Phosphoproteomic Analysis of Paper Mulberry Reveals Phosphorylation Functions in Chilling Tolerance

Paper mulberry is a valuable woody
species with a good chilling
tolerance. In this study, phosphoproteomic analysis, physiological
measurement, and mRNA quantification were employed to explore the
molecular mechanism of chilling (4 °C) tolerance in paper mulberry.
After chilling for 6 h, 427 significantly changed phosphoproteins
were detected in paper mulberry seedlings without obvious physiological
injury. When obvious physiological injury occurred after chilling
for 48 h, a total of 611 phosphoproteins were found to be significantly
changed at the phosphorylation level. Several protein kinases, especially
CKII, were possibly responsible for these changes according to conserved
sequence analysis. The results of Gene Ontology analysis showed that
phosphoproteins were mainly responsible for signal transduction, protein
modification, and translation during chilling. Additionally, transport
and cellular component organization were enriched after chilling for
6 and 48 h, respectively. On the basis of the protein–protein
interaction network analysis, a protein kinase and phosphatases hub
protein (P1959) were found to be involved in cross-talk between Ca2+, BR, ABA, and ethylene-mediated signaling pathways. We also
highlighted the phosphorylation of BpSIZ1 and BpICE1 possibly impacted
on the CBF/DREB-responsive pathway. From these results, we developed
a schematic for the chilling tolerance mechanism at phosphorylation
level.