Effects of active silicon uptake by rice on 29Si fractionation in various plant parts

Rice (Oryza sativa L.) accumulates large amounts of silicon which improves its growth and health due to enhanced resistance to biotic and abiotic stresses. Silicon uptake and loading to xylem in rice are predominantly active processes performed by transporters encoded by the recently identified genes Lsi1 (Si influx transporter gene) and Lsi2 (Si efflux transporter gene). Silicon deposition in rice during translocation to upper plant tissues is known to discriminate against the heavier isotopes (29)Si and (30)Si, resulting in isotope fractionation within the plant. We analyzed straw and husk samples of rice mutants defective in Lsi1, Lsi2 or both for silicon content and delta(29)Si using isotope ratio mass spectrometry (IRMS) and compared these results with those for the corresponding wild-type varieties (WT). The silicon content was higher in husk than in straw. All the mutant rice lines showed clearly lower silicon content than the WT lines (4-23% Si of WT). The delta(29)Si was lower in straw and husk for the uptake defective mutant (lsi1) than for WT, albeit delta(29)Si was 0.3 per thousand higher in husk than in straw in both lines. The effect of defective efflux (lsi2) differed for straw and husk with higher delta(29)Si in straw, but lower delta(29)Si in husk while WT showed similar delta(29)Si in both fractions. These initial results show the potential of Si isotopes to enlighten the influence of active uptake on translocation and deposition processes in the plant.