Transcriptomic, morphological, and metabolomic differences in fir trees
from a peri-urban forest under chronic ozone exposure
Urbanization modifies ecosystem conditions and evolutionary processes.
This includes air pollution, mostly tropospheric ozone (O3), which leads
to urban and peri-urban forest decline. Such is the case of fir (Abies
religiosa) forests in the peripheral mountains west of Mexico City,
which have been severely affected by O3 pollution since the 1970s.
Interestingly, some young individuals with low O3—related damage have
been observed within a zone of great O3 incidence, which might suggest
rapid tolerance/adaptation to this pollutant. We compared asymptomatic
and damaged individuals of the same age (≤15 years old; n = 10) using
histological, metabolomic and transcriptomic methods. Plants were
sampled during days of high (170 ppb) and moderate (87 ppb) O3
concentration periods. After verifying that all individuals clustered
within the same local genetic group when compared to a species-wide
panel (Admixture analysis with ~1.5K SNPs), we observed
thicker epidermis and more collapsed cells in the palisade parenchyma in
needles from damaged than from asymptomatic individuals; such
differences increased with needle age. Furthermore, damaged individuals
had lower concentrations of various terpenoids (ß-Pinene, ß-Caryophylene
Oxide, α-Caryophylene, ß-α-Cubebene and α-Muurolene) than asymptomatic
trees, as evidenced through GC-MS. Finally, transcriptomic analyses
revealed differential expression for thirteen genes related to
carbohydrate metabolism, plant defense, and gene regulation. Our results
indicate a rapid and contrasting phenotypic response between plants,
likely modeled by standing genetic variation and/or plastic mechanisms.
They open the door to future evolutionary studies for understanding how
O3 tolerance develops in urban environments, and for identifying
tolerant germplasm for forest restoration.