Characterization, costs, cues and future perspectives of phenotypic plasticity

Abstract

Background: Plastic responses of plants to the environment are ubiquitous. Phenotypic plasticity occurs in many forms and at many biological scales, and its adaptive value depends on the specific environment and interactions with other plant traits and organisms. Even though plasticity is the norm rather than the exception, its complex nature has been a challenge in characterizing the expression of plasticity, its adaptive value for fitness and the environmental cues that regulate its expression.

Scope: This review discusses the characterization and costs of plasticity and approaches, considerations, and promising research directions in studying plasticity. Phenotypic plasticity is genetically controlled and heritable; however, little is known about how organisms perceive, interpret and respond to environmental cues, and the genes and pathways associated with plasticity. Not every genotype is plastic for every trait, and plasticity is not infinite, suggesting trade-offs, costs and limits to expression of plasticity. The timing, specificity and duration of plasticity are critical to their adaptive value for plant fitness.

Conclusions: There are many research opportunities to advance our understanding of plant phenotypic plasticity. New methodology and technological breakthroughs enable the study of phenotypic responses across biological scales and in multiple environments. Understanding the mechanisms of plasticity and how the expression of specific phenotypes influences fitness in many environmental ranges would benefit many areas of plant science ranging from basic research to applied breeding for crop improvement.

Keywords: Phenotypic plasticity; environmental cues; genotype x environment interaction; phenotypic variation.

Fig. 1.

Signal versus noise detection. Genotypes must discriminate between relevant (cue) and irrelevant (noise) environmental information. In general, there is a threshold value of the strength of the cue (dashed lines) in which the organism will express plasticity (in this case, phenotype X or Y) depending on if it is perceived as a signal or noise. The optimal location of the threshold shifts depending on the costs and benefits of correctly responding to a signal (D), incorrectly responding to a signal (B), incorrectly responding to noise as a signal (C) or correctly not responding to noise (A). Modified from Pfennig (2021).

Fig. 2.

(A) Cryptic genetic variation. In ancestral and familiar environments, genotypes 1 and 2 do not display genotype by environment interactions as their reaction norms are similar and parallel. However, genotypes 1 and 2 diverge in evolutionarily novel environments, demonstrating hidden reaction norms and cryptic genetic variation. (B) The norms of reaction for three genotypes in three environments. For a specific trait, each genotype is phenotyped in each environment and these points are joined to form a reaction norm. (3) a genotype varies continuously and linearly, (4) a genotype response with a few discrete alternatives (i.e. polyphenism), (5) a genotype is fixed or relatively canalized across environments. Genotypes 3, 4 and 5 display genotype by environment interactions as their reaction norms are not parallel. Modified from Sultan (2015).

Fig. 3.

Multiple environments are needed to accurately characterize reaction norms. (A) Description of reaction norms (dashed lines) of two genotypes using only two environmental points (solid lines and points); (B) description of reaction norms using three environments captures much more of the underlying reaction norm shape. Fitting a linear line to two environments may severely over- or under-estimate the plastic response. Modified from Arnold et al. (, b).

Fig. 4.

Key considerations for the study of phenotypic plasticity. Multiple environmental factors and factor levels are needed to examine plastic responses in multiple traits. The quantification method of plastic responses must be carefully considered as many methods generate different outputs, which complicate the analysis and interpretation of plasticity, particularly in comparative studies and multiple environments.