Cloning and characterization of full-length cDNA encoding cell-wall invertase from maize.

Invertases (P-fructofuranosidase, EC 3.2.1.26) have been studied in many organisms, including plants. At least two forms of the enzyme, soluble and particulate, are a common feature to a11 organisms. The soluble form is predominantly localized to vacuoles and cytoplasm and the particulate form, in a11 cases tested, is ionically bound to the cell wall (hence, cell-wall-bound form) and is readily extractable with high salt concentration (Sturm and Chrispeels, 1990; von Schaewen et al., 1990; Weil and Rausch, 1990). Each of the two forms of invertase is known to have severa1 isozymes (Jaynes and Nelson, 1971; Unger et al., 1994). Physiological role($ of invertases in various tissues is not well understood, although it is believed to play an important role in Suc mobilization between source and sink organs of the plant. In both tomato and tobacco transgenic plants (Dickinson et al., 1991; von Schaewen et al., 1990, respectively), expression of a chimeric invertase gene in apoplastic regions led to serious interruption in SUC export and much inhibition of growth. These data suggest that cell-wall invertase plays an important role in phloem loading. Consistent with this interpretation are the genetic data from cultivated and wild relatives of tomato that show that accumulation of Suc or hexoses in ripening fruits is entirely dependent on a monogenic trait related to invertase expression in this tissue (Chetelat et al., 1993). Similarly in maize (Zea mays L.), both cell-wall-bound and soluble forms of invertases play a rate-limiting role in the normal development of seed. In particular, a single-gene mutation miniature 1 (mn 1) seed marked by a loss of 80% of the seed weight is associated with invertase deficiency in developing seed (Miller and Chourey, 1992). Genetic data suggest that invertase deficiency is the causal basis of miniature seed phenotype (Miller and Chourey, 1992; Cheng and Chourey, 1994). At the molecular level, the most detailed analyses of invertases in plants are done in carrot (Sturm and