Sucrose synthase expression pattern in young maize leaves: implications for phloem transport.

To study whether sucrose synthase (SuSy, EC 2.4.1.13) plays a role in phloem loading/unloading, we have directly assessed the distribution of SuSy transcripts and protein in young maize (Zea mays L.) leaves. Our data show that at the RNA and protein levels there is no expression of SuSy in phloem of sink tissue and that SuSy expression is specifically excluded from this tissue. These data, in conjunction with others in the literature, indicate that the localisation of SuSy to phloem may be limited to source tissue, whereas a non-vascular expression is linked with sink tissue activity.