Plasma membrane aquaporins function in moisture regulation during seed germination and leaf hydration in eelgrass

Eelgrass (Zostera marina) is a marine angiosperm distributed in shallow seas that has a unique mechanism for regulating water content. Water transfer across the plasma membrane is facilitated by aquaporins, which are membrane proteins. In this study, we investigated the physiological functions of plasma membrane intrinsic proteins (PIPs) in eelgrass. ZoPIP1;1 and ZoPIP2;1 in eelgrass leaves were amplified by RT-PCR. Both ZoPIPs localized to the plasma membrane, and only ZoPIP2;1 showed water permeability. ZoPIP1;1 and ZoPIP2;1 were expressed in all tissues except male flowers. ZoPIPs were expressed during seed and fruit development, and their expression levels were low in mature seeds and elevated in germinating seeds. Moisture content decreased in the later phases of seed development and increased during seed germination. These findings imply that ZoPIPs participate in water uptake during seed germination. In leaves, ZoPIP expression was upregulated by dehydration but downregulated by rehydration. Water reabsorption of leaves was inhibited by HgCl₂, an inhibitor of aquaporins. These findings indicate that ZoPIPs are upregulated by dehydration during extremely low tides and promote water uptake when the tide turns. Therefore, eelgrass PIPs function in moisture regulation during growth, seed germination, and the flood-ebb tidal cycle in seawater.