Leaf margin morphogenesis in crucifer plants
Keywords: leaf
PIN1
KNOX
serration
leaflet
Auxin
CUC2

A key question in developmental biology is how form is generated. The model species Arabidopsis thaliana produces simple leaves with marginal outgrowths termed serrations. Serration development in A. thaliana requires both the transcription factor CUP-SHAPED COTYLEDON2 (CUC2) and the auxin efflux facilitator PIN-FORMED1 (PIN1), which regulates polar auxin transport by forming convergence points (Hay et al., 2006; Nikovics et al., 2006; Scarpella et al., 2006). In Chapter 3, I investigate how CUC2, PIN1 and auxin interact to control serration development. I demonstrate that CUC2 promotes PIN1 convergence point and auxin activity foci formation along the margin of the leaf, whilst high auxin activity represses CUC2 expression. Furthermore, interspersed peaks of CUC2 and auxin activity pattern serration development along the proximo-distal axis of the leaf. Thus, auxin, PIN1 and CUC2 form a negative feedback loop that patterns serration development.
CUC genes and PIN1 are required for leaflet development in Cardamine hirsuta (Barkoulas et al., 2008; Blein et al., 2008), a close relative of A. thaliana that produces compound leaves subdivided into units termed leaflets. However, it is unclear how CUC and PIN1 interact to control leaflet development. In Chapter 4, I demonstrate that similar to A. thaliana, CUC genes promote PIN1 convergence point and auxin activity foci formation at the C. hirsuta leaf margin, whilst high auxin activity represses CUC2 expression. These genetic interactions likely create interspersed peaks of CUC2 and auxin activity that pattern leaflet development. Thus, the same negative feedback loop between CUC, PIN1 and auxin patterns both leaflet development in C. hirsuta and serration development in A. thaliana.
KNOTTED1-LIKE HOMEOBOX (KNOX) genes are expressed in C. hirsuta leaves, and interact with ChCUC and PIN1 in positive and negative feedback loops, respectively, to control leaflet development (Barkoulas et al., 2008; Blein et al., 2008). KNOX genes are not expressed in A. thaliana leaves, but deeply lobed margins reminiscent of leaflets develop in association with ectopic KNOX expression in leaves (Chuck et al., 1996; Hay et al., 2006). However, it is unclear whether regulatory interactions of PIN1, CUC and KNOX which occur in C. hirsuta leaflets are employed during KNOX-induced lobe development in A. thaliana. In Chapter 5, I demonstrate that CUC2 and polar auxin transport are required for ectopic KNOX expression. Conversely, I show that KNOX misexpression up-regulates CUC2 expression in A. thaliana leaves. Thus, interactions between KNOX, CUC and PIN1 that occur in leaflet development in C. hirsuta also occur in association with KNOX-induced lobe development in A. thaliana.
In addition to investigating the regulatory interactions between known components of leaf development pathways, I sought to identify novel genes that mediate CUC2-dependent serration development in A. thaliana. In Chapter 6, I identify a suppressor of the smooth margin phenotype of cuc2 leaves that partially restores PIN1 localisation in the absence of functional CUC2. Finally, in the General Discussion I evaluate how interlinking feedback loops between CUC, KNOX and auxin pattern serration and leaflet development. I then discuss why interlinking feedback loops may have been deployed to control outgrowths in both plant and animal systems.