Epigenetic variation is likely to contribute to the phenotypic plasticity and adaptative capacity of plant species, and may be especially important for long‐lived organisms with complex life cycles, including forest trees. Diverse environmental stresses and hybridization/polyploidization events can create reversible heritable epigenetic marks that can be transmitted to subsequent generations as a form of molecular “memory”. Epigenetic changes might also contribute to the ability of plants to colonize or persist in variable environments. In this review, we provide an overview of recent data on epigenetic mechanisms involved in developmental processes and responses to environmental cues in plant, with a focus on forest tree species. We consider the possible role of forest tree epigenetics as a new source of adaptive traits in plant breeding, biotechnology, and ecosystem conservation under rapid climate change.

Woody plants exhibit significant and predictable patterns of change in morphology and physiology as they become older and larger. Four models of potential pathways controlling these changes are presented: a stimulus–response model in which fully developed organs respond to changes in environment (defined here as everything external to the organ); an extrinsic model in which the attributes of developing organs are determined by environmental factors; an intrinsic model in which changes are a result of programmed changes in gene expression; and an extrinsic–intrinsic model in which changes in gene expression are induced by environmental factors. We review evidence that a genetic component is involved in controlling age- and size-related changes in foliar morphology and physiology and discuss the possibility of complex interactions among model pathways.

Differential display reverse transcription-polymerase chain reaction was used to detect the induction of gene expression during adventitious root formation in loblolly pine (Pinus taeda) after treatment with the exogenous auxin indole-3-butyric acid. A BLAST search of the GenBank database using one of the clones obtained revealed very strong similarity to the α-expansin gene family in angiosperms. A near-full-length loblolly pine α-expansin sequence was obtained using 5′- and 3′-rapid amplification of cDNA end cloning, and the deduced amino acid sequence was highly conserved relative to those of angiosperm expansins. Northern analysis indicates that α-expansin mRNA expression increases 50- to 100-fold in the base of hypocotyl stem cuttings from loblolly pine seedlings in response to indole-3-butyric acid, with peak expression occurring 24 to 48 h after induction.

We characterized SCARECROW-LIKE genes induced by auxin in rooting-competent cuttings of two distantly related forest species (Pinus radiata D. Don and Castanea sativa Mill.) before the activation of cell division that results in adventitious root formation. The predicted protein sequences contain domains characteristic of the GRAS protein family and show a strong similarity to the SCARECROW-LIKE proteins, indicating conserved functions of these proteins. Quantitative RT-PCR analysis showed that these genes are expressed at relatively high levels in roots. Induction of increased mRNA levels in rooting-competent cuttings of both species in response to exogenous auxin was observed within the first 24 h of the root induction process, a time when cell reorganization takes place, but before the resumption of cell division and the appearance of adventitious root primordia. These results suggest that SCARECROW-LIKE genes play a role during the earliest stages of adventitious root formation.

Adventitious rooting is an organogenic process by which roots are induced from differentiated cells other than those specified to develop roots. In forest tree species, age and maturation are barriers to adventitious root formation by stem cuttings. The mechanisms behind the respecification of fully differentiated progenitor cells, which underlies adventitious root formation, are unknown. Here, the GRAS gene family in pine is characterized and the expression of a subset of these genes during adventitious rooting is reported. Comparative analyses of protein structures showed that pine GRAS members are conserved compared with their relatives in angiosperms. Relatively high GRAS mRNA levels were measured in non-differentiated proliferating embryogenic cultures and during embryo development. The mRNA levels of putative GRAS family transcription factors, including Pinus radiata’s SCARECROW (SCR), PrSCR, and SCARECROW-LIKE (SCL) 6, PrSCL6, were significantly reduced or non-existent in adult tissues that no longer had the capacity to form adventitious roots, but were maintained or induced after the reprogramming of adult cells in rooting-competent tissues. A subset of genes, SHORT-ROOT (PrSHR), PrSCL1, PrSCL2, PrSCL10 and PrSCL12, was also expressed in an auxin-, age- or developmental-dependent manner during adventitious root formation. The GRAS family of pine has been characterized by analyzing protein structures, phylogenetic relationships, conserved motifs and gene expression patterns. Individual genes within each group have acquired different and specialized functions, some of which could be related to the competence and reprogramming of adult cells to form adventitious roots.