A Potential Biofertilizer—Siderophilic Bacteria Isolated From the Rhizosphere of Paris polyphylla var. yunnanensis

Abstract

The increasing demands for crop production have become a great challenge while people also realizing the significance of reductions in synthetic chemical fertilizer use. Plant growth-promoting rhizobacteria (PGPR) are proven biofertilizers for increasing crop yields by promoting plant growth <i>via</i> various direct or indirect mechanisms. Siderophilic bacteria, as an important type of PGPR, can secrete siderophores to chelate unusable Fe³⁺ in the soil for plant growth. Siderophilic bacteria have been shown to play vital roles in preventing diseases and enhancing the growth of plants. <i>Paris polyphylla</i> var. <i>yunnanensis</i> (<i>PPVY</i>) is an important traditional Chinese herb. However, reports about its siderophilic bacteria are still rare. This study firstly isolated siderophilic bacteria from the rhizosphere soil of <i>PPVY</i>, identified by morphological and physio-biochemical characteristics as well as 16S rRNA sequence analysis. The dominant genus in the rhizobacteria of <i>PPVY</i> was <i>Bacillus</i>. Among 22 isolates, 21 isolates produced siderophores. The relative amount of siderophores ranged from 4 to 41%. Most of the isolates produced hydroxamate siderophores and some produced catechol. Four isolates belonging to <i>Enterobacter</i> produced the catechol type, and none of them produced carboxylate siderophores. Intriguingly, 16 strains could produce substances that have inhibitory activity against <i>Candida albicans</i> only in an iron-limited medium (SA medium). The effects of different concentrations of Fe³⁺ and three types of synthetic chemical fertilizers on AS19 growth, siderophore production, and swimming motility were first evaluated from multiple aspects. The study also found that the cell-free supernatant (CFS) with high siderophore units (SUs) of AS19 strain could significantly promote the germination of pepper and maize seeds and the development of the shoots and leaves of <i>Gynura divaricata</i> (Linn.). The bacterial solution of AS19 strain could significantly promote the elongation of the roots of <i>G. divaricata</i> (Linn.). Due to its combined traits promoting plant growth and seed germination, the AS19 has the potential to become a bioinoculant. This study will broaden the application prospects of the siderophilic bacteria-AS19 as biofertilizers for future sustainable agriculture.