Optimal defense in plants: assessment of resource allocation costs

Abstract

Genetic and environmental variation of functional traits within populations might be maintained by natural selection when resource allocation costs (RACs) balance trait benefits. Despite the intuitive appeal of optimization models, empirical tests have failed to support the importance of RACs for plant traits that confer resistance against pests. To address this discrepancy, we modified an early model by allowing the cost function to vary across a resource gradient as predicted for RACs and by assuming that the benefits depend on variation in the pest population for susceptibility. Instead of the intermediate defense optimum of the original model, defenses were predicted to be either high or absent, depending on resource availability and history. This result is not supported by empirical tests for ecological or evolutionary outcomes, including our own examination of glucosinolate toxins from sib-families of Boechera stricta (Brassicaceae) grown across an NPK fertilizer gradient. Although we detected an apparent cost of defense in the absence of herbivores, the cost did not increase as resources became more limiting. Also defense production did not vary across the resource gradient as predicted by the modified model. Thus, a model based on explicit expectations of RACs produced predictions that are not supported. Instead, other kinds of costs, such as ecological (indirect) costs may be more important. Alternatively, general conflicts in gene expression and antagonistic crosstalk among signaling pathways may underlie apparent costs.

Resource availability alters fitness trade‐offs: implications for evolution in stressful environments

“…We applied two nutrient treatments once weekly as a mixture of water-soluble Peters Professional General Purpose Fertilizer 20N-10P-20K (Scott's Co., Marysville, OH, USA) at 110 ppm N (1×; 1 ppm = 1 mg/L), and 1100 ppm (10×). We selected fertilizer concentrations to be stressful vs. moderate (low vs. medium) based on previous studies of nutrient stress in B. stricta (Siemens et al, 2010(Siemens et al, , 2012. We applied high nutrient levels early in life history to simulate nutrient pulses that occur at snowmelt when seedlings first emerge (Groffman et al, 2001(Groffman et al, , 2011Gavazov et al, 2017) and to enhance seedling establishment (Villar-Salvador et al, 2012;Gan et al, 2015).…”

“…Alternatively, individuals can vary in their capacity to acquire resources, such that some individuals have more resources to allocate to multiple functions, which would generate positive relationships between fitness components like growth and survival (Van Noordwijk and de Jong, ; Zera and Harshman, ). Trade‐offs have emerged in response to altered conditions associated with climate change, such as new soil moisture levels (Rosenzweig and Hillel, ; Sheffield and Wood, ) and nutrient availability (Kuzyakov, ; Siemens et al., ). Drought stress has altered both the sign and magnitude of phenotypic and genetic correlations associated with fitness in barley ( Hordeum vulgare ; Shakhatreh et al., ).…”

Boechera, a model system for ecological genomics

“…Several studies have examined these questions in Boechera . Siemens et al. (2010) identified a significant cost of defence when resistant plants were grown in the absence of herbivores.…”

“…Several studies have examined these questions in Boechera. Siemens et al (2010) identified a significant cost of defence when resistant plants were grown in the absence of herbivores. This cost of defence was independent of resource availability, and glucosinolate chemical defences did not vary across a resource gradient.…”

Evolutionary constraint on low elevation range expansion: Defense‐abiotic stress‐tolerance trade‐off in crosses of the ecological model Boechera stricta

“…This population genetic differentiation and variation also include the high and low elevation range margin populations studied here (Siemens & Haugen, ). There is also significant quantitative genetic variation for the relevant traits (resistance to herbivores, GS content, abiotic stress‐tolerance) in low elevation range margin populations of B. stricta (Siemens et al, ; Siemens, Keck, & Ziegenbein, ).…”