Evidence for Symplastic Phloem Unloading with Concomitant High Activity of Acid Cell Wall Invertase in Agrobacterium tumefaciens‐Induced Plant Tumors*

Abstract

Abstract:In stems of Ricinus communis and leaves of Kalanchoë daigremontiana, rapidly growing tumors were induced by the wild type strains of Agrobacterium tumefaciens C58 and A281 (p35 Sgusint). Transformed cells, monitored by histochemical β‐glucuronidase (GUS) staining, showed GUS activity in K. daigremontiana tumors in up to 100% of the tissue. In R. communis tumors, however, GUS activity was patchy, probably due to interference in gus expression from highly active phenolic compounds. Functionality of the sieve elements within the vascular bundles of the tumor and their connection with host stem bundles were shown by applying fluorescein to source leaves as a tracer of phloem‐mobile solutes. The transport pathway within the tumor and the mechanism of phloem unloading were investigated by iontophoretic injection of Lucifer yellow CH into sieve tubes. Apparent symplastic solute unloading into parenchyma cells was confirmed by localizing common primary pit fields by staining them with aniline blue. In spite of the evidence for symplastic unloading, the activity of acid cell wall invertase (CWI) was about tenfold higher in tumor than in the adjacent host stem tissue. These results indicate primary independence of phloem unloading of CWI in tumors.