Endophytic Pseudomonas sp. from Agave palmeri Participate in the Rhizophagy Cycle and Act as Biostimulants in Crop Plants

Abstract

Plant growth-promoting bacteria are generating increasing interest in the agricultural industry as a promising alternative to traditional chemical fertilizers; however, much of the focus has been on rhizosphere bacteria. Bacterial endophytes are another promising source of plant growth-promoting bacteria, and though many plants have already been prospected for beneficial microbes, desert plants have been underrepresented in such studies. In this study, we show the growth-promoting potential of five strains of endophytic Pseudomonas sp. isolated from Agave palmeri, an agave from the Sonoran Desert. When inoculated onto Kentucky bluegrass, clover, carrot, coriander, and wheat, endophytic Pseudomonas sp. increased seedling root lengths in all hosts and seedling shoot lengths in Kentucky bluegrass, carrot, and wheat. Transformation of the Pseudomonas sp. strain P3AW to express the fluorescent protein mCherry revealed that Pseudomonas sp. becomes endophytic in non-native hosts and participates in parts of the rhizophagy cycle, a process by which endophytic bacteria cycle between the soil and roots, bringing in nutrients from the soil which are then extracted through reactive oxygen-mediated bacterial degradation in the roots. Tracking of the Pseudomonas sp. strain P3AW also provided evidence for a system of endophyte, or endophyte cell content, transport via the vascular bundle. These results provide further evidence of the rhizophagy cycle in plants and how it relates to growth promotion in plants by biostimulant bacteria.