Feed Your Friends: Do Plant Exudates Shape the Root Microbiome?

Plant health in natural environments depends on interactions with complex and dynamic communities comprising macro- and microorganisms. While many studies have provided insights into the composition of rhizosphere microbiomes (rhizobiomes), little is known about whether plants shape their rhizobiomes. Here, we discuss physiological factors of plants that may govern plant-microbe interactions, focusing on root physiology and the role of root exudates. Given that only a few plant transport proteins are known to be involved in root metabolite export, we suggest novel families putatively involved in this process. Finally, building off of the features discussed in this review, and in analogy to well-known symbioses, we elaborate on a possible sequence of events governing rhizobiome assembly

Function of Root Border Cells and their Exudates on Plant Defense in Hydroponic Systems

Controlled environment agriculture offers a solution to challenges including less available land, water deficits, and consumer demand for pesticide free produce. However, control of soil-borne diseases is a major limiting factor. The goal of this dissertation was to examine predictions of the hypothesis that border cells function to protect plant health by controlling microorganisms associated with plants grown in hydroponic culture. Border cells separate from root tips upon immersion in water, and appear to have important roles in the defense mechanisms of plant roots. The general objectives were (1) to study the delivery of border cells in hydroponics; (2) to evaluate interactions between border cells and microorganisms in hydroponics; and (3) to explore approaches to alter border cell production for improved root disease control. In this study it was confirmed that border cells can be released continuously into the solution of hydroponic culture suggesting that plants grown in this system may use extra energy in the production of new border cells. Free border cells interacted with microorganisms present in the hydroponic solution by secreting an extracellular capsule. Previous studies showed that proteins are a key component of this capsule, including lectins. The interaction of pea lectin and Nectria haematococca spores therefore was explored. Results demonstrated that pea lectin agglutinates fungal spores in a hapten-specific manner, and inhibits their germination. Lectin had no negative effect on root development suggesting that it could be used as a potential control for soil-borne diseases in hydroponics. To control the production of border cells, subsequent studies measured the impact of a transient exposure of root tips to different metabolites secreted by root caps and border cells. Exposure to specific metabolites altered the production of border cells without measurable effects on root growth and development. This is in contrast to results obtained with altered gene expression. For example, gene silencing of a border cell specific gene resulted in altered root growth

Extracellular Trapping of Soil Contaminants by Root Border Cells: New Insights into Plant Defense

Soil and water pollution by metals and other toxic chemicals is difficult to measure and control, and, as such, presents an ongoing global threat to sustainable agriculture and human health. Efforts to remove contaminants by plant-mediated pathways, or “phytoremediation”, though widely studied, have failed to yield consistent, predictable removal of biological and chemical contaminants. Emerging research has revealed that one major limitation to using plants to clean up the environment is that plants are programmed to protect themselves: Like white blood cells in animals, border cells released from plant root tips carry out an extracellular trapping process to neutralize threats and prevent injury to the host. Variability in border cell trapping has been found to be correlated with variation in sensitivity of roots to aluminum, and removal of border cell results in increased Al uptake into the root tip. Studies now have implicated border cells in responses of diverse plant roots to a range of heavy metals, including arsenic, copper, cadmium, lead, mercury, iron, and zinc. A better understanding of border cell extracellular traps and their role in preventing toxin uptake may facilitate efforts to use plants as a nondestructive approach to neutralize environmental threats

Border cell counts of Bollgard3 cotton and extracellular DNA expression levels

In a world where there is growing pressure to grow more with a smaller environmental footprint, alternative forms of plant protection are needed. The root tips of most plants produce border cells in a mucilage that also contains extracellular DNA (exDNA), which is known to be involved in plant defence. A decade after we first demonstrated that there was varietal difference in the number of border cells in Australian cotton cultivars, we enumerated current commercial cultivars and assessed the level of exDNA produced by individual root tips. The results exposed that there has been a change in the number of border cells per root tip, that cultivar variation still exists and that the recovered levels of exDNA also differs. However, there was no correlation between border cell number and disease resistance to two of the major wilt pathogens and the levels of exDNA did not change when a root tip suspension was incubated with spores of fungal pathogens. The results imply that, while there is potential for border cells and root tip properties to be incorporated into breeding programmes, we still need to develop a better understanding of how root tips are able to influence disease epidemiology if we are to capitalise on this phenotypic property

Border cell counts of Bollgard3 cotton and extracellular DNA expression levels

In a world where there is growing pressure to grow more with a smaller environmental footprint, alternative forms of plant protection are needed. The root tips of most plants produce border cells in a mucilage that also contains extracellular DNA (exDNA), which is known to be involved in plant defence. A decade after we first demonstrated that there was varietal difference in the number of border cells in Australian cotton cultivars, we enumerated current commercial cultivars and assessed the level of exDNA produced by individual root tips. The results exposed that there has been a change in the number of border cells per root tip, that cultivar variation still exists and that the recovered levels of exDNA also differs. However, there was no correlation between border cell number and disease resistance to two of the major wilt pathogens and the levels of exDNA did not change when a root tip suspension was incubated with spores of fungal pathogens. The results imply that, while there is potential for border cells and root tip properties to be incorporated into breeding programmes, we still need to develop a better understanding of how root tips are able to influence disease epidemiology if we are to capitalise on this phenotypic property.Cotton Research and Development Corporation12 month embargo; published: 19 August 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Drought-Tolerant Barley: II. Root Tip characteristics in Emerging Roots

Reduced water resources are of increasingly urgent global concern. One potential strategy to address the crisis is the use of drought tolerant crops in agriculture. Barley varieties developed for reduced irrigation (“Solum” and “Solar”) use significantly less water than conventional varieties (“Cochise” and “Kopious”). The underlying mechanism of this drought tolerance is unknown but root structure and function play a key role in plant water uptake. In this study, an empirical survey compared early root development between drought tolerant and conventional varieties. Traits associated with root meristem-regulated cell division including rate of seed germination, border cell number and root cap mucilage production, and root hair emergence were quantified during root emergence. Preliminary results revealed that drought tolerant varieties exhibited faster seed germination and root hair production than conventional varieties. Border cell number and mucilage production in the drought tolerant varieties also were higher than in the conventional variety “Kopious,” but lower than in “Cochise”. Each trait, if found to be linked to the observed drought tolerance, could yield a simple, rapid, and inexpensive tool to screen for new crop varieties. Further detailed studies are needed