Part IV: Hormones & Signaling
Chemical Coordination in Plants
Plants coordinate development, growth, and responses to environmental stress using a suite of small-molecule hormones that act at nanomolar to micromolar concentrations. Unlike animals, plants produce hormones in virtually all tissues, and the same compound can have opposite effects depending on concentration, tissue type, and developmental stage.
Secondary metabolites — the vast chemical diversity of terpenes, phenolics, and alkaloids — serve as chemical defenses, pollinator attractants, UV screens, and antimicrobials. Their biosynthesis branches from primary metabolic pathways, particularly the shikimate pathway (phenolics), the MEP and MVA pathways (terpenoids), and amino acid catabolism (alkaloids).
8+
Major plant hormone classes
>200,000
Known plant secondary metabolites
~20%
Fixed carbon allocated to phenylpropanoids
Chapters in Part IV
Chapter 10: Plant Hormones & Signaling
Auxin (IAA, TIR1 receptor, polar transport), cytokinin, gibberellin (DELLA repressors), ABA (stomatal closure, SnRK2), ethylene (ACC synthase, ETR1), brassinosteroids, jasmonate, salicylate.
Chapter 11: Secondary Metabolites Overview
Terpenes, phenolics, alkaloids — ecological roles (defense, pollination, UV protection), biosynthetic origins from shikimate, MEP/MVA pathways, amino acid catabolism.
Chapter 12: Phenylpropanoid Pathway
PAL, C4H, 4CL, CHS, CHI, monolignol synthesis (coniferyl, sinapyl, p-coumaryl alcohols), lignin polymerization, coumarin biosynthesis, stilbene synthase (resveratrol).
Key Reactions in Part IV
Auxin biosynthesis: \(\text{Trp} \xrightarrow{TAA1} \text{IPA} \xrightarrow{YUC} \text{IAA}\)
Ethylene synthesis: \(\text{SAM} \xrightarrow{ACS} \text{ACC} \xrightarrow{ACO} \text{Ethylene} + HCN + CO_2\)
PAL: \(\text{L-Phe} \xrightarrow{PAL} \text{trans-Cinnamate} + NH_3\quad \Delta G^{\circ\prime} = -5\text{ kJ mol}^{-1}\)