The Effects of High CO2 and Strigolactones on Shoot Branching and Aphid-Plant Compatibility Control in Pea

Abstract

Elevated atmospheric CO₂ concentrations (eCO₂) regulate plant architecture and susceptibility to insects. We explored the mechanisms underpinning these responses in wild type (WT) peas and mutants defective in either strigolactone (SL) synthesis or signaling. All genotypes had increased shoot height and branching, dry weights and carbohydrate levels under eCO₂, demonstrating that SLs are not required for shoot acclimation to eCO₂. Since shoot levels of jasmonic acid (JA) and salicylic acid (SA) tended to be lower in SL signaling mutants than the WT under ambient conditions, we compared pea aphid performance on these lines under both CO₂ conditions. Aphid fecundity was increased in the SL mutants compared to the WT under both ambient and eCO₂ conditions. Aphid infestation significantly decreased levels of JA, isopentenyladenine, trans-zeatin and gibberellin A4 and increased ethylene precursor ACC, gibberellin A1, gibberellic acid (GA₃) and SA accumulation in all lines. However, GA₃ levels were increased less in the SL signaling mutants than the WT. These studies provide new insights into phytohormone responses in this specific aphid/host interaction and suggest that SLs and gibberellins are part of the network of phytohormones that participate in host susceptibility.

Keywords: aphids; climate change; high atmospheric carbon dioxide; phytohormones; plant architecture; strigolactones.

Figure 1

The effect of high CO₂ on plant height in wild type peas and mutants defective either in strigolactone (SL) synthesis (rms1-2 and rms5-3) or signaling (rms3-1 and rms4-1). Plants were grown for up to 32 days either under ambient (420 ppm) or high CO₂ (750 ppm). Different letters indicate a statistically significant difference (p < 0.05) between genotype at each time point, while a significant difference between [CO₂] within genotype is indicated by an asterisk (p < 0.05). Day 7 and day 14, n = 6; day 32, n = 7. Mean indicated as +.

Figure 2

The effect of high CO₂ on the branching of the shoots of wild type peas and mutants defective either in strigolactone (SL) synthesis (rms1-2 and rms5-3) or signaling (rms3-1 and rms4-1). Plants were grown for up to 32 days either under ambient (420 ppm) or high CO₂ (750 ppm). Different letters indicate a statistically significant difference (p ≤ 0.05) between genotype at each time point, while a significant difference between [CO₂] within genotype is indicated as: * p < 0.05; *** p < 0.001. Day 7 and day 14, n = 6; day 32, n = 7. Mean indicated as +.

Figure 3

The effect of high CO₂ on the fresh weight/dry weight ratios (A) and dry weights (B) of wild type peas and mutants defective either in strigolactone (SL) synthesis (rms1-2 and rms5-3) or signaling (rms3-1 and rms4-1). Plants were grown for 28 days (fresh weight/dry weight) or 32 days (dry weights) either under ambient (420 ppm CO₂) or high CO₂ (750 ppm). Different letters indicate a statistically significant difference (p < 0.05) between genotype at each time point, while a significant difference between [CO₂] within genotype is indicated as: * p < 0.05; ** p < 0.01 (n = 5). Mean indicated as +.

Figure 4

The effect of high CO₂ on the levels of glucose, fructose, sucrose and starch in wild type peas and mutants defective either in strigolactone (SL) synthesis (rms1-2 and rms5-3) or signaling (rms3-1 and rms4-1). Plants were grown for 28 days either under ambient (420 ppm CO₂; bottom row) or high (750 ppm CO₂; top row). Data shown as mean ± SD of three replicates. Significant differences between [CO₂] within genotype are indicated as: * p <0.05; ** p < 0.01; *** p < 0.001; **** p < 0.0001.

Figure 5

Aphid fecundity on wild type peas and mutants defective either in strigolactone (SL) synthesis (rms1-2 and rms5-3) or signaling (rms3-1 and rms4-1) grown under ambient CO₂ conditions. A single pea aphid nymph was placed on each 5-day-old plant. Plants were then grown in air for 15 days before aphid numbers were counted. Significant differences between genotypes are indicated as: * p < 0.05. Strigolactone mutants as a group were also significantly different from the wildtype. Wildtype (WT) and rms3-1, n = 19; rms4-1, n = 18; rms2-1 and rms5-3, n = 9.

Figure 6

The effect aphid infestation on the levels of different phytohormones in wild type peas and mutants defective in strigolactone signaling (rms3-1 and rms4-1) under high CO₂ growth conditions. Data shown as mean ± SD (n = 3). Different letters indicate a statistically significant difference (p < 0.05) between genotype means (aphid-exposed and non-exposed grouped), while a significant difference between aphid exposure groups within genotype is indicated as: * p < 0.05; ** p < 0.01; *** p < 0.001; **** p < 0.0001.

Figure 7

The number of aphids on each plant was significantly correlated (p = 0.018) with shoot gibberellic acid concentration (ng/g DW) with a single relationship explaining variation across all genotypes (n = 30, n = 10 per genotype).