Effect of Phosphorus Levels on the Protein Profiles of Secreted Protein and Root Surface Protein of Rice

Plant roots are complicated organs
that absorb water and nutrients
from the soil. Roots also play an essential role in protecting plants
from attack by soil pathogens and develop a beneficial role with some
soil microorganisms. Plant-derived rhizosphere proteins (e.g., root
secretory proteins and root surface binding proteins) are considered
to play important roles in developing mutual relationships in the
rhizosphere. In the rhizosphere, where plant roots meet the surrounding
environment, it has been suggested that root secretory protein and
root surface binding protein are important factors. Furthermore, it
is not known how the physiological status of the plant affects the
profile of these proteins. In this study, rice plants were grown aseptically,
with or without phosphorus nutrition, and proteins were obtained from
root bathing solution (designated as root secretory proteins) and
obtained using 0.2 M CaCl2 solution (designated as root
surface binding proteins). The total number of identified proteins
in the root bathing solution was 458, and the number of root surface
binding proteins was 256. More than half of the proteins were observed
in both fractions. Most of the proteins were categorized as either
having signal peptides or no membrane transport helix sites. The functional
categorization suggested that most of the proteins seemed to have
secretory pathways and were involved in defense/disease-related functions.
These characteristics seem to be unique to rhizosphere proteins, and
the latter might be part of the plants strategy to defeat pathogens
in the soil. The low phosphorus treatment significantly increased
the number of pathogenesis-related proteins in the root secretory
proteins, whereas the change was small in the case of the root surface
binding proteins. The results suggested that the roots are actively
and selectively secreting protein into the rhizosphere.