POLLINATION - ANTHER DEHISCENCE

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POLLINATION

 

Pollination is commonly defined as the process of pollen transfer from anther to stigma of a flower. pollen grains are formed in the pollen sac which are completely enclosed by multi-layered anther wall. Therefore, the first obvious requisite for pollination is the opening of anther sacs to release the pollen grains (anther dehiscence).


ANTHER DEHISCENCE:

Anther dehiscence to release the pollen grains is critical for the male fertility of plants. Anthers with viable pollen that do not dehisce timely are functionally as good as male sterile plants that do not produce viable pollen. Anther dehiscence is a multistage process involving localized differentiation and degeneration, combination with changes in structure and water status of the anther to facilitate complete opening of anther to release pollen.

Anther dehiscence involves three types of specialized cells:

  1. Stomium
  2. Septum
  3. Endothecium

STOMIUM:

The stomium differentiates before the microspore mother cells enter meiosis. It comprises of small specialized epidermal cells and, at anther maturation, splits to facilitate anther dehiscence. The distribution of thickening in endothecium around the stomium determines the form of stomium opening.

SEPTUM:

The septum that separates the two lobes of anther, breaks down at a later stage and the two sporangia of an anther lobe become joined to forms a single locule.

ENDOTHECIUM:

The endothecium is the hypodermal layer of the anther wall, which after the release of microspores from the tetrads undergoes expansion and deposition of ligno-cellulosic secondary thickening that arise from the inner tangential walls and run outward and upward ending near the outer wall of each cell. The outer tangential wall remains thin. The thickening may be annular-rib type, reticulate-rib type or palmate-rib type depending on the species. In the anther which open by longitudinal slits the endothecial cells around the junction of the two sporangia lack these thickenings. Endothecium secondary thickening is essential for providing mechanical force for anther dehiscence.

After the release of microspores from the tetrads, tangential swelling of the epidermis and endothecium increases the circumference of the anther locule wall. However, due to thickening, inner wall of the endothecium does not expand. The unequal expansion of the inner and outer wall of the endothecium causes tension to develop and inward bending of the locule, resulting in the disruption of the stomium cells. In the final stage of anther development ,dehydration of the anther walls causes the locules to bend outwards. The septum separating the two locules is enzymatically lysed and undergoes programmed cell death (PCD). Around the pollen maturation stage the PCD starts from the tapetum and extends to the outer tissues of the anther including the middle layers and stomium. At this point dehydration of epidermis and endothecium causes shrinkage of the tangential outer wall, which is limited by secondary thickening, resulting in an increased tension on the stomium region. As the pressure increases, the stomium splits and the anther walls retract. In rice the force created by anther desiccation is not sufficient for stomium and septum rupture. Swelling of the pollen generates additional force for septum breakdown.

Jasmonic acid (JA)seems to be a critical signal for anther dehiscence. Other hormones, particularly auxin, are also involved in this process.


 

KEY DEVELOPMENTAL EVENTS IN ANTHER DEHISCENCE ARE:

Microspore release:

  • Accumulation of auxin in the anther
  • Endothecium expansion
  • Secondary thickening formation in endothecium.

Tapetal breakdown and stomium split:

  • Enzymatic lysis of septum
  • PCD in stomium region
  • JA role in stomium degeneration
  • Swelling of endothecial cells and pollen
  • Expression of acquaporins
  • Localized changes in sucrose transporters, K+ ions
  • Dehydration of anther walls.

Filament extension / anther and flower opening:

  • Localized changes in sucrose transporters
  • Dehydration of anther walls – active transport and evaporation
  • Filament extension – JA biosynthesis in the filament
  • Auxin transport in the filament – role in elongation
  • GA role in the filament elongation.

 


REFERENCE :

THE EMBROLOGY OF ANGISPERMS 6th EDITION Author; SS BHOJWANI, SP BHATNAGAR ,PK DANTU.  

 

 

 

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