Phloem loading in rice leaves depends strongly on the apoplastic pathway.

Phloem loading is the first step in sucrose transport from source leaves to sink organs. The phloem loading strategy in rice remains unclear. To determine the potential phloem loading mechanism in rice, yeast invertase (INV) was overexpressed specifically in the cell wall by 35S promoter to block sugar transmembrane loading in rice. The transgenic lines exhibited obvious phloem loading suppression characteristics accompanied by the accumulation of sucrose and starch, restricted vegetative growth and decreased grain yields. The decreased sucrose exudation rate with p-chloromercuribenzenesulfonic acid (PCMBS) treatment also indicated that rice actively transported sucrose into phloem. Moreover, the expression level of OsSUT1 was much higher than that of other plasma membrane localized OsSUTs in the source leaf. Cross sections of the GUS transgenic plant showed that the signals of OsSUT1 and OsSUT5 occurred in the phloem companion cells. The ossut1 and ossut4 mutants presented a decrease of grain yield, implying important roles of OsSUTs in phloem loading. Based on these results, we conclude that rice uses the apoplastic loading as a major phloem loading strategy.