Seed Coat: Associated Invertases of Fava Bean Control Both Unloading and Storage Functions: Cloning of cDNAs and Cell Type: Specific Expression

Abstract

We have studied the molecular physiology of photosynthate unloading and partitioning during seed development of fava bean (Vicia faba). During the prestorage phase, high levels of hexoses in the cotyledons and the apoplastic endospermal space are correlated with activity of cell wall-bound invertase in the seed coat. Three cDNAs were cloned. Sequence comparison revealed genes putatively encoding one soluble and two cell wall-bound isoforms of invertase. Expression was studied in different organs and tissues of developing seeds by RNA gel analysis, in situ hybridization, enzyme assay, and enzyme activity staining. One extracellular invertase gene is expressed during the prestorage phase in the thin-walled parenchyma of the seed coat, a region known to be the site of photoassimilate unloading. We propose a model for an invertase-mediated unloading process during early seed development and the regulation of cotyledonary sucrose metabolism. After unloading from the seed coat, sucrose is hydrolyzed by cell wall-bound invertases. Thus, invertase contributes to establish sink strength in young seeds. The resultant hexoses are loaded into the cotyledons and control carbohydrