Assimilate export inhibition in Sugarcane yellow leaf virus-infected sugarcane is not due to less transcripts for sucrose transporters and sucrose-phosphate synthase or to callose deposition in sieve plates

Sink leaves, source leaves and internodes of four Hawaiian sugarcane cultivars were tested, whether the inhibition of SCYLV-caused assimilate export was due to lower sucrose synthesis activity, less sucrose transporter content or block of solute flow by callose deposition. The transcript levels of sucrose-phosphate synthase (SPSII) and sucrose transporter ShSUT4 were constant throughout sugarcane tissues, whereas the sucrose transporter ShSUT1A was highest in sink leaves and lowest in internodes. No differences in transcript levels were observed between the SCYLV-susceptible and -resistant cultivars and between SCYLV-infected and SCYLV-free sugarcane line. Callose deposition at the sieve plates and the pit fields between sieve tubes and companion cells, visualized by aniline blue-staining, was similar between SCYLV-infected and -free sugarcane. It is concluded that the reduction of assimilate export is neither due to lower sucrose transporter levels nor to a physical block of sieve tubes.