Molecular analysis of physiological responses to changes in nitrogen in a marine macroalga, Porphyra yezoensis (Rhodophyta)

The rhodophyte seaweed Porphyra yezoensis, known more commonly world-wide as nori, is an important commercial crop in Japan. Cultivation of nori in Japan is often affected by outbreaks of iroochi, a discoloration of the thalli due to a decrease in inorganic nutrients in the culture area that in turn decreases the amount of photosynthetic pigments in the thalli. Treating thalli with inorganic nitrogen can reverse iroochi. In this paper, we report on the characterization of three P. yezoensis genes, a nitrate transporter (PyNRT2) and two urea transporters (PyUT1 and PyUT2), which may be involved in reversing iroochi. The predicted length of the PyNRT2 protein was 479 amino acids (AA), while PyUT1 and PyUT2 were 740 and 680 AA, respectively. PyNRT2 was more similar to NRT2 from a chromophyte than to NRTs from Chlamydomonas and higher plants. The two P. yezoensis UTs had 56% AA identity to each other, and showed the closest relationship to higher plant and yeast DUR3 proteins which formed a subfamily of the sodium-solute symporter protein family. Hydrophobicity plots of the AA sequences showed that the PyNRT2, PyUT1, and PyUT2 included 12, 15, and 16 transmembrane domains, respectively. Southern blot analysis indicated that the P. yezoensis genome has a single NRT2-encoding gene and at least four UT-encoding genes. Expression analysis of PyNRT2 and PyUT genes showed that the messenger RNA level of the PyNRT2 gene reached a maximum after 48 h in the nitrate starvation condition and was then restored to the constitutive level, while expression of the PyUT genes was induced in proportion to treatment times in the nitrate starvation condition. These results suggest that the PyNRT2 and PyUT are responsible for the high-affinity nitrate/urea transport systems that operate under low external nitrate concentrations.