Apoplasmic loading in the rice phloem supported by the presence of sucrose synthase and plasma membrane-localized proton pyrophosphatase

Although Oryza sativa (rice) is one of the most important cereal crops, the mechanism by which sucrose, the major photosynthate, is loaded into its phloem is still a matter of debate. Current opinion holds that the phloem loading pathway in rice could involve either a symplasmic or an apoplasmic route. It was hypothesized, on the basis of a complementary body of evidence from arabidopsis, which is an apoplasmic loader, that the membrane specificity of proton pyrophosphatases (H(+)-PPases; OVPs) in the sieve element-companion cell (SE-CC) complexes of rice source leaves would support the existence of either of the aforementioned phloem loading mechanisms. Additionally, it was contended that the presence of sucrose synthase in the SE-CC complexes would be consistent with an apoplasmic sucrose loading route in rice.