“Leveraging omics for seagrass biology, management and restoration”
– By Prof. Jeanine Olsen 2016
Author: Kelcie L. Chiquillo
Seagrasses are a unique group of species that have returned to the sea from a freshwater ancestor. This is a rare event to be able to reinvade the salty seawater conditions from a terrestrial environment. Sequencing the genome of Zostera marina gives Olsen et al. a picture of how these organisms have evolved, since most plants cannot survive in a fully submerged, salty environments.
So how did they do it?
First they underwent a genome duplication around 65 – 70 million years ago. Genome duplications provide advantages to organisms where one set of genes will continue with cellular function and the other set can be used for adaptation and speciation. Olsen found 2 events where the “copia” genes—young, dominant genes associated with gaining new genes and gene modification– invaded the genome, possibly allowing for the adaptation of seagrasses to the marine environment.
Seagrasses have also gained genes called sulfotranferases, which facilitate the retention of water and ion homeostasis in the cell wall. They have increased the number of metallothiones (MTs) to resist stress and use late embryogenesis abundant (LEA) proteins to tolerate high salinities.
Contrastingly, seagrasses have lost the ability to protect themselves from UV light, and have lost phytochrome genes which are associated with red light sensing. This is not surprising since most seagrasses are subtidal and intertidal species and red light gets lost very quickly in very shallow water.
However, they did gain a light harvesting complex A (LHCA) and light harvesting complex B (LHCB) genes, which allows them to photosynthesize more efficiently.
Interestingly, all fresh water plants and seagrasses lack a stomata, but unlike fresh water plants– which contain stomata genes but are not expressed– seagrasses have completely lost the genes to build stomata, and in essence can never reinvade the terrestrial realm. For many terrestrial plants the stomata is a site for gas exchange, yet these pores can be vulnerable to pathogens, however terrestrial plants use volatile points of signaling as a defense mechanism. Yet, seagrasses have lost the volatile defense mechanism genes, for example the R genes– defense associated nucleotide binding type gene family—and turpinoids are highly reduced. Even parts of the ethylene pathway are gone, since the synthesis of EIN2 protein has disappeared. However, its interesting because seagrasses have retained the EIN3 protein which indicates that there may be an alternative signaling pathway for seagrasses to defend themselves.
As many of us are aware, seagrasses are in decline, yet are one of the most important ecosystems in the marine realm providing many functions and services to both marine organisms and humans. For example, seagrasses use sucrose synthase genes to sequester high amounts of carbon dioxide and store 90% of fixed carbon in the form of sucrose. Understanding Omics, or genomics, has given scientists a key insight into how genes play in certain traits. And we can use omics as a tool to jump start and restore seagrass beds, as well as identify warning indicators of threatened seagrass beds before the loss of shoots, counts and densities.