Specificity of the accumulation of mRNAs and proteins of the plasma membrane and tonoplast aquaporins in radish organs

Plant aquaporins occur in multiple isoforms and are distributed in both plasma membrane and tonoplast. We cloned cDNAs for plasma-membrane aquaporins (PAQ1, 1b, 1c, 2, 2b, and 2c) of radish (Raphanus sativus L.). The amino acid sequences of the PAQs showed on average 63% sequence identity. Their sequences were 23% identical to those of tonoplast aquaporins (γ- and δ-VM23). A comprehensive investigation of the aquaporin mRNAs, including VM23, in seedlings, plants, flowers and seeds of radish showed a marked accumulation of all the mRNAs in hypocotyls and growing taproots. In other organs, the mRNA level of each isoform varied according to the organ. In petals, stamens, pistils and sepals of flowers, the levels of PAQ1, 1b, 1c and γ-VM23 mRNAs were high, and mRNAs of all aquaporins except for δ-VM23 were detected at high levels in pericarps. The protein levels of aquaporins on the basis of the membrane protein were determined by immunoblotting. Proteins PAQ1 and VM23 were detected in every organ except for the mature petiole. The PAQ2 protein level was especially high in green cotyledons and leaves, but was extremely low in seedling cotyledons and hypocotyls. Proteins PAQ1, PAQ2 and VM23 were highly accumulated in growing pericarps, but not in the immature seeds. These results indicate that the gene expression of the aquaporin isoforms was individually regulated in an organ- and tissue-specific manner, and that the amounts of aquaporin protein, especially PAQ2, are regulated in certain tissues at the translational level and by the rate of protein turnover.