Temporal release of fatty acids and sugars in the spermosphere: impacts on Enterobacter cloacae-induced biological control

Abstract

The aim of this study was to determine the temporal release of fatty acids and sugars from corn and cucumber seeds during the early stages of seed germination in order to establish whether sugars found in exudate can prevent exudate fatty acid degradation by Enterobacter cloacae. Both saturated (long-chain saturated fatty acids [LCSFA]) and unsaturated (long-chain unsaturated fatty acids [LCUFA]) fatty acids were detected in corn and cucumber seed exudates within 15 min after seed sowing. LCSFA and LCUFA were released at a rate of 26.1 and 6.44 ng/min/seed by corn and cucumber seeds, respectively. The unsaturated portion of the total fatty acid pool from both plant species contained primarily oleic and linoleic acids, and these fatty acids were released at a combined rate of 6.6 and 0.67 ng/min/seed from corn and cucumber, respectively. In the absence of seed exudate sugars, E. cloacae degraded linoleic acid at rates of 29 to 39 ng/min, exceeding the rate of total fatty acid release from seeds. Sugars constituted a significant percentage of corn seed exudate, accounting for 41% of the total dry seed weight. Only 5% of cucumber seed exudate was comprised of sugars. Glucose, fructose, and sucrose were the most abundant sugars present in seed exudate from both plant species. Corn seeds released a total of 137 microg/seed of these three sugars within 30 min of sowing, whereas cucumber seeds released 0.83 microg/seed within the same time frame. Levels of glucose, fructose, and sucrose found in corn seed exudate (90 to 342 microg) reduced the rate of linoleic acid degradation by E. cloacae to 7.5 to 8.8 ng/min in the presence of either sugar, leaving sufficient concentrations of linoleic acid to activate Pythium ultimum sporangia Our results demonstrate that elevated levels of sugars in the corn spermosphere can prevent the degradation of LCUFA by E. cloacae, leading to its failure to suppress P. ultimum sporangial activation, germination, and subsequent disease development.

FIG. 1.

Corn (A) and cucumber (B) water uptake in glass beads (• [A] and ⧫ [B]), sand (□), and water (▴). Each marker represents the mean of at least six observations from at least two replicate experiments, and the error bars represent the standard deviations. Note the different y axes.

FIG. 2.

Oleic acid (A) and LA (B) released from germinating corn (•) and cucumber (○) seeds during the first 6 h after sowing. Each marker represents the mean of six observations.

FIG. 3.

Saturated FA (palmitic acid [□], stearic acid [○], and others [•]) released from germinating corn (A) and cucumber (B) seeds during the first 6 h after sowing. Other saturated FA include caprylic (8:0), capric (10:0), lauric (12:0), myristic (14:0), and behenic (22:0) acids. Each marker represents the mean of six observations.

FIG. 4.

Glucose (○), fructose (□), sucrose (•), and other sugars (▪) released by germinating corn (A) and cucumber (B) seeds during the first 6 h after sowing. Other sugars include arabinose, cellobiose, galactose, lactose, maltose, mannitol, mannose, raffinose, rhamnose, ribose, trehalose, and xylose for corn. The same sugars except lactose and mannose are included in the same category for cucumber. Each marker represents the mean of six observations. Note the different y axes.

FIG. 5.

Germination of P. ultimum sporangia in response to cell-free LA solutions that have been treated with E. cloacae. E. cloacae cells were incubated in 200-μg/ml (•) or 350-μg/ml (○) solutions of LA for 24 h. Cells were then removed, and the supernatant was assayed. Each marker represents the mean of three observations ± standard deviation.

FIG. 6.

Germination of P. ultimum sporangia in response to cell-free LA solutions amended with different concentrations of glucose (A), fructose (B), or sucrose (C) that have been treated with E. cloacae for 0 (•), 4 (○), or 10 (▪) h. Each marker represents the mean of three observations ± standard deviation.

FIG. 7.

Levels of remaining LAs (as measured by GC with a FID) in solutions amended with 0 (•), 18 (○), 90 (▴), or 180 (▪) μg glucose/100 μl (A) or 0 (•), 36 (○), or 180 (▴) μg sucrose/100 μl (B) that have been incubated with E. cloacae for various periods of time. Each bar represents the mean of three observations ± standard deviation.