Chapter 31: Plant Responses to Internal and External Signals

Plant hormones function as chemical messengers operating at extremely low concentrations to coordinate growth, development, and stress adaptation across tissues. Auxin mediates directional growth responses including phototropism and apical dominance while promoting cell elongation through the acid-growth hypothesis mechanism. Cytokinins regulate cell division and work synergistically with auxin to determine cell fate and tissue differentiation patterns. Gibberellins control stem elongation, seed germination, and fruit development through multiple physiological pathways. Abscisic acid coordinates drought tolerance and seed dormancy by triggering stomatal closure and other stress-protective mechanisms, while ethylene regulates fruit maturation, leaf abscission, senescence, and mechanical stress responses including the triple response. Light serves as a critical environmental signal transduced through multiple photoreceptor systems that enable plants to monitor day length and light quality. Blue-light photoreceptors including cryptochromes and phototropins work alongside phytochromes, which exist in interconvertible red and far-red light-absorbing forms to regulate seed germination, shade-avoidance responses, circadian clock entrainment, and photoperiodic flowering responses. The discovery of florigen as a mobile flowering signal demonstrated how leaves sense photoperiod and transmit developmental signals to growing meristems. Plants also respond dynamically to mechanical stimuli, gravity, and multiple abiotic stresses through specialized sensory mechanisms. Gravitropism involves statolith sedimentation and auxin redistribution to orient root and shoot growth correctly, while thigmotropism and thigmomorphogenesis encompass growth alterations triggered by touch and wind through turgor-mediated movements and electrical signal transduction. Responses to abiotic stresses including drought, flooding, salt accumulation, excessive heat, and cold exposure involve coordinated adjustments to hormone signaling, membrane composition, and protective protein synthesis. Plant defense against herbivory combines physical deterrents, toxic secondary metabolites, and volatile signaling to recruit natural enemies, while pathogen resistance operates through pattern-recognition immunity and effector-triggered immunity mechanisms involving hypersensitive cell death responses and systemic acquired resistance signaling mediated by salicylic acid. These interconnected signaling networks reveal plants as sophisticated environmental sensors capable of integrating multiple stimuli and executing coordinated adaptive responses.