A SIMILAR DICHOTOMY OF SUGAR MODULATION AND DEVELOPMENTAL EXPRESSION AFFECTS BOTH PATHS OF SUCROSE METABOLISM - EVIDENCE FROM A MAIZE INVERTASE GENE FAMILY

Invertase and sucrose synthase catalyze the two known paths for the fi rst step in carbon use by sucrose-importing plant cells. The hypothesi s that sugar-modulated expression of these genes could provide a means of import adjustment was initially suggested based on data from sucro se synthases alone; however, this hypothesis remained largely conjectu ral without critical evidence for invertases. Toward this end, a famil y of maize invertases was cloned and characterized. Here, we show that invertases are indeed sugar modulated and, surprisingly, like the suc rose synthase genes, fall into two classes with contrasting sugar resp onses, In both families, one class of genes is upregulated by increasi ng carbohydrate supply (Sucrose synthase1 [Sus1] and Invertase2 [Ivr2] ), whereas a second class in the same family is repressed by sugars an d upregulated by depletion of this resource (Shrunken1 [Sh1] and Inver tase1 [Ivr1]), The two classes also display differential expression du ring development, with sugar-enhanced genes (Sus1 and Ivr2) expressed in many importing organs and sugar-repressed, starvation-tolerant gene s (Sh1 and Ivr1) upregulated primarily during reproductive development . Both the Ivr1 and Ivr2 invertase mRNAs ate abundant in root tips, ve ry young kernels, silk, anthers, and pollen, where a close relationshi p is evident between changes in message abundance and soluble invertas e activity. During development, patterns of expression shift as assimi late partitioning changes from elongating silks to newly fertilized ke rnels, Together, the data support a model for integrating expression o f genes differentially responsive to carbohydrate availability (i.e., feast and famine conditions) with developmental signals, The demonstra tion that similar regulatory patterns occur in both paths of sucrose m etabolism indicates a potential to influence profoundly the adjustment of carbon resource allocation.