Forest trees face threats from many insect pest species, underscoring the importance of understanding their defense mechanisms for survival. In a North American conifer species Picea glauca, white spruce, a defense-related gene, βglu1, is responsible for releasing phenolic compounds (acetophenones) to defend against its insect defoliator, Choristoneura fumiferana, the eastern spruce budworm. βglu1 is also expressed in a Eurasian conifer species Picea abies, Norway spruce, although no major insect defoliator is present within the species’ natural range. We compared range-wide variation of βglu1 transcript levels from foliage samples of P. glauca in North America and P. abies in Europe using RT-qPCR and targeted transcriptome sequencing. βglu1 transcript levels were highly correlated between the two methods, with wide ranges of variation being detected within and between populations in both species. We found a significant longitudinal gradient in βglu1 transcript levels in P. glauca, with one βglu1 gene form being differentially expressed across populations, but not in P. abies. The expression level differences in P. glauca are consistent with the historically higher C. fumiferana outbreak frequency and severity in eastern compared to western populations, with C. fumiferana defoliation severity being a significant explanatory variable for βglu1 transcript levels. Climate per se was not a significant explanatory factor in either species. Overall, these results enhance our understanding of potential adaptive variation in acetophenone defenses in P. glauca, while the factors influencing βglu1 transcript variation in P. abies require further investigation.

Climate change poses one of the greatest threats to forest ecosystem integrity. An improved understanding of how trees respond to extreme climatic events is crucial to find new ways of managing forests in the face of global warming. In this work we look at the genetic mechanisms governing the production of the plant hormone abscisic acid (ABA), which safeguards plant’s water status by the means of two divergent modes across different conifer species. We find that conifers from evolutionary ancient families adopt a conservative water strategy during drought by accumulating high levels of ABA in their leaves, which we describe as Rising types, while more derived species accumulate the hormone in a transient manner and allow for greater water loss, accordingly to a Peaking type. Moreover, we provide evidence that these contrasting strategies may be controlled by divergent gene expression, including sequences involved in the biosynthetic and catabolic pathways of ABA, and especially nine- cis-epoxycarotenoid dioxygenases ( NCEDs). Our results help to clarify the genetic and physiological bases of iso/anisohydric responses. We believe that studying these and other related genes that regulate plant water status, such as those involved in ABA storage and mobilisation, may help foresters develop and grow more resistant trees.