Plant adaptation or acclimation to rising CO₂? Insight from first multigenerational RNA-Seq transcriptome

Atmospheric carbon dioxide (CO₂) directly determines the rate of plant photosynthesis and indirectly effects plant productivity and fitness and may therefore act as a selective pressure driving evolution, but evidence to support this contention is sparse. Using Plantago lanceolata L. seed collected from a naturally high CO₂ spring and adjacent ambient CO₂ control site, we investigated multigenerational response to future, elevated atmospheric CO₂. Plants were grown in either ambient or elevated CO₂ (700 μmol mol⁻¹), enabling for the first time, characterization of the functional and population genomics of plant acclimation and adaptation to elevated CO₂. This revealed that spring and control plants differed significantly in phenotypic plasticity for traits underpinning fitness including above-ground biomass, leaf size, epidermal cell size and number and stomatal density and index. Gene expression responses to elevated CO₂ (acclimation) were modest [33–131 genes differentially expressed (DE)], whilst those between control and spring plants (adaptation) were considerably larger (689–853 DE genes). In contrast, population genomic analysis showed that genetic differentiation between spring and control plants was close to zero, with no fixed differences, suggesting that plants are adapted to their native CO₂ environment at the level of gene expression. An unusual phenotype of increased stomatal index in spring but not control plants in elevated CO₂ correlated with altered expression of stomatal patterning genes between spring and control plants for three loci (YODA, CDKB1;1 and SCRM2) and between ambient and elevated CO₂ for four loci (ER, YODA, MYB88 and BCA1). We propose that the two positive regulators of stomatal number (SCRM2) and CDKB1;1 when upregulated act as key controllers of stomatal adaptation to elevated CO₂. Combined with significant transcriptome reprogramming of photosynthetic and dark respiration and enhanced growth in spring plants, we have identified the potential basis of plant adaptation to high CO₂ likely to occur over coming decades.