This essay builds on ideas developed in our recent paper (Tims et al., 2025), with new analysis and commentary. One of my coauthors, John Courie, is active on Substack. You should give him a read.
Plant hormones are molecules produced in response to pulsed changes in plants, which may be the product of either internal development or external elicitation. Below are some data points for specific plant hormones that influence Cannabis trichomes, the seat of most of both cannabinoid and terpene production. This becomes important in generating increased levels of the active compounds, but also influencing the final ratio of both terpenes and cannabinoids.
Gibberellins (GA) stimulate plant cell division and elongation. Mansouri et al. (2009) noted that GA treatment increased the number of cannabis trichomes and reduced the amount of Δ9-THC produced. This suggests a strategic use in growing hemp to reduce Δ9-THC levels.
Cytokinin (CK) induces floral induction and trichome formation in cannabis, which are intimately connected through similar genetic pathways (Matías-Hernández et al., 2016).
Jasmonic acid (JA) is involved in the formation of plant memory of herbivory, with population-level damage cues creating heterogeneity in JA bursts (Stork et al., 2009). The hormone mediates the progression of the endoreduplication cycle that increase the number of multicellular trichomes or trichome size (Li et al., 2021) and density (Boughton et al., 2005; Yan et al., 2017). It also increases terpene production (Gabotti et al., 2019; Moreno-Rodríguez et al., 2020).
A related hormone, methyl jasmonic acid (MeJA), increased Δ9-THC by 21% when applied (Apicella et al., 2022).
Salicylic acid (SA) increased THCA expression, while decreasing CBDA and metabolic precursors for terpenes. The hormone acts antagonistically to JA stimulation by reducing trichome formation (Jalali et al., 2019).
Abscisic acid is considered a plant stress hormone. Direct application increased Δ9-THC levels (Mansouri et al., 2009).
Ethylene, a gaseous plant hormone, acts less as a primary elicitor of herbivore-induced defense, but modulates other defense signals, including JA (Schmelz et al., 2003; Onkokesung et al., 2010). Ethylene produced in response to herbivore attack dramatically exceeds that produced by simple mechanical wounding (von Dahl et al., 2007). This speaks to an herbivore saliva-based signaling that leads to accumulation of defense molecules, including phenolics (Hudgins et al., 2004) and release of volatile terpenes thought to function as indirect defenses (Horiuchi et al., 2001; Schmelz et al., 2003).
Rarely do plant hormones work in isolation, often forming positive and negative feedback loops. We see cross talk between plant hormones serving an oscillatory function, balancing multiple trophic influences on plant development and phytochemical expression patterns. Gibberellins (GAs) and cytokinins (CKs) work synergistically to increase density of trichomes (Traw & Dawson, 2003), while SA can reduce trichome formation, antagonistic to JA stimulation (Jalali et al., 2019).
Negative crosstalk occurs with hormones playing differing roles between stimulating growth and eliciting plant defense (Pieterse et al., 2009; Wang, 2014). Microbial symbionts and/or endophytes can provide defense-stimulating hormones that uncouple the normal trade-off in plants of hormone expression to promote growth versus protection against plant consumers (Bennett et al., 2006; Pangesti et al., 2013). Microbial endophytes have been shown to produce GA, indoleacetic acid (IAA), SA, AA, and JA (Cosoveanu, 2021; Waqas et al., 2012).
In vitro and in vivo research has shown that both SA and JA elicit the synthesis of plant secondary metabolites (Jan et al., 2021) as well as regulate trichome initiation and development (Li et al., 2021). Investigating the leaves and inflorescences of medical cannabis chemotypes, Garrido et al. (2022) found that the foliar application of SA, MeJA, and GABA increased leaf and inflorescence production of CBDA and THCA. They did not report any significant increase in trichome density.
It is worth noting that exogenous application of phytohormones of microbial origin increased plants’ abiotic and biotic stress tolerance (Egamberdieva et al., 2017) and that the actual ecosystem stressor elicits a more robust response than the application of hormones playing a role in the underlying plant response to stress.
Weyers and Paterson (2001) suggested a focus on the “whole plant” aspects of hormonal control and a need for clear delineation of spatial and temporal activity of these phytochemicals. Plant systems react to multiple phytochemical signals, and that each compound appears to impact multiple systems (Matthysse, 1984). This process allows plants to respond within an existing balance of compounds present in specific tissue (Wareing, 1977). In opposition to the design of many experiments, concentration of these compounds does not always drive plant response; rather, changes in plant physiological activity appears to be based on spatial differences or temporal changes in the tissue’s responsiveness to a set of plant hormones (Trewavas, 1981).
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