Photoassimilation, assimilate translocation and plasmodesmal biogenesis in the source leaves of arabidopsis thaliana grown under an increased atmospheric CO₂ Concentration

Using 18-day-old Arabidopsis thaliana seedlings grown under increased (780 p.p.m., experimental plants) or ambient (390 p.p.m., control plants) CO ₂ conditions, we evaluated ¹⁴CO₂ photoassimilation in and translocation from representative source leaves. The total ¹⁴CO₂ photoassimilation amounts increased in the third leaves of the experimental plants in comparison with that found for the third leaves of the control plants, but the rates were comparable for the first leaves of the two groups. In contrast, translocation of labeled assimilates doubled in the first leaves of the experimental group, whereas translocation was, at best, passively enhanced even though photoassimilation increased in their third leaves. The transcript levels of the companion cell-specific sucrose:H⁺ symporter gene SUC2 were not significantly affected in the two groups of plants, whereas those of the sucrose effluxer gene SWEET12 and the sieve element-targeted sucrose:H⁺ symporter gene SUT4 were up-regulated in the experimental plants, suggesting up-regulation of SUT4-dependent apoplastic phloem loading. Compared with SUC2, SUT4 is a minor component that is expressed in companion cells but functions in sieve elements after transfer through plasmodesmata. The number of aniline blue-stained spots for plasmodesma-associated callose in the midrib wall increased in the first leaf of the experimental plants but was comparable in the third leaf between the experimental and control plants. These results suggest that A. thaliana responds to greater than normal concentrations of CO₂ differentially in the first and third leaves in regards to photoassimilation, assimilate translocation and plasmodesmal biogenesis.