student profile: Mr David Jeon


Thesis work

Thesis title: Genes Involved in Initiation of the Shoot Apical Meristem and Leaf Development

Supervisors: Charles WARREN , Mary BYRNE

Thesis abstract:

The above-ground body of a plant is generated from pluripotent cell population called the shoot apical meristem (SAM), which gives rise to organs such as the leaves and flowers. The SAM is established during embryogenesis. Following germination the SAM gives rise to leaves and secondary (axillary) meristems. The axillary meristems form branches and flowers. Variation in the timing and arrangement of organs and axillary meristems from the SAM produce the vast array of plant architectures seen in nature.

Underlying mechanisms for maintenance of SAM activity and leaf development have been studied for decades. In Arabidopsis, KNOTTED-like homeobox (KNOX) genes are crucial for maintenance of stem cell functions in SAM. SHOOT MERISTEMLESS (STM), one of the four class I KNOX genes in Arabidopsis, is required for early embryogenesis and maintenance of vegetative SAM through hormone regulation. STM induces the level of cytokinin (CK) through the activation of isopentenyl transferase (IPT) to maintain SAM activity (Hay and Tsiantis, 2010). Loss-of-function mutant stm showed failure to establish of the SAM and this phenotype can be suppressed partially by exogenous CK or the expression of STM::IPT. Furthermore, it is identified that phytohormone gibberellin (GA) is negatively regulated by KNOX through the direct regulation of GA 20-oxidase1 (GA20ox1; biosynthetic) and GA 2-oxidase1 (GA2ox1; Catabolic) in tobacco and maize, respectively (Hay and Tsiantis, 2010). These previous works showed that SAM activity is maintained by plant hormonal regulation mediated by KNOX gene expression. KNOX genes are also implicated in organogenesis. The flanks of the SAM where there is an auxin maximum generated through the auxin efflux carrier PIN-FORMED 1 (PIN1) can be initiated to further organs like leaf primordial, and in this region there is exclusion of expression of KNOX genes. This repression of KNOX genes in the organization of leaf is mediated by the MYB transcription factor ASYMMETRIC LEAVES1 (AS1) and the LOB domain protein ASYMMETRIC LEAVES2 (AS2). Early of leaf primordial has accumulated AS1 and AS2 expression and AS1-AS2 complex represses the expression of class I KNOX genes, BREVIPEDICELLUS (BP) and KNOTTED-like from Arabidopsis thaliana 2 (KNAT2), by direct binding to their promoter regions. In this way there is a distinction between cells that make up the SAM and cells that will differentiate into an organ.

There is an intimate link between the SAM and development of leaf polarity, where the leaf develops a top (adaxial) and a bottom (abaxial) side. AS1 and AS2 repress the level of genes determining abaxial leaf polarity such as KANADI, ARF3/ETT and YABBY. These abaxial genes repress genes required for adaxial fate including class III HD-ZIP transcription factor genes PHB, PHV and REV. In addition to these transcription factors, genes encoding components of the ribosome and the proteasome influence leaf adaxial fate indicating that regulation of protein synthesis and protein turn-over are also important for leaf development.

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