THE ROLE OF HORMONES IN PLANT DEVELOPMENT

 

HORMONES ARE NATURALLY OCCURRING ORGANIC SUBSTANCES THAT, AT LOW CONCENTRATIONS, HAVE A PROFOUND INFLUENCE ON PHYSIOLOGICAL PROCESSES

 

   ANIMAL HORMONES:

 

   1.   SYNTHESIZED IN DESCRETE

      ORGANS AND TISSUES

 

   2.   TRANSPORTED IN THE BLOOD          STREAM TO SPECIFIC TARGET

      TISSUES

 

   3.   AFFECT A PHYSIOLOGICAL            PROCESS IN A CONCENTRATION-    DEPENDENT MANNER

 

 

 

 

 

 

   PLANT HORMONES:

 

   1.   SOMES MAY BE PRODUCED BY          THE ORGAN WHERE THEY WILL       HAVE AN EFFECT OR,

 

   2.   MAY BE TRANSPORTED TO WHERE

      THEY WILL HAVE AN EFFECT

  

   3.   a.   EFFECTS MAY BE                      CONCENTRATION DEPENDENT

 

      b.   EFFECTS MAY BE THE                  RESULT OF CHANGING                  SENSITIVITIES OF THE               TARGET CELLS TO THE               HORMONE

 

   4.   MAY HAVE MORE THAN ONE            AFFECT

 

 

 

 

 

 

THE PLANT HORMONES

 

   1.   AUXINS

   2.   GIBBERELLINS

   3.   CYTOKININS

   4.   ABSCISIC ACID (ABA)

   5.   ETHYLENE

 

   6.   BRASSINOSTEROIDS

   7.   POLYAMINES

 

AUXINS

 

   1.   FIRST PLANT HORMONES TO BE

      DISCOVERED

 

   2.   SYNTHESIZED IN THE STEM AND

      ROOT APICES

 

   3.   TRANSPORTED THROUGH THE            PLANT AXIS

 

   4.   STIMULATE:

 

      a.   CELL ELONGATION

 

      b.   ROOT INITIATION

 

      c.   VASCULAR DIFFERENIATION

 

      d.   TROPIC RESPONSES

 

      e.   BUD DEVELOPMENT

 

NATURAL AND SYNTHETIC AUXINS

 

   NATURAL:

 

   1. INDOLE-3-ACETIC ACID (IAA)

 

   2.   INDOLE-3-ETHANOL

 

   3.   INDOLE-3-ACETALDEHYDE

 

   4.   INDOLE-3-ACETONITRILE

 

2,3 AND 4 ARE PRECURSORS OF IAA

 

   5.   INDOLE BUTYRIC ACID (IBA)

 

 

 

   6.   4-CHLOROINDOLEACETIC ACID

      (4-CHLOROLIAA)

 

   7.   PHENYLACETIC ACID (PAA)

 

5,6 AND 7 WERE ONCE ONLY THOUGHT

TO BE SYNTHETIC AND NOT FOUND TO BE NATURALLY OCCURRING

 

DISCOVERY OF IAA

 

EXPERIMENTAL BEGINNINGS OF PLANT HORMONE RESEARCH CAN BE TRACED TO WORK BY CHARLES DARWIN.

 

DARWIN STUDIED PHOTOTROPISM, THE

MOVEMENT OF PLANTS TOWARD UNIDIRECTIONAL LIGHT

 

DARWIN USED CANARY GRASS COLEOPTILES IN HIS STUDY

 

   A COLEOPTILE IS A COVERING OR

   SHEATH COVERING THE PRIMARY

   LEAVES OF A SEEDLING

 

DARWIN OBSERVED THAT WHEN THE

COLEOPTILE TIPS WERE COVERED TO

EXCLUDE LIGHT, OR WHEN THEY WERE

REMOVED, THE STEMS DID NOT GROW TOWARD THE LIGHT

 

