Pg. Caimi et al., FRUCTAN ACCUMULATION AND SUCROSE METABOLISM IN TRANSGENIC MAIZE ENDOSPERM EXPRESSING A BACILLUS-AMYLOLIQUEFACIENS SACB GENE, Plant physiology, 110(2), 1996, pp. 355-363
Over 40,000 species of plants accumulate fructan, beta-2-1- and beta-2
-6-linked polymers of fructose as a storage reserve. Due to their high
fructose content, several commercial applications for fructans have b
een proposed. However, plants that accumulate these polymers are not a
gronomically suited for large-scale cultivation or processing. This st
udy describes the transformation of a Bacillus amyloliquefaciens SacB
gene into maize (Zea mays L.) callus by particle bombardment. Tissue-s
pecific expression and targeting of the SacB protein to endosperm vacu
oles resulted in stable accumulation of high-molecular-weight fructan
in mature seeds. Accumulation of fructan in the vacuole had no detecta
ble effect on kernel development or germination. Fructan levels were f
ound to be approximately 9-fold higher in sh(2) mutants compared to wi
ld-type maize kernels. In contrast to vacuole-targeted expression, sta
rch synthesis and endosperm development in mature seeds containing a c
ytosolically expressed SacB gene were severely affected. The data demo
nstrate that hexose resulting from cytosolic SacB activity was not uti
lized for starch synthesis. Transgenic seeds containing a chimeric Sac
B gene provide further evidence that the dominant pathway for starch s
ynthesis in maize endosperm is through uridine diphosphoglucose cataly
zed by the enzyme sucrose synthase.