Increasing atmospheric CO2 levels may reduce extraction of nutrients from soil microbes in plant roots
Creators: name: White, James
name: Verma, Rajan
name: Kingsley, Kathryn
name: Qiuwei Zhang
name: Nkolika Obi
name: Kowalski, Kurt P.
Publisher: name: F1000 Research Limited
Year: 2019

description: Summary.<br><br>This research was presented at the Ecological Society of America annual conference in Louisville, Kentucky on August 14, 2019. In this presentation we report the following: 1) Elevated CO2 inhibits superoxide formation in plant root cells; 2) Without superoxide, microbes do not convert to protoplasts within root cells; 3) Without protoplast phases in root cells, root hairs do not elongate; 4) In elevated CO2 , the rhizophagy cycle in roots is suppressed and nutrient absorption into roots is reduced. <br><br>Background/Question/Methods.<br> <br>The rhizophagy cycle is a nutritional process where nutrients are extracted from microbes oxidatively in roots. In the rhizophagy cycle, microbes alternate between a free-living phase in soil and a plant-dependent protoplast phase inside plant root cells. Microbes acquire nutrients (protein and minerals) in the soil phase, and nutrients are extracted from microbes in plant root cells through exposure to reactive oxygen (superoxide) produced on the root cell plasma membrane. In the endophytic protoplast phase microbes are also increased in numbers prior to expulsion into the soil from elongating root hairs. Carbon dioxide has been shown to be a universal inhibitor of reactive oxygen. We hypothesize that increasing levels of CO<sub>2 </sub>in the atmosphere will reduce the efficiency of nutrient extraction from microbes by inhibiting the production of reactive oxygen in roots. To test this hypothesis, seedlings of tall fescue grass (<i>Festuca arundinacea</i>), wheat (<i>Triticum aestivum</i>) and tomato (<i>Lycopersicum esculentum</i>) were germinated on agarose then placed in gas chambers that contained air with approximately 0.04% CO<sub>2 </sub>(current level in atmosphere) or air with significantly elevated levels of CO<sub>2 </sub>(0.06%). After several days, Petri dishes containing seedlings were flooded with stains nitro blue tetrazolium (to detect superoxide) or 3, 3-diaminobenzidine (to detect hydrogen peroxide) and then incubated for 15 hours at lab ambient temperature. Seedling roots were then examined microscopically to detect microbes in root cells and presence of reactive oxygen staining around microbes. To assess for inhibition of nutrients absorbed into plant tissues in elevated CO<sub>2</sub> atmospheres, tomato seedlings were germinated and grown for 3 weeks in chambers containing air with non-elevated or elevated CO<sub>2</sub>. Seedlings were harvested and assessed for their content in several key plant nutrients.<br> <br>Results/Conclusions.<br> <br>Seedlings of all species grown in chambers containing air with 0.06% CO<sub>2 </sub>showed reduced numbers of microbe protoplasts in roots and reduced staining for superoxide and hydrogen peroxide around microbes compared to seedlings grown in chambers containing air with 0.04% CO<sub>2</sub>. Due to the lack of microbe protoplasts within root cells, root hair elongation and root exudation was suppressed in roots grown in the elevated CO<sub>2</sub> atmospheres. Our experimental results point to CO<sub>2 </sub>inhibition of the rhizophagy cycle as a possible explanation of reduced nutrient absorption in crop plants subjected to increased levels of CO<sub>2</sub>. In experiments with tomato seedlings, we found that potassium was substantially reduced in seedlings grown in the elevated CO<sub>2</sub> atmosphere. Calcium and sulfur were also slightly reduced in seedlings grown in elevated CO<sub>2. </sub>Increasing CO<sub>2 </sub>levels in the air may reduce efficiency of nutrient acquisition in the rhizophagy cycle by suppressing reactive oxygen secretion in plant roots around microbes, resulting in failure to strip cell walls from microbes and replicate them prior to expulsion from root hairs.