EFB530 Plant Physiology

Hormones and hormone receptors

What makes a compound a hormone?

• usually effective in very low concentrations
• produced in one cell or tissue, transported to its site of action (target cell)
• not usually a metabolic intermediate, special compound (usually small molecule)
• can initiate a cascade of events (signal amplification)
• perceived by a specific receptor in the target cell

Hormones are perceived by receptor proteins

• bind at high affinity and specificity (hormone=ligand)
• are activated upon binding, inactive when not binding ligand
• initiate the cascading response
• can be either cytoplasmic or membrane-bound (cell surface)

Three types of cell surface hormone receptors

• ion channel-linked
• G protein-linked
• enzyme-linked

Receptors trigger a signaling cascade of events

• within then cell, the cascade often involves changes in levels of special molecules=second messengers (the hormone was the first messenger)
• these act to amplify and diversify the hormone signal, resulting in multiple simultaneous responses, including changes in gene expression, enzyme activity, cytoskeletal structure
• this cascade may be modulated by other regulatory systems in the cell
• changes in gene expression represent a longer-term response than changes in the activity of an enzyme already present in the cell

Second messengers can activate/inactivate many enzymes as the next step in a cascade

• Ca²⁺ and cAMP can act as second messengers (often cGMP is used in plant cells)
• Ca²⁺ levels are released from internal stores when Ca²⁺ channels are opened by IP₃ (produced by the enzyme phospholipase C)
• cAMP is produced by the enzyme adenylate cyclase
• commonly, kinases and/or phosphatases (i.e. Ca²⁺-dependent protein kinase, cAMP-dependent protein kinase, calmodulin-dependent protein kinase) are involved, which change the phosphorylation state of proteins to activate or inactivate them

Any activation cascade initiated by the binding of the hormone must be reversed when the hormone levels decrease and the hormone is released from the receptor

• an inactivation cascade reverses the response to return the cell to its previous state
• can involve a phosphatase which dephosphorylates a protein that a kinase has acted on previously

Five main hormones (or hormone families) in plants

• auxins
• gibberellins (gibberellic acid)
• cytokinins
• ethylene
• abscisic acid (ABA)

Two relatively new hormones

• brassinolide
• systemin  (also involves jasmonic acid)

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