BIOCHEMISTRY AND MODE OF ACTION OF HORMONES

 

HOW DO HORMONES AFFECT THE PHYSIOLOGY OF A CELL?

 

THREE STEPS TO DETERMINING HOW HORMONES WORK

 

   1.   SIGNAL PERCEPTION

 

   2.   A SIGNAL TRANSDUCTION             PATHWAY

 

   3.   FINAL RESPONSE

 

SIGNAL PERCEPTION

 

   THE TARGET CELL (THE CELL THAT    RESPONDS TO A HORMONE) MUST BE    CAPABLE OF DETECTION THE    HORMONES PRESENCE

 

 

 

 

 

   THE HORMONE RECEPTOR IS:

 

      1.   SPECIFIC FOR THE HORMONE

     

      2.   CHARACTERISTIC OF THE

         TARGET CELL

 

   THE RECEPTOR IS A GLYCOPROTEIN

   THAT BINDS REVERSIBLY TO THE

   HORMONE

 

   ONCE BOUND, THE RECEPTOR

   CHANGES SHAPE TO ASSUME AND

   ACTIVATED STATE

 

   THIS IS A HORMONE-REACTOR

   COMPLEX

 

   THE PRESENCE OR ABSENCE OF A

   PARTICULAR RECEPTOR IS IMPORT-

   ANT IN THAT:

 

      1.   IT DETERMINES WHICH                CELLS ARE ABLE TO                   RESPOND TO A PARTICULAR             HORMONE

 

      2.   DIFFERENT CELL TYPES MAY

         CONTAIN DIFFERENT                RECEPTORS THAT ELLICIT             RESPONSES TO THE SAME          HORMONE

 

SIGNAL TRANSDUCTION

 

ACTIVATED HORMONE-RECEPTOR COMPLEXES START A CASCADE OF BIOCHEMICAL EVENTS

 

   1ST MESSENGER - HORMONE

 

   2ND MESSENGER -   WHAT IS PRO-

                     DUCED IN                            RESPONSE TO A                    HORMONE

 

HOW HORMONES WORK IN PLANTS?

 

   1.   ARE THERE HORMONE-BINDING

      PROTEINS IN PLANTS?

 

   2.   DO PLANT HORMONE RESPONSES        INVOLVE SECOND MESSENGERS?

 

   3.   DO PLANT HORMONES ALTAR             GENE EXPRESSION?

 

HORMONE BINDING PROTEINS IN PLANTS

 

   1.   BINDING MUST BE SPECIFIC

 

   2.   THE RECEPTOR SHOULD HAVE A

      HIGH AFFINITY FOR THE            HORMONE

 

   3.   RECEPTORS CAN BE SATURATED

      BY ADDING MORE HORMONE

 

   4.   THE HORMONE MUST BIND

      REVERSIBLY

 

HORMONE TYPES

 

1.   AUXIN-BINDING PROTEINS

 

  

   LOCATED IN:

 

   1.   CALLUS CULTURE OF TOBACCO

      PITH

 

   2.   MAIZE COLEOPTILES

 

   THREE-CLASSES OF IAA-BINDING

   PROTEINS

 

   - 2 ASSOCIATED WITH THE MEM-

   BRANE

 

   - 1 DIVIDED BETWEEN THE CYTO-

   PLASMIC AND NUCLEAR FRACTIONS

 

2.   CYTOKININ-BINDING PROTEINS

 

   CBF-1 PROTEIN - CYTOKININ

   BINDING FACTOR

 

      ASSOCIATED WITH RIBOSOMES

 

      MAY HAVE A ROLE IN                   REGULATING PROTEIN                   TRANSLATION

 

3.   GIBBERELLIN- AND ABSCISIC    ACID-BINDING PROTEINS

 

   NO CONFIRMED RECEPTORS FOR GA

   OR ABA

 

SECOND MESSENGERS IN PLANTS

 

1.   CALCIUM -   CALMODULIN    COMPLEXES WITH CALCIUM TO

   FORM A CaM.Ca2+ COMPLEX

 

   a.   CaM.Ca2+ ACTIVATES CERTAIN

      ENZYMES

 

      1.   NAD KINASE TO                   PHOSPHORYLATE   NAD TO             NADP

 

  

 

 

 

2.   PHOSOHOINOSITIDES

 

   a.   INOSITOL TRIPHOSPHATE             SYSTEM

 

      1.   ACTIVATES PHOSPHOLIPASE            C

 

         a.   MAY ACT THROUGH A G-

            PROTEIN TO BREAK DOWN

            PHOSPHOTIDYLINOSITOL

            BISPHOSPHATE (PIP2) TO

            INOSITOL TRIPHOSPHATE

            (IP3) AND                           DIACYLGLYCEROL (DAG)

 

MECHANISMS OF HORMONE ACTION

 

AUXIN AND CELL EXPANSION

 

   AUXIN-INDUCED CELL EXPANSION

   CAN BEGIN WITHIN TEN MINUTES    OF AUXIN APPLICATION AN CAN

   INCREASE GROWTH 10-FOLD

 

 

   POSSIBLE EXPLANATIONS:

 

      a.   AUXIN ACTIVATES THE               GENES FOR CERTAIN                   PROTEINS THAT ARE                  NECESSARY FOR CELL                GROWTH

 

      b.   ACID GROWTH THEORY.                AUXIN INDUCED PROTON              EXCRETION

 

         1.   AUXIN CAUSES THE

            ACIDIFICATION OF THE

            CELL WALL ENVIRONMENT

            BY STIMULATING THE

            CELLS TO RELEASE                   PROTONS

         2.   THE LOWER pH                         STIMULATES WALL                   LOOSENING ENZYMES

         3.   TURGOR PRESSURE                     AGAINST THE LOOSER                 WALL CAUSES IT TO                   EXPAND

 

         4.   NEW CELL WALL                   MATERIAL IS LAYED                   DOWN SO THE CELL WALL          DOES NOT BECOME                    THINNER AS IT                      STRETCHES

 

PHOTOMORPHOGENESIS

 

PLANTS RESPONSES TO LIGHT

 

PLANTS PERCEIVE LIGHT BY HAVING PHOTORECEPTORS

 

PHOTORECEPTORS

 

   1.   PHYTOCHROME

  

      a.   ABSORBS IN THE RED AND

         FAR-RED REGIONS

 

   2.   CRYPTOCHROME

 

      a.   ABSORBS BLUE AND UV-A

 

 

   3.   UV-B RECEPTOR