Detritivorous crustaceans become herbivores on jasmonate-deficient plants

Abstract

The jasmonate signal pathway is known to control defenses against herbivores, such as leaf eaters (folivores). Does the reach of the pathway extend to defense against other types of animal? Among the arthropods attracted to seed baits placed below flowering Arabidopsis thaliana plants are 2 largely nocturnal isopod crustaceans generally considered as detritivores: Porcellio scaber and Armadillidium vulgare. Parallel laboratory experiments identified the isopods as being capable of predation on intact plants. Isopod feeding was strongly facilitated in jasmonate-deficient Arabidopsis and rice plants. The feeding activity of isopods revealed potentially detritivore-sensitive, jasmonate-protected Achilles' heels in these architecturally different plants (petioles and inflorescence stems in Arabidopsis, and lower stem and mesocotyl in rice). The work addresses the question of what stops 2 detritivores from attacking living plants and provides evidence that it is, in part, the jasmonate signal pathway. Furthermore, senescent leaves from an Arabidopsis jasmonate mutant were consumed more rapidly than senescent wild-type leaves, suggesting that past activity of the jasmonate signal pathway in leaves may slow carbon recycling through detritivory.

Fig. 1.

Field observation of organisms drawn to baits of 50–100 seeds deposited in a natural population of A. thaliana. (A) Juvenile earwigs (Forficula sp.). (B) The beetle Amara aenea. (C) Ants (T. caespitum) removing seed. (D) Two isopod crustacea. The upper individual was tentatively identified as A. vulgare. (E) the isopod P. scaber and an earthworm (yellow arrowhead; not observed to remove seeds). (F) P. scaber visiting seed bait near a compost heap.

Fig. 2.

Laboratory observation of seed predation by P. scaber. (A) P. scaber juvenile feeding on WT seed. (B–E) Stages in seed consumation. (F and G) Effect of jasmonates on seed predation by small isopods in the laboratory. In both cases, feeding on WT and homozygous aos seeds (produced by hand-pollinating aos plants with pollen from other aos plants sprayed with MeJA to rescue male fertility) was compared to an experimental design shown in the Inset for F. The circle diameter was 2.3 cm, and 50 seeds were scattered evenly in each sector (2 sectors for each genotype, total of 100 seeds per genotype). (F) Feeding of small (4–6.5 mm) P. scaber on WT and aos seeds (results from 8 filters; paired t test: t = 3.87, df = 7, P = 0.006). (G) Similar experiments with small (3.5–5 mm) A. vulgare; results were from 3 filters (paired t test: t = 4.6, df = 2, P = 0.044). The results in F and G show that aos seeds were damaged or consumed more readily than WT seeds by the small isopods.

Fig. 3.

Damage patterns inflicted by isopods on Arabidopsis plants. (A) P. scaber climbing on the underside of a cauline leaf from a WT plant. Note a severed inflorescence stem (yellow arrowhead). (B) Petiole-severing activity observed on a WT Arabidopsis plant (yellow arrowhead). (C and D) Comparison of feeding on WT Arabidopsis plants and aos plants compromised in jasmonate production. Arabidopsis was grown for 5 weeks and then transplanted into fresh soil. Plants were photographed before the addition of P. scaber that had been starved for 2 days. Seven days later the plants were again photographed. P. scaber consumed all aboveground tissues of aos, even destroying the leaf midribs. Note that plants were in pairs. (E) Arabidopsis plants (WT and aos) were grown for 6 weeks in the same container before introducing P. scaber. Leaf damage was monitored.

Fig. 4.

Damage patterns inflicted by isopods on rice plants. (A and B) Seven-day-old rice plants were transferred for 24 h into soil containing A. vulgare. Arrowhead indicates damage to stem of the homozygous hebiba (heb) mutant but not to plants that did not segregate for the homozygous mutation (WT). The center of the elongated mesocotyl (m) in hebiba is indicated with an arrow. (C) Pairs of 5-leaf-stage rice plants that did not segregate for the hebiba phenotype (WT) and the hebiba phenotype plants (heb) plants after culture with P. scaber. P. scaber felled the hebiba plants.

Fig. 5.

Jasmonate synthesis restrains feeding by an isopod crustacean on dying leaves. (A) Chlorophyll measurements on representative batches of detached and senescing leaves (9 to 10) used in feeding experiments (B). The photo shows representative leaves of both genotypes (WT and aos), with WT on the left of each pair. Chlorophyll values are in micrograms per milligram dry weight (±SD) and are shown for freshly detached leaves (day 0) and for leaves 3 and 9 days after incubation on humid soil in the dark at 23–24°C. (B) Leaves (–8) from 4.5- to 5-week-old WT and aos plants were detached and either offered immediately to P. scaber (fresh) or allowed to senesce for 3 or 9 days before feeding. Experiments (fresh, 3 days, and 9 days) were performed separately. Standard deviations were calculated from n = 3 (day 0), n = 4 (day 3 and day 9). In all cases, the leaves of aos were more susceptible to P. scaber than were WT leaves. Approximately 8 woodlice per liter of soil were used for the assays.