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Discuss the mechanisms responsible for polarity acquisition in lateral organs of plants. What evidence supports your views?

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Discuss the mechanisms responsible for polarity acquisition in lateral organs of plants. What evidence supports your views? Plant development is based on the formation of modular sections, lateral organs are formed in a reiterative pattern from the periphery of the shoot apical meristem (SAM). Te establishment of polarity in the lateral organs is essential for asymmetric development and research to date indicates it involves both intrinsic and extrinsic factors. Molecular studies have revealed genes are thought to assist in establishing this polarity with many of them being spatially localised transcription factors. In this essay I shall describe a proposed model of organ polarity acquisition and support this with evidence from experiments that have been performed. Lateral organ development starts with the formation of the SAM. Initially formed during embryogenesis the SAM is the progenitor of the above ground portion of the plant. In order to achieve this the SAM must serve three primary functions: 1. Lateral organs, such as leaves are produced from the peripheral regions of the SAM 2. The basal regions of the SAM are responsible for the creation of the stem. 3. The stem cells of the SAM must replenish those regions from which cells have been recruited and maintain the pool of pluripotent cells necessary for growth. ...read more.


Such a feedback would create a reference point for adaxial positioning thus removing the need for an external ligand allowing it to grow away from the original source of the ligand. This ties in well with the observation that whilst younger leaf primordia are unable to develop adaxial polarity older leaf primordia can autonomously develop into phenotypically normal leaves. However studies have not yet been performed looking at a loss of PHB function which if this hypothesis hold should display an abaxialised phenotype. Another interesting observation has been made regarding the role that adaxial promoting genes play in SAM maintenance. It was noted by McConnell et al (2001) that the mutants found in the mutagenesis screen not only exhibited adaxialised leaves but also overgrown meristems and even ectopic meristems on the leaf margins. Also studies on the Antirrhinum gene PHANTASTICA revealed loss of function mutations that resulted in abaxialisation and an arrested meristem. These pieces of suggest that adaxial promoting factors signal the SAM to regenerate the cells used when the lateral organ develops. In contrast to PHANTASTICA and PHABULOSA which appear to promote adaxial cell identity members of the YABBY gene family act in a redundant manner to KANADI to promote abaxial development. Kerstetter et al (2001) screened Arabidopsis for mutants which displayed trichomes (leaf hairs) on the abaxial surface of the first two leaves. This was the KANADI (KAN) ...read more.


There is also some evidence that KNOX genes may be involved in setting up initial domains for polarity and that some difference occurs between the way orthologues (specifically ROUGH SHEALTH 2 and PHANTASTICA) of monocots and eudicots play a part in control of leaf axis. Summary Given the current evidence Eshed et al (2001) produced a model of polarity establishment in lateral organs with the spatial and temporal aspects mapped onto a potato apical meristem. Factors, possibly sterol or lipid are produced by the SAM which in turn activate PHABULOSA thus promoting adaxial cell fate, regeneration of the meristem and by indirect factors inhibits KANADI. KANADI is believed to be involved in promoting abaxial cell fate and possible direct inhibitory action on the stability of the PHABULOSA transcripts. YABBY is thought to act in a parallel pathway with KANADI to promote abaxial cell fate but the exact interaction between the two pathways has not been elucidated. Finally the collaboration between the adaxial and the abaxial factors establish polarity within the leaf and facilitate a normal phenotypic development. By assessing all this evidence I believe there is a long way to go before the entire pathway is elucidated. Although evidence exists for some components of the pathway in only a few cases, such as KANADI and PHABULOSA am I convinced of their roles in the pathway although the mechanisms behind them are still evading the scientific community. ...read more.

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