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.

Diversity of Serratia marcescens Strains Associated with Cucurbit Yellow Vine Disease in Georgia

Cucurbit yellow vine disease (CYVD), caused by the squash bug (Anasa tristis)-transmitted bacterium Serratia marcescens, was first identified in Oklahoma and Texas in 1988 and in Georgia in 2012. S. marcescens is a highly diverse species found in many ecological niches. In previous studies, CYVD strains of S. marcescens formed a closely related group separate from non-CYVD strains based on biological and molecular characterization techniques. Multilocus sequence analysis (MLSA) of six housekeeping genes and repetitive elements-based polymerase chain reaction (rep-PCR) using the BOX and ERIC primers were used to assess the genetic diversity of CYVD strains of S. marcescens collected in Georgia together with a strain from Texas and seven non-CYVD strains of S. marcescens. rep-PCR results revealed genetic diversity among CYVD strains while MLSA results showed a 100% similarity across the six loci for all but one of the CYVD strains, which differed at the icd locus by five polymorphisms. For both methods, CYVD strains clustered separately from nonplant-pathogenic S. marcescens strains and were most similar to a rice endophyte strain. One CYVD strain isolated from a squash bug shared genetic similarities with non-CYVD strains, and may be the result of a recombination event between CYVD and non-CYVD strains.

First Report of the Cucurbit Yellow Vine Disease Caused by Serratia marcescens in Watermelon and Yellow Squash in Alabama

More than 3,000 acres of watermelon were planted in Alabama in 2010 with a production value more than $4 million (J. Kemble, personal communication). Symptoms typical of cucurbit yellow vine disease (CYVD) were observed in a 2-ha watermelon field in Crawford, AL on 8 June 2010. Watermelon, cv. Jubilee, exhibited a yellow appearance and some plants were completely wilted. Incidence of affected plants was estimated at 25%. On 24 June, plant samples were collected from a 1-ha watermelon (cv. Jubilee) field near Dadeville, AL. Approximately 30% of the plants exhibited yellowing and wilting, which is symptomatic of CYVD. Samples were also collected from a small planting of yellow crooked-neck squash at the same location. Approximately 20% of the squash plants had symptoms typical of CYVD. Cross-sections of belowground stem and primary root revealed a honey-brown phloem discoloration and a healthy appearing xylem, symptoms consistent with CYVD caused by the phloem-colonizing bacterium, Serratia marcescens Bizio (1). Isolations were made from the crown on four symptomatic watermelon and two squash plants. Approximately 2.5-mm3 tissue pieces from the phloem were excised, surface sterilized in 10% sodium hypochlorite, and ground in 1-ml PBS (phosphate buffer with saline). A 10-μl aliquot of slurry was plated onto nutrient agar (NA) (Difco, Detroit, MI) and the plates were stored at room temperature for 4 days. Individual colonies were selected and purified by serial dilution plating. Isolates from watermelon and squash were consistent with S. marcescens in colony morphology, color, and texture. Three isolates obtained from watermelon were grown on NA and suspended in sterile water at 108 cells per ml for mechanical transmission experiments on ‘Lemondrop’ squash. Sterile water served as a negative control. After 28 days, plants were cross-sectioned at the juncture of the root and stem and observed for phloem discoloration. Of the 56, 58, and 62 plants inoculated in three replicate studies, 78.6, 56.9, and 62.9% developed CYVD symptoms, respectively, while none of the controls were positive. Cultured bacteria from six of the symptomatic, greenhouse-inoculated plants representing the three watermelon isolates were subjected to multiplex end-point PCR using primer sets YV1/YV4, specific for the species S. marcescens, and a79F/R, which amplifies only the CYVD strains of S. marcescens (3). All six bacteria cultures along with the positive control (reference isolate W01 obtained from watermelon in Texas) were positive, while the negative PBS control was negative. Although rhizosphere-inhabiting and plant growth promoting endophytic strains of S. marcescens have been reported from Alabama (2), to our knowledge, this is the first known report of CYVD and phytopathogenic S. marcescens in Alabama cucurbits. References: (1) B. D. Bruton et al. Plant Dis. 87:937, 2004. (2) J. A. McInroy and J. W. Kloepper. Plant Soil 173:333, 1995. (3) Q. Zhang et al. Appl. Environ. Microbiol. 71:7716, 2005.

Genomic Comparison of Plant Pathogenic and Nonpathogenic Serratia marcescens Strains by Suppressive Subtractive Hybridization

ABSTRACT Cucurbit yellow vine disease (CYVD) is caused by disease-associated Serratia marcescens strains that have phenotypes significantly different from those of nonphytopathogenic strains. To identify the genetic differences responsible for pathogenicity-related phenotypes, we used a suppressive subtractive hybridization (SSH) strategy. S. marcescens strain Z01-A, isolated from CYVD-affected zucchini, was used as the tester, whereas rice endophytic S. marcescens strain R02-A (IRBG 502) was used as the driver. SSH revealed 48 sequences, ranging from 200 to 700 bp, that were present in Z01-A but absent in R02-A. Sequence analysis showed that a large proportion of these sequences resembled genes involved in synthesis of surface structures. By construction of a fosmid library, followed by colony hybridization, selection, and DNA sequencing, a phage gene cluster and a genome island containing a fimbrial-gene cluster were identified. Arrayed dot hybridization showed that the conservation of subtracted sequences among CYVD pathogenic and nonpathogenic S. marcescens strains varied. Thirty-four sequences were present only in pathogenic strains. Primers were designed based on one Z01-A-specific sequence, A79, and used in a multiplex PCR to discriminate between S. marcescens strains causing CYVD and those from other ecological niches.