An emerging alternative to synthetic fertilizers involves harnessing the growth-promotional ability of bacterial endophytes. Although microbial benefits to plant nutrient uptake have been commonly attributed to nutrient solubilization in soil, this explanation may not fully explain uptake patterns observed for several important nutrients, such as P and Zn. The rhizophagy cycle is a newer model to explain nutrient delivery from bacterial endophytes to their plant hosts. This model hypothesizes that bacteria scavenge for nutrients from the rhizosphere, move to carbon-rich plant root tips, and are oxidatively degraded in roots for their nutrients. Bacteria are expelled from root hairs back into the rhizosphere, where they reform their cell walls and resume nutrient scavenging. The particular nutrients involved in this model are unknown. In this study, we provide evidence to support the involvement of P and Zn in a rhizophagy cycle. In a series of magenta box experiments, we cultivated unsterilized rice seeds and surface-sterilized rice seeds in growth chambers with variable CO2 levels. High atmospheric CO2 levels have been predicted to inhibit the rhizophagy cycle. We monitored plant growth parameters and nutritional status (N, P, K, Ca, S, Zn, Fe) for 3 weeks.
Results/Conclusions
Our data shows that sterilized rice seeds grew much shorter roots, with fewer and shorter root hairs, indicative of an impaired endophytome. Elevated CO2 improved plant growth, but reduced uptake of several nutrients. P and Zn concentrations in unsterilized rice subjected to CO2 treatment decreased to levels comparable to sterilized rice in ambient air. This finding suggests that, in the context of P and Zn uptake, CO2-driven inhibition of the rhizophagy cycle negates nutritive promotion by endophytes. In the pursuit of more sustainable agricultural practices in the face of climate change, understanding the mechanisms of nutrient delivery will have great implications for designing effective crop biostimulants.