Contribution of tissue layers to plant patterning

Phyllotaxis is defined by the divergence angle between two successive leaves, the most common angle being 137.50. Leaf initiation and development are dictated by auxin dynamics at the shoot apical meristem (SAM). Polar auxin transport by auxin efflux carrier PIN1 (Reinhardt, et al. 2003) forms auxin maxima within the meristem where new primordia arise. Current phyllotactic models are also based on auxin fluxes within the L1 layer of the SAM (Smith, et al. 2006). However, recent work has demonstrated that primordium formation and vein development occur almost simultaneously in the SAM and thus indicate the involvement of inner tissues, within and outside of the SAM. In order to address the contributions of different tissues to the phyllotactic patterning, we are using laser ablation to kill and remove the effect of a given cell. Initial experiments separating the youngest primordia (P1) and incipient primordia from the meristem by superficial layer ablation resulted in the loss of phyllotactic patterning. Similarly, ablation of L1 cells in incipient primordia resulted in loss of phyllotactic patterning. We are currently developing ground tissue ablation method using 2-photon laser. We will ablate the ground tissue cells and observe and effect on the primordium. This approach will complement the L1 layer experiments and give us a better view if the influence of L1 versus ground tissue fluxes on phyllotaxis.