HE SURMIZED THAT THE “INFLUENCE” WAS PRODUCED IN THE TIP AND

TRANSPORTED DOWN THE STEM WHERE THE BENDING OCCURRED

 

OTHER RESEARCHERS STUDYING THIS PHENOMENON WERE:

 

   1910 -   BOYCE-JENSEN FOUND

            THAT THE STIMULUS                  WOULD PASS THROUGH AN             AGAR BLOCK

 

   1918 -   PAAL OBSERVED THAT                WHEN THE TIP WAS                  REMOVED AND REPLACED,          ASYMMETRICALLY, THE               STEM BENT EVEN IN THE

            DARK

 

   1928 -   THE ACTIVE SUBSTANCE

            WAS ISOLATED BY FRITZ

            WENDT.

 

FRITZ W. WENDT WAS A GRADUATE STUDENT WORKING IN HIS FATHERS LAB IN HOLLAND

 

   FRITZ WENDT DID THIS BY:

 

   1.   PLACING EXCISED COLEOPTILE

      TIPS ON SMALL SQUARES OF          AGAR FOR A PERIOD OF TIME

 

   2.   THEN, PLACING THE AGAR

      BLOCKS, ASYMMETRICALLY, ON

      DECAPITATED SEEDLINGS HE

      OBSERVED THAT THE PLANT             BENT AWAY FROM THE SIDE THE

      BLOCK WAS ON

 

   3.   THE ACTIVE SUBSTANCE COULD

      BE ISOLATED FROM THE AGAR

      BLOCKS

 

 

RESEARCHERS THOUGHT THAT FEMALE

SEX HORMONES MIGHT HAVE AN EFFECT OF PLANT GROWTH.

 

ONLY URINE FROM PREGNANT WOMEN HAD A HORMONE LIKE EFFECT

 

THE COMPOUND ISOLATED FROM URINE WAS THE SAME AS THAT FOUND IN THE AGAR BLOCKS

 

EFFECTS INCLUDE:

 

   1.   CELL ELONGATION IN EXCISED

      STEMS

 

   2.   VASCULAR DIFFERENTIATION -

 

      MORE XYLEM ELEMENTS AND

      PHLOEM SIEVE TUBES   DEVELOPE    IN THE PRESENCE OF    AUXIN

 

 

 

 

 

   3.   SHOOT AND ROOT DEVELOPMENT:

 

      a.   AXILLARY BUD GROWTH IS

         INHIBITED BY AUXIN

 

         1.   REMOVAL OF THE TIP

            CAUSES DEVELOPMENT OF

            THE AXILLARY BUDS

 

         2.   THE APICAL MERISTEM

            EXERTS APICAL                    DOMINANCE

     

      b.   ABSCISSION

 

         THE PROCESS OF SHEDDING

         ORGANS SUCH AS LEAVES

         AND FRUITS

 

         1.   AN ABSCISSION LAYER

            FORMS AT THE BASE OF

            A PETIOLE

 

         2.   AS THE ORGAN AGES,

            THE WALLS OF THE                  CELLS IN THE                        ABSCISSION LAYER                   WEAKEN AND SEPARATE

 

      c.   ROOT ELONGATION AND

         DEVELOPMENT

 

         1.   IAA PROMOTES ROOT                GROWTH AT VERY LOW                  CONCENTRATIONS

 

         2.   IAA INHIBITS ROOT

            GROWTH AT HIGHER

            CONCENTRATIONS

        

         3.   IAA STIMULATES                      LATERAL ROOT                      FORMATION

 

      d.   FLOWER AND FRUIT FORMA-

         TION

 

         1.   EXOGENOUS IAA                     INHIBITS FLOWER                    FORMATION

 

         2.   IF FLOWERS HAVE                   FORMED, IAA PROMOTES               FEMALENESS

 

         3.   IAA PROMATES                        PARTHENOCARPY                      (DEVELOPMENT OF                      FRUITS WITHOUT                      FERTILIZATION)