ROOT-MICROBE INTERACTIONS

 

MICROBES ASSOCIATED WITH ROOTS MAY EITHER:

 

   1.   INVADE THE ROOTS

   2.   BE FREE-LIVING IN THE SOIL

 

THE ASSOCIATION MAY BE EITHER:

 

   1.   BENEFICIAL

   2.   PATHOGENIC

 

BACTERIA

 

BACTERIA ARE ATTRACTED TO ROOTS

BECAUSE OF THE NUTRIENTS EXUDED BY THE ROOTS

 

THE RHIZOSPHERE (SOIL SURROUNDING THE ROOTS) MAY

CONTAIN 50% MORE BACTERIA THAN SOIL OUTSIDE THE RHIZOSPHERE

 

 

 

ROOTS SECRETE MUCILAGE.  MUCILAGE IS A POLYSACCHARIDE SECRETED BY GOLGI VESSELS IN CELLS NEAR THE GROWING TIP.

 

MUCILAGE ATTRACTS BACTERIA, WHICH SECRETE MUCOPOLY-SACCHARIDES, COLLOIDIAL MINERALS AND ORGANIC MATTER WHICH MAKE UP

MUCIGEL

 

BACTERIA ARE INSTRUMENTAL IN NITROGEN FIXATION.  SOME NITROGEN-FIXING BACTERIA INVADE THE ROOT WHILE OTHERS REMAIN FREE-LIVING

 

BACTERIA SOLUBALIZE INSOLUBLE FORMS OF PHOSPHORUS SO THAT PLANTS CAN UTALIZE WHAT WAS AN INSOLUBLE RESOURCE

 

BACTERIA CAN FUNCTION IN THE FORMATION OF PROTEOID ROOTS

 

  

 

   PROTEOID ROOTS ARE:

 

   1.   LARGE NUMBERS OF LATERAL

      ROOTS

 

   2.   FORMED IN AREAS OF HEAVY          AGGREGATION OF BACTERIA

 

   3.   FORMED NEAR THE SOIL

      SURFACE

 

   4.   COULD BE RELATED TO IAA

      PRODUCTION BY THE BACTERIA

 

MYCORRHIZA

 

AN ASSOCIATION BETWEEN ROOTS AND FUNGUS.  A ROOT INFECTED WITH A FUNGUS IS CALLED A MYCORRHIZA

 

THIS IS A FORM OF MUTUALISM.  BOTH THE PLANT AND FUNGUS BENEFIT FROM THE ASSOCIATION.

 

MORE THAN 80% OF ROOTS FORM MYCORRHIZAL ASSOCIATIONS

 

TWO MAJOR FORMS OF MYCORRHIZAE EXIST:

 

   1.   ECTOTROPHIC

 

      a.   ECTOMYCORRHIZEA

     

         SHORT, HIGHLY BRANCHED,

         ENSHEATHED IN A MANTLE 

         OF FUNGAL HYPHAE

 

         FOUND IN A FEW FAMILIES

         (PINACEAE, FAGACEAE)

        

         PENETRATES THE                     APOPLASTIC SPACE OF THE

         ROOT CORTEX, FORMING A

         HARTIG NET

 

   2.   ENDOTROPHIC

 

      a.   ENDOMYCORRHIZAE

 

         FOUND IN SOME SPECIES OF

         EVERY ANGIOSPERM FAMILY

         AND MOST GYMNOSPERMS

         (EXCEPT THE PINACEAE)

 

         HYPHAE DEVELOPE WITHIN

         THE CELLS OF THE CORTEX

 

         1.   VESICULAR-ARBUSCULAR

            MYCORRHIZAE (VAM)

 

            a.   HYPHAE GROW BETWEEN

               AND INTO CORTICAL

               CELLS AND BRANCH

               INTO “TREELIKE”

               STRUCTURES CALLED

               ARBUSCULES

 

            b.   ARBUSCULES DO NOT

               ENTER THE CELL

 

            c.   SOME ARBUSCULES

               FORM VESICLES

 

ROOTS INFECTED WITH MYCORRHIZAE GROW BETTER THAN UNINFECTED ROOTS. WHY?

 

   THE SURFACE AREA OF THE ROOT    IS INCREASED BY MYCORRHIZAL    INFECTION

 

   INCREASE IN SURFACE AREA

   ALLOWS FOR MORE UPTAKE OF

   NUTRIENTS.  PHOSPHORUS UP-

   TAKE IS GREATLY ENHANCED.

 

   IF SOILS ARE NUTRIENT POOR,

   PLANTS INFECTED WITH    MYCORRHIZAE ARE BETTER ABLE

   TO SURVIVE DUE TO ENHANCED

   NUTRIENT UPTAKE

 

NUTRIENT DEPLETION ZONE

 

   THE AREA OF SOIL FROM WHICH

   THE PLANT IS ABLE TO EXTRACT

   NUTRIENT ELEMENTS.  THE

   SIZE OF THE DEPLETION ZONE IS

   DIFFERENT FOR EACH NUTRIENT

 

 

 

 

 

nutrient           

depletion              nutrient

zone w/o                  depletion

mycorrhizae           w/ myco-

                        rrhizae

                 

 

 

 

MANY MYCORRHIZAE ARE HOST-

SPECIFIC SPECIES

 

MANY ATTEMPTS AT REVEGETATION

FAIL DUE TO LACK OF THE PROPER

MYCORRHIZAE IN THE SOIL