Long-distance transport of the maize pathogenesis-related PRms protein through the phloem in transgenic tobacco plants

The PRms protein is a pathogenesis-related (PR) protein involved in the defense response of maize (Zea mays L.) plants against fungal pathogens. We previously reported that PRms overexpression in transgenic tobacco (Nicotiana tabacum) plants results in the activation of sucrose export from photosynthetically active leaves and accumulation of higher levels of sucrose in sink tissues [I. Murillo, R. Roca, C. Bortolotti, B. San Segundo, Engineering photoassimilate partitioning in tobacco plants improves growth and productivity and provides pathogen resistance, Plant J. 36 (2003) 330–341]. By using immunoelectron microscopy, here we demonstrate that PRms localizes to plasmodesmata of the sieve element/companion cell complex in leaves of PRms tobacco plants. PRms was also detected at the lumen of the sieve elements, as well as at the sieve-pore area between the sieve elements. The presence of PRms in phloem exudates collected from leaves of PRms tobacco plants was confirmed. Moreover, grafting experiments revealed that PRms is transmitted from PRms rootstocks to non-transgenic scions. Our findings provide experimental evidence that PRms traffics from companion cells into the sieve element via plasmodesmata. Once loaded into the sieve element, PRms is freely mobile within the phloem and it is translocated from source to sink tissues. Collectively, these results reveal the long-distance transport of the PRms protein through the phloem in transgenic tobacco plants.