The Root: Structure and development

Roots

The four major functions of roots are 1)  Absorption 2)  Conduction 3)  Storage 4)  Anchorage

 wheat,  Tri$cum,  germ  Electron  microscopy  

Root systems  

• A primary (taproot) root system consists of: ü  A primary root and secondary roots. ü Present mainly in dicots ü Radicle of seedling gives rise to the root.

• Fibrous (adventitious) root system: ü There is not a tap root ü Present in monocots, ferns and some dicots. ü The roots arise from the stem ü The radicle is short-lived

Secale cereale Root system including root hairs = 639 m2

occupying only 6 liters of soil

Medicago can extend its roots 10 m or more

Root extension

The three primary meristems occur near to the apical meristem Phloem sieve tubes reach maturity earlier

Root tissues

Columella  Controls  gravitropism      Contains  amyloplasts  

stele  

Mucigel is a slimy substance made by dictyosomes. 1.  lubrication 2.  It contains allelopathic

compounds 3.  Water and nutrient

absorption 4.  help establish beneficial

symbioses with soil microbes.

Mucigel  cells  

Casparian strip or Casparian tissue

The endodermal cell walls, contain the casparian strip with suberin, a waxy water repellant substance.

Secondary growth

Mainly consist in the formation of secondary vascular tissues and periderm (cork tissue) Not capable of extension. Does not undergo reversible changes in thickness. True pits present. No plasmodesmata present.

Secondary growth

In some cases dead roots are replaced immediately by young roots If the cortex is retained, their walls will be covered with a suberin lamella, lignin and cellulose

Origin  of  lateral  roots  Branch roots originate in the pericycle, so they are endogenous. Never happen in the elongation section of the root

0 I II III IV V

High  auxine  concentra9on  

Credit:  Yvon  Jaillais  (Thierry  Gaude  lab)  

Arabidopsis lateral root development

Origin  of  adven33ous  roots,  process  of  rhizogenesis.  

Primordial cells differentiated from the cambium. Radicles are generated with a formed apical meristem.

Modifications of the roots

Potamogeton  and    Myriophyllum  have  air  chambers  extending  from  the  shoots  and  leaves  to  the  roots.  

Specialized roots Pneumatophores are structures with spongy tissues Negative geotropism Aerating roots which are found in some plants found growing in water-logged, badly aerated soils.

Swamp  Cypress  (Taxodium  dis$chium)  has  characteris9c  "knees”  

Pneumatophores  

Banyan tree with stilt roots.

Prop roots are adventitious roots

Aerial roots

Aerial roots

There are many species of epiphytes. (i.e. plants which grow attached to other plants) Velamen: multi-layered epidermis on the outside surfaces of the roots.

Common in large tropical trees. Buttress roots are immense exaggerated wall-like structures at the interface between the trunk and the ground.

Bu<ress  roots  

Tuberous roots are storage organs which act as perennating organs in environments which have extreme fluctuations of either temperature or moisture.

Ipomoea sp. (Sweet Potato)

Storage  roots  

Attendance

Define dictyosome What are the four main functions of the Mucigel cells? What is suberin? What is the velamen? __________ is the hormone related with the root elongation