Whether rice uses an energy-dependent apoplasmic or a passive symplasmic strategy of phloem loading is still not resolved.
Regmi et al. hypothesized that the membrane-specific localization of proton pyrophosphatases (H+-PPases) in the sieve element–companion cell (SE-CC) complexes in minor veins would reveal whether rice uses either of the two phloem loading mechanisms. Like in the apoplasmic loading species Arabidopsis, immunogold labelling revealed that H+-PPases in rice were preferentially localized at the plasma membrane in the SE-CC complexes, suggesting that rice also uses an active sucrose loading scheme. Moreover, ultrastructural localization of sucrose synthase at the SE-CC complexes supported this interpretation.
Kamesh C. Regmi, Shangji Zhang, Roberto A. Gaxiola, 2015, 'Apoplasmic loading in the rice phloem supported by the presence of sucrose synthase and plasma membrane-localized proton pyrophosphatase', Annals of Botany, p. mcv174 http://dx.doi.org/10.1093/aob/mcv174