Transgenerational impacts of herbivory and inbreeding on reproductive output in Solanum carolinense

PREMISE

Plant maternal effects on offspring phenotypes are well documented. However, little is known about how herbivory on maternal plants affects offspring fitness. Furthermore, while inbreeding is known to reduce plant reproductive output, previous studies have not explored whether and how such effects may extend across generations. Here, we addressed the transgenerational consequences of herbivory and maternal plant inbreeding on the reproduction of Solanum carolinense offspring.

METHODS

Manduca sexta caterpillars were used to inflict weekly damage on inbred and outbred S. carolinense maternal plants. Cross-pollinations were performed by hand to produce seed from herbivore-damaged outbred plants, herbivore-damaged inbred plants, undamaged outbred plants, and undamaged inbred plants. The resulting seeds were grown in the greenhouse to assess emergence rate and flower production in the absence of herbivores. We also grew offspring in the field to examine reproductive output under natural conditions.

RESULTS

We found transgenerational effects of herbivory and maternal plant inbreeding on seedling emergence and reproductive output. Offspring of herbivore-damaged plants had greater emergence, flowered earlier, and produced more flowers and seeds than offspring of undamaged plants. Offspring of outbred maternal plants also had greater seedling emergence and reproductive output than offspring of inbred maternal plants, even though all offspring were outbred. Moreover, the effects of maternal plant inbreeding were more severe when plant offspring were grown in field conditions.

CONCLUSIONS

This study demonstrates that both herbivory and inbreeding have fitness consequences that extend across generations even in outbred progeny.

Cry1Ac production is costly for native plants attacked by non-Cry1Ac-targeted herbivores in the field

Plants are the primary producers in most terrestrial ecosystems and have complex defense systems to protect their produce. Defense-deficient, high-yielding agricultural monocultures attract abundant nonhuman consumers, but are alternatively defended through pesticide application and genetic engineering to produce insecticidal proteins such as Cry1Ac (Bacillus thuringiensis). These approaches alter the balance between yield protection and maximization but have been poorly contextualized to known yield-defense trade-offs in wild plants. The native plant Nicotiana attenuata was used to compare yield benefits of plants transformed to be defenseless to those with a full suite of naturally evolved defenses, or additionally transformed to ectopically produce Cry1Ac. An insecticide treatment allowed us to examine yield under different herbivore loads in N. attenuata's native habitat. Cry1Ac, herbivore damage, and growth parameters were monitored throughout the season. Biomass and reproductive correlates were measured at season end. Non-Cry1Ac-targeted herbivores dominated on noninsecticide-treated plants, and increased the yield drag of Cry1Ac-producing plants in comparison with endogenously defended or undefended plants. Insecticide-sprayed Cry1Ac-producing plants lagged less in stalk height, shoot biomass, and flower production. In direct comparison with the endogenous defenses of a native plant, Cry1Ac production did not provide yield benefits for plants under observed herbivore loads in a field study.

What happens in the pith stays in the pith: tissue-localized defense responses facilitate chemical niche differentiation between two spatially separated herbivores

Herbivore attack is known to elicit systemic defense responses that spread throughout the host plant and influence the performance of other herbivores. While these plant-mediated indirect competitive interactions are well described, and the co-existence of herbivores from different feeding guilds is common, the mechanisms of co-existence are poorly understood. In both field and glasshouse experiments with a native tobacco, Nicotiana attenuata, we found no evidence of negative interactions when plants were simultaneously attacked by two spatially separated herbivores: a leaf chewer Manduca sexta and a stem borer Trichobaris mucorea. T. mucorea attack elicited jasmonic acid (JA) and jasmonoyl-l-isoleucine bursts in the pith of attacked stems similar to those that occur in leaves when M. sexta attacks N. attenuata leaves. Pith chlorogenic acid (CGA) levels increased 1000-fold to levels 6-fold higher than leaf levels after T. mucorea attack; these increases in pith CGA levels, which did not occur in M. sexta-attacked leaves, required JA signaling. With plants silenced in CGA biosynthesis (irHQT plants), CGA, as well as other caffeic acid conjugates, was demonstrated in both glasshouse and field experiments to function as a direct defense protecting piths against T. mucorea attack, but not against leaf chewers or sucking insects. T. mucorea attack does not systemically activate JA signaling in leaves, while M. sexta leaf-attack transiently induces detectable but minor pith JA levels that are dwarfed by local responses. We conclude that tissue-localized defense responses allow tissue-specialized herbivores to share the same host and occupy different chemical defense niches in the same hostplant.

Herbivory-induced jasmonates constrain plant sugar accumulation and growth by antagonizing gibberellin signaling and not by promoting secondary metabolite production

Plants respond to herbivory by reconfiguring hormonal networks, increasing secondary metabolite production and decreasing growth. Furthermore, some plants display a decrease in leaf energy reserves in the form of soluble sugars and starch, leading to the hypothesis that herbivory-induced secondary metabolite production and growth reduction may be linked through a carbohydrate-based resource trade-off. In order to test the above hypothesis, we measured leaf carbohydrates and plant growth in seven genetically engineered Nicotiana attenuata genotypes that are deficient in one or several major herbivore-induced, jasmonate-dependent defensive secondary metabolites and proteins. Furthermore, we manipulated gibberellin and jasmonate signaling, and quantified the impact of these phytohormones on secondary metabolite production, sugar accumulation and growth. Simulated herbivore attack by Manduca sexta specifically reduced leaf sugar concentrations and growth in a jasmonate-dependent manner. These effects were similar or even stronger in defenseless genotypes with intact jasmonate signaling. Gibberellin complementation rescued carbohydrate accumulation and growth in induced plants without impairing the induction of defensive secondary metabolites. These results are consistent with a hormonal antagonism model rather than a resource-cost model to explain the negative relationship between herbivory-induced defenses, leaf energy reserves and growth.

Changes in cytokinins are sufficient to alter developmental patterns of defense metabolites in Nicotiana attenuata

Plant defense metabolites are well known to be regulated developmentally. The optimal defense (OD) theory posits that a tssue's fitness values and probability of attack should determine defense metabolite allocations. Young leaves are expected to provide a larger fitness value to the plant, and therefore their defense allocations should be higher when compared with older leaves. The mechanisms that coordinate development with defense remain unknown and frequently confound tests of the OD theory predictions. Here we demonstrate that cytokinins (CKs) modulate ontogeny-dependent defenses in Nicotiana attenuata. We found that leaf CK levels highly correlate with inducible defense expressions with high levels in young and low levels in older leaves. We genetically manipulated the developmental patterns of two different CK classes by using senescence- and chemically inducible expression of CK biosynthesis genes. Genetically modifying the levels of different CKs in leaves was sufficient to alter ontogenic patterns of defense metabolites. We conclude that the developmental regulation of growth hormones that include CKs plays central roles in connecting development with defense and therefore in establishing optimal patterns of defense allocation in plants.