Root system structure is an expression of genetically-defined developmental navigation of a variable habitat. Along a precipitation gradient, roots show stereotypical adaptations.  Root architectural and anatomical responses to resource insufficiency is likewise conserved and predictable. For example, nitrogen is mobile, and insufficiencies result in longer roots compared to the plants insufficient in the  immobile nutrient potassium.  Adaptation of Poaceae root architecture and anatomy to variable habitat conditions have been well-documented, and many of the traits implicated in drought adaptation have the potential to affect plant interactions with the microbiome. 

Experimental evaluation of variation in the root structure (anatomy and architecture) of invasive plants is uncommon, despite increasing recognition that belowground competition is integral to the transformative effects of invasion and that conditions affecting  competition vary between habitats.

Immediately upon extension of the radicle, the embryonic root, into the soil substrate, the plant begins to interface with the local microbiome. The root secretes organic molecules, including a substantial proportion of carbon-rich photosynthetic product that provides a rich resource for soil microbes. The region surrounding the root is abundant with microbial life, with magnitudes greater microbial biomass in the soil space proximal to roots. This establishes a competitive economy among microbes, over which the plant exerts substantial selective potential. Not only can composition of root exudates be modified to enrich for particular resources, but small molecules also act as functional signals that shape community selection in a more refined manner.

Thus, the root-rhizosphere interface involves two examples of genotype-by-environment interactions, where the plant root responds dynamically to variable biogeochemical conditions while also influencing these conditions via exudates.

Plant control over microbiome community composition and function serves as a mechanism to increase host fitness, either by direct specific interactions (impairment of a growth-reducing microbe or enhancement of a mutualist) or by indirect interactions wherein changes in the soil microbiome have a secondary effect on the growth potential of neighboring plants. In the context of nutrient economies, this can occur by changing microbially-mediated cycling functions (ie ammonia oxidization, nitrification, denitrification) such that nitrogen is found in forms preferred by the invasive species, or by disruption of taxa that might otherwise support native competitors. 

Invasive plants are frequently associated with changes in bioavailable nitrogen and shifts in the soil microbiotic community, but substantiating causation is challenging. Invaders do not increase nitrogen access uniformly. Further, the microbiome is susceptible to spatial variation in soil matrix micro-habitat, so logistical restrictions on sampling necessarily confounds prediction of microbiome trends even within one invasion.

Our work considers variation in root structure and the rhizosphere microbiome, and how differences in nitrogen-cycling interactions might result in variable belowground characteristics. 

Cheatgrass is regarded as one of the worst plant invaders internationally, due to its competitive potential and the deleterious effects of its presence. In addition to increasing the frequency of wildfire by an order of magnitude in the arid and semi-arid regions of continental America, dried seeds cause injury to livestock, pets, and hikers, and it is an herbicide-resistant weed in cool-season crops. US growers report winter wheat losses and contamination costing more than $2 billion USD annually.

Like other plant invaders, cheatgrass is associated with an increase in bioavailable nitrogen and shifts in nitrogen-cycling soil microbial functional groups. No allelopathic compounds have been found, so cheatgrass domination of arid and semi-arid systems is attributed to seed survival of wildfire and early-season acquisition of soil resources.

Although it was evident that cheatgrass invasion was associated with shifts in biogeochemical cycling, habitat variation between studies meant that no generalizations could be made. Cheatgrass influenced the nitrogen cycle in different ways in different ecosystems.

A GWAS performed by Dr. Diana Gamba supported previous information that there were multiple ancestries of cheatgrass, that these ancestries are associated with different strategies to increase fitness based on the aridity and temperature of the original site, and found that this variation influenced life history phenotypes. 

Flowering time is an important indicator of developmental pace, and is related to variable nitrogen acquisition dynamics and to habitat condition. 

Rhizobox Trial 1, Jan 2024

Three entangled root systems, extracted from soil

Sincere thanks to undergrad researchers Natalia V. Ali, Rosalyn Towler, Fatimah Almaskin, Jonny Hur

Washed and Scanned Roots