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Published on March 25, 2009; 10.1104/pp.109.137034

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Received February 12, 2009
Accepted March 19, 2009

Studies of abphyl1 phyllotaxy mutants of maize indicate complex interactions between auxin and cytokinin signaling in the shoot apical meristem

Byeong-ha Lee , Robyn Johnston , Yan Yang , Andrea Gallavotti , Mikiko Kojima , Bruno A. N. Travencolo , Luciano da F. Costa , Hitoshi Sakakibara , and David Jackson *

Cold Spring Harbor Laboratory, 1 Bungtown Rd. Cold Spring Harbor, NY 11724, USA; Department of Life Science, Sogang University, Seoul, 121-742, Korea; University of Amsterdam, Kruislaan 318, Amsterdam, 1098SM, The Netherlands; Section of Cell and Development Biology, University of California, San Diego, La Jolla, California 92093-0116, USA; Plant Science Center, RIKEN (The Institute of Physical and Chemical Research), Suehiro 1-7-22, Tsurumi, Yokohama, 230-0045, Japan; Instituto de Fisica de Sao Carlos, Universidade de Sao Paulo, Av. Trabalhador Sao Carlense 400, PO Box 369, Sao Carlos, Sao Paulo, 13560-970, Brazil

* Corresponding author; email: jacksond{at}cshl.edu.

One of the most fascinating aspects of plant morphology is the regular geometric arrangement of leaves and flowers, called phyllotaxy. The shoot apical meristem (SAM) determines these patterns, which vary depending on species and developmental stage. Auxin acts as an instructive signal in leaf initiation, and its transport has been implicated in phyllotaxy regulation in Arabidopsis. Altered phyllotactic patterns are observed in a maize mutant, abphyl1 (abph1, aberrant phyllotaxy1), and ABPH1 encodes a cytokinin-inducible type A response regulator, suggesting that cytokinin signals are also involved in the mechanism by which phyllotactic patterns are established. We therefore investigated the interaction between auxin and cytokinin signaling in phyllotaxy. Treatment of maize shoots with a polar auxin transport inhibitor, 1-naphthylphthalamic acid, strongly reduced ABPH1 expression, indicating that auxin or its polar transport is required for ABPH1 expression. Immunolocalization of the PINFORMED1 (PIN1) polar auxin transporter revealed that PIN1 expression marks leaf primordia in maize, similarly to Arabidopsis. Interestingly, maize PIN1 expression at the incipient leaf primordium was greatly reduced in abph1 mutants. Consistently, auxin levels were reduced in abph1, and the maize PIN1 homolog was induced not only by auxin but also by cytokinin treatments. Our results indicate distinct roles for ABPH1 as a negative regulator of SAM size and a positive regulator of PIN1 expression. These studies highlight complex interaction between auxin and cytokinin signaling in the specification of phyllotactic patterns, and suggest an alternative model for the generation of altered phyllotactic patterns in abph1 mutants. We propose that reduced auxin levels and PIN1 expression in abph1 mutant SAMs delays leaf initiation, contributing to the enlarged SAM and altered phyllotaxy of these mutants.






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