Molecular basis of the increase in invertase activity elicited by gravistimulation of oat-shoot pulvini

An asymmetric (top vs. bottom) increase in invertase activity is elicited by gravistimulation in oat-shoot pulvini starting within 3 h after treatment. In order to analyze the regulation of invertase gene expression in this system, we examined the effect of gravistimulation on invertase mRNA induction. Total RNA and poly (A)+RNA, isolated from oat pulvini, and two oligonucleotide primers, corresponding to two conserved amino-acid sequences (NDPNG and WECPD) found in invertase from other species, were used for the polymerase chain reaction (PCR). A partial-length cDNA (550 base pairs) was obtained and characterized. There was a 52% deduced amino-acid sequence homology to that of carrot beta-fructosidase and a 48% homology to that of tomato invertase. Northern blot analysis showed that there was an obvious transient accumulation of invertase mRNA elicited by gravistimulation of oat pulvini. The mRNA was rapidly induced to a maximum level at 1 h following gravistimulation treatment and gradually decreased afterwards. The mRNA level in the bottom half of the oat pulvinus was significantly higher (five-fold) than that in the top half of the pulvinus tissue. The induction of invertase mRNA was consistent with the transient enhancement of invertase activity during the graviresponse of the pulvinus. These data indicate that the expression of the invertase gene(s) could be regulated by gravistimulation at the transcriptional and/or translational levels. Southern blot analysis showed that there were four genomic DNA fragments hybridized to the invertase cDNA. This suggests that an invertase gene family may exist in oat plants.