Intercellular trafficking of a KNOTTED1 green fluorescent protein fusion in the leaf and shoot meristem of Arabidopsis

Dominant mutations in the maize homeobox gene knotted1 (kn1) act nonautonomously during maize leaf development, indicating that Kn1 is involved in the generation or transmission of a developmental signal that passes from the inner layers of the leaf to epidermal cells. We previously found that this nonautonomous activity is correlated with the presence of KN1 protein in leaf epidermal cells, where KN1 mRNA could not be detected. Furthermore, KN1 protein expressed in Escherichia coli and labeled with a fluorescent dye can traffic between leaf mesophyll cells in microinjection assays. Here we show that green fluorescent protein (GFP)-tagged KN1 is able to traffic between epidermal cells of Arabidopsis and onion. When expressed in vivo, the GFP-KN1 fusion trafficked from internal tissues of the leaf to the epidermis, providing the first direct evidence, to our knowledge, that KN1 can traffic across different tissue layers in the leaf. Control GFP fusions did not show this intercellular trafficking ability. GFP-KN1 also trafficked in the shoot apical meristem, suggesting that cell-to-cell trafficking of KN1 may be involved in its normal function in meristem initiation and maintenance.