Bacterial Growth in Milpa Polyculture and Monoculture Soils [Emory University]

Polycultures, or multicrops, are groupings of plants that grow more prolifically when planted together as compared to when planted alone as monocultures. One of the best known and widely utilized polycultures is the milpa cropping system - the cultivation of maize, beans, and squash together as “the three sisters.” Milpa has been utilized by the indigenous population of Central America for millennia due to its consistent abundant harvests; today it remains a cornerstone of the region’s tradition, diet, and economic growth. Likely contributing to this legacy is the known association of polycultures and heightened resistance to disease, yet the mechanism underlying this relationship in milpa has largely been unexplored. To assess the health of farm soil exposed to milpa monocrops, bicrops, and multicrops, we measured the growth of two bacterial strains: a Burkholderia strain symbiotic of Anasa tristis (the squash bug, an agricultural pest) and a plant pathogenic Serratia strain that is the primary causal agent of cucurbit yellow vine disease (CYVD). We found that after one week in both the polyculture (corn, bean, and squash) soil and the corn monoculture soil, the growth of Burkholderia was significantly inhibited. However, in both corn & bean and bean & squash biculture soils, the growth of the strain was significantly enhanced. The growth of the Serratia strain did not yield any significant increase or decrease after one week in any milpa soil. We conclude that the cultivation of milpa in its polyculture configuration demonstrates antibiotic activity towards the Burkholderia strain SQ4A. Our investigation supports findings that certain multicrop systems are less susceptible to disease than monocultures possibly due to their greater microbial biomass; thus we can infer a higher amount of root exudates present in the soil, of which a substantial amount may be anti-microbials.

Involvement of Quinolinate Phosphoribosyl Transferase in Promotion of Potato Growth by a Burkholderia Strain

ABSTRACT Burkholderia sp. strain PsJN stimulates root growth of potato explants compared to uninoculated controls under gnotobiotic conditions. In order to determine the mechanism by which this growth stimulation occurs, we used Tn5 mutagenesis to produce a mutant, H41, which exhibited no growth-promoting activity but was able to colonize potato plants as well as the wild-type strain. The gene associated with the loss of growth promotion in H41 was shown to exhibit 65% identity at the amino acid level to the nadC gene encoding quinolinate phosphoribosyltransferase (QAPRTase) in Ralstonia solanacearum. Complementation of H41 with QAPRTase restored growth promotion of potato explants by this mutant. Expression of the gene identified in Escherichia coli yielded a protein with QAPRTase activities that catalyzed the de novo formation of nicotinic acid mononucleotide (NaMN). Two other genes involved in the same enzymatic pathway, nadA and nadB, were physically linked to nadC. The nadA gene was cotranscribed with nadC as an operon in wild-type strain PsJN, while the nadB gene was located downstream of the nadA-nadC operon. Growth promotion by H41 was fully restored by addition of NaMN to the tissue culture medium. These data suggested that QAPRTase may play a role in the signal pathway for promotion of plant growth by PsJN.

Identification of a Putative P-Transporter Operon in the Genome of a Burkholderia Strain Living inside the Arbuscular Mycorrhizal Fungus Gigaspora margarita

ABSTRACT This article reports the identification of a putative P-transporter operon in the genome of a Burkholderia sp. living in the cytoplasm of the arbuscular mycorrhizal fungus Gigaspora margarita. Its presence suggests that Burkholderiasp. has the potential for P uptake from this environment. This finding raises new questions concerning the importance of intracellular bacteria for mycorrhizal symbiosis.