Proteomics of Nitrogen Remobilization in Poplar Bark

Seasonal
nitrogen (N) cycling in temperate deciduous trees involves
the accumulation of bark storage proteins (BSPs) in phloem parenchyma
and xylem ray cells. BSPs are anabolized using recycled N during autumn
leaf senescence and later become a source of N during spring shoot
growth as they are catabolized. Little is known about the catabolic
processes involved in remobilization and reutilization of N from BSPs
in trees. In this study, we used multidimensional protein identification
technology (MudPIT) and spectral counting to identify protein changes
that occur in the bark during BSP catabolism. A total of 4,178 proteins
were identified from bark prior to and during BSP catabolism. The
majority (62%) of the proteins were found during BSP catabolism, indicating
extensive remodeling of the proteome during renewed shoot growth and
N remobilization. Among these proteins were 30 proteases, the relative
abundances of which increased during BSP catabolism. These proteases
spanned a range of families including members of the papain-like cysteine
proteases, serine carboxypeptidases, and aspartyl proteases. These
data identify, for the first time, candidate proteases that could
potentially provide hydrolase activity required for N remobilization
from BSPs and provide the foundation for research to advance our knowledge
of poplar N cycling.