Characterization of the role of apoplastic invertase in plant development

The cell wall acid invertase has been proposed to be involved in establishing metabolic sinks by maintaining a steep sucrose concentration gradient between source and sink organs. Sucrose unloaded to the apoplast is converted to hexoses by cell wall invertase, thereby allowing the continued efflux of sucrose from the phloem. Hexose molecules are further transported to sink tissues by hexose/H+ symporters. The cooperation of hexose transporters and extracellular cell wall invertase is important for providing non photosynthetic tissues with organic compounds. ^ Using reverse genetics the role of cell wall invertase in higher plants has been investigated. Transgenic carrots expressing carrot cell wall invertase in both sense and antisense orientation or a reporter gene under the direction of either an ethylene inducible promoter or a tissue specific promoter were generated. The transgenic carrot calli expressing sense construct under the ethylene inducible promoter show reduced apoplastic invertase activity relative to the control. The reduction of cell wall invertase could be due to cosuppression. ^ A second generation of Arabidopsis antisense constructs were generated. Defined regions within the third exon of Arabidopsis cell wall invertase isoform 1 (AINV1) and isoform 2 (AINV2) were cloned in antisense orientation behind the CaMV 35S promoter. Two independently transformed lines of AINV1 antisense plants display a short silique phenotype. Anthers in these two transgenic lines fail to extend beyond the stigmas. As a result siliques of these two transformants contain a large number of unfertilized ovules and a small number of developing seeds due to a perturbation in the development of the stamens. Data on reciprocal crosses between wild-type and transgenic lines indicate that the antisense transgene has no effect on the functionality of pollen grains and carpel development after pollination. Western analyses indicate that apoplastic invertase was increased in flowers and siliques of these two transformants. Northern blot data showed that this increase was due to a higher expression of AINV2 transcripts. The cause of this stimulation may be a physiological compensation in AINV1 antisense plants. The obtained data provide evidence for the function of invertase in sink tissue development and the interrelationships of invertase isoforms in higher plants.