How a leaf gets its shape

Leaves are formed from a group of initial cells within the meristem. One of the earliest markers of leaf initiation is the down-regulation of KNOX genes in initial cells. Polar auxin activity, MYB and LOB domain transcription factors function to keep KNOX out of the initiating leaf. If KNOX genes are expressed in initial cells, leaves fail to form. As the leaf grows away from the meristem, its shape is determined by growth in three axes, proximal–distal, abaxial–adaxial and medial–lateral. HD-ZIPIII, KANADI and the small RNA pathway play a significant role in the latter two axes. KNOX proteins play a role in the proximal–distal axis. Although genetic networks are conserved between monocots and dicots, the outcome in leaf shape often differs.

Research highlights▶ Class I KNOX transcription factors maintain the SAM and repressed at the incipient leaf primordia. ▶ Auxin and AS1/AS2 complex repress KNOX expression. ▶ Adaxial–abaxial axis is established by antagonistic relationship between the HD-ZIPIII and KAN genes. ▶ tasiRNA and miR165/166 maintain the adaxial–abaxial polarity by opposite polar distribution. ▶ KNOX activity elaborates different leaf shapes depending on the timing and location of expression.