In instructive interaction, 

a signal from the inducing cell is necessary for initiating new gene expression in the responding cell.

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#Unit 6. System Physiology – Plant

UVR8 lacks a prosthetic chromophore

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#Unit 6. System Physiology – Plant

Upon absorbing UV-B photons, the tryptophans undergo structural changes that break the salt bridges, leading to the dissociation of the two functionally active monomers. The monomers then interact with COP1–SPA complexes to activate gene expression

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#Unit 6. System Physiology – Plant

The UVR8- COP1-SPA complex then activates the transcription of the major transcription factor HY5, which controls the expression of many of the genes induced by UV-B.

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#Unit 6. System Physiology – Plant

Plant immunity involves

Anatomical/Mechanical Barriers & Constitutive Immunity

Inducible Défense

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#Unit 6. System Physiology – Plant

Thorns are modified branches, as in citrus and acacia; spines

are modified leaves

Prickles are derived mainly from the epidermis and thus can be easily snapped off the stem, protect plants from larger herbivores

Trichomes, or hairs, provide a much more effective defense against insect pests

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#Unit 6. System Physiology – Plant

Trichomes of nettles (Urtica dioica) have “cocktail” of secondary metabolites that can cause severe irritation of the skin of animals.