2A increases phosphatase activity in vivo. Plant J 20, 389-399.
Garbers, C., DeLong, A., Deruère, J., Bernasconi, P., and Soll, D. (1996). A mutation in protein phosphatase 2A regulatory subunit A affects auxin transport in Arabidopsis. EMBO J 15, 2115-2124.
Muday, GK, Brady1, SR, Argueso, C., Deruère, J, Kieber, JJ, and DeLong, A. (2006) RCN1-regulated phosphatase activity and EIN2 modulate hypocotyl gravitropism by a mechanism that does not require ethylene signaling. Plant Physiology: 141: 1617-1629
Rashotte, AM, DeLong, A, and Muday, GK (2001) Genetic and chemical reductions in protein phosphatase activity alter auxin transport, gravity response and lateral root elongation. Plant Cell 13: 1683-1697