The Halophyte Seashore Paspalum Uses Adaxial Leaf Papillae for Sodium Sequestration

Salinity is a growing issue worldwide, with nearly 30% of arable land predicted to be lost due to soil salinity in the next 30 years. Many grass crops that are vital to sustain the world's caloric intake are salt sensitive. Studying mechanisms of salt tolerance in halophytic grasses, plants that thrive in salt conditions, may be an effective approach to ultimately improve salt-sensitive grass crops. Seashore paspalum (Paspalum vaginatum) is a halophytic Panicoid grass able to grow in salt concentrations near that of seawater. Despite its widespread cultivation as a sustainable turfgrass, the mechanism underlying its ability to retain high Na⁺ concentrations in photosynthetic tissue while maintaining growth remains unknown. We examined the leaf structure and ion content in P. vaginatum 'HI10', which shows increased growth under saline conditions, and Paspalum distichum 'Spence', which shows reduced growth under salt, to better understand the superior salt tolerance of cv HI10. A striking difference between cv HI10 and cv Spence was the high steady-state level of K⁺ in cv HI10. Imaging further showed that the adaxial surface of both cv HI10 and cv Spence contained dense costal ridges of papillae. However, these unicellular extensions of the epidermis were significantly larger in cv HI10 than in cv Spence. The cv HI10 papillae were shown to act as Na⁺ sinks when plants were grown under saline conditions. We provide evidence that leaf papillae function as specialized structures for Na⁺ sequestration in P. vaginatum, illustrating a possible path for biotechnological improvement of salt-sensitive Panicoid crops with analogous leaf structures.