Why Do Creosote Bushes Have Endophytes?

The Creosote Bush

• Creosote bush is among the most drought-tolerant of desert shrubs in the Southwest, allowing it to successfully colonize – and dominate – huge expanses of desert flatlands. An evergreen, it nonetheless sheds some of its small, paired, point-tipped leaves during parched times. It produces yellow flowers of twisted sepals during extended periods of bloom. The name stems from the strong odor the shrub emits when its leaves are broken or after a big rain, a trait also reflected in a Mexican moniker for creosote bush, “hediondilla,” which means “little stinker.” In prime growing conditions creosote bush may grow 12 feet tall, but is typically much shorter.

Endophytes

• Endophytes refer to fungi and bacteria that colonize plant tissues in a case of symbiotic mutualism -- that is, both host and occupant glean benefit from the interaction. The endophytes have a relatively secure, nutrient-rich environment as their home. Plants, meanwhile, may respond to endophytic interaction with bolstered root and shoot growth and greater resistance to pests and environmental stressors. Most land plants may engage with endophytes to some degree, and the creosote bush is no exception.

Creosote Bush Endophytes

• The Creosote bush and endophytic fungi form symbiotic relationships known as arbuscular mycorrhizae, meaning that the fungi tap into the plant’s root cells. Such root-living fungi are extremely common in the plant kingdom; a 2002 paper in “Western North American Naturalist” by Jonathan H. Titus et al. suggested over 90 percent of higher plants are mycorrhizal, mostly in arbuscular (root-tapping) form. That same paper showed that all indigenous perennial species investigated, including creosote bush, were mychorrhizal – suggesting that these relationships are widespread and may impart important benefits to both plant and fungi.

Seasonality

• A study by Martha E. Apple et al., published in a 2005 edition of “Symbiosis,” showed that arbuscular mycorrhizal fungi colonized two Mojave shrubs -- the creosote bush and white bur sage (or burro bush) -- in seasonal cycles. The highest rates of colonization were associated with major rainfall events, with decreases observed during intervals of drought. The study also suggested that fungal symbiosis declined during springtime spurts of fresh root, shoot and flower growth, possibly because the plants were dedicating more carbon for this production, with less available to mycorrhizal fungi. Exploring such seasonal variations in endophytic interaction may shed further light on the benefits the creosote bush might extract from this symbiosis.