Tissue-specific induction of the mRNA for an extracellular invertase isoenzyme of tomato by brassinosteroids suggests a role for steroid hormones in assimilate partitioning

Brassinosteroids (BRs) induce various growth responses when applied exogeno usly to plant tissues, and the analysis of biosynthetic mutants reveals an essential role for plant growth and development. Only a few BR-regulated ge nes have been identified so far, and the corresponding gene products are as sumed to be involved in cell elongation. The present study shows that BR gr owth responses are linked to the regulation of carbohydrate metabolism by i nduction of the mRNA for the key enzyme of an apoplastic phloem-unloading p athway. Addition of BRs to autotrophic tomato suspension culture cells spec ifically elevates the activity of cell-wall-bound invertase, whereas the in tracellular invertase activities were not affected. This enhanced enzyme ac tivity was shown to correlate with the induction of the mRNA of extracellul ar invertase Lin6, whereas the mRNA levels of the other three extracellular invertase isoenzymes were not affected. The induction level induced by dif ferent BRs correlates with their growth-promoting activity. The physiologic al significance of this regulation is further supported by the low concentr ations and short incubation times required to induce Lin6 mRNA. This regula tory mechanism results in an elevated uptake of sucrose via the hexose mono mers, and thus an increased supply of carbohydrates to the BR-treated cells . Experiments with tomato seedlings showed that the localized BR-dependent growth response of the hypocotyl elongation zone was accompanied by a speci fic induction of Lin6 mRNA that is restricted to the corresponding tissues. This study demonstrates a role of BRs in tissue-specific source/sink regul ation.