A maize invertase clone

--Karen E. Koch, Jian Xu and Donald R. McCarty

Invertase catalyzes one of two enzymatic reactions that can cleave sucrose (sucrose + H2O -> glucose + fructose). The other is mediated by the reversible sucrose synthase reaction (sucrose + UDP <-> UDP-glucose + fructose). Sucrose breakdown is essential to growth and development of kernels and other non-photosynthetic tissues which survive on sucrose imported from phloem. Both invertase and sucrose synthase are considered critical to sucrose import, although their respective significance varies depending on the tissue and stage of development. In kernels, for example, invertase is very active in the pedicel area throughout development, where it hydrolyzes much, if not most, of the sucrose enroute into the growing grain. Invertase is also active in maize root tissues where its function may be related to sucrose import/retrieval, symbiotic relationships, or osmotic adjustment.

A cDNA clone was obtained from a maize root tip cDNA library (Clonetech, Palo Alto, CA) by probing with a tomato invertase clone from Ellen Klan, E and Alan Bennett, A (UC-Davis) (Klan and Bennett, Plant Physiol., in press). Partial sequence of the 1.2kb maize clone showed 85% homology at the amino acid level to the tomato invertase, 60% to that of carrot root, and 45% to yeast invertase. The high degree of homology to the tomato gene and the presence of other highly conserved residues in the deduced amino acid sequence suggest strongly that this clone encodes a plant invertase. Eleven other clones ranging from 1.2 to 2.5kb were subsequently isolated from the same library through homology to the initial 1.2kb maize clone.

Northern blot hybridizations to root tip RNA showed a broad band at ca. 2.5kb, corresponding to the length of the longest homologous cDNA clone. The size of this mRNA suggests a protein with a sizeable extension relative to other plant invertases. Its structure and function will be of interest.

The existence of two closely related, unlinked invertase genes in maize was indicated by Southern blot analysis of DNA from two parent lines and F2 progeny (independent assortment of two strongly hybridizing bands was evident). It is not yet known whether both genes are expressed.

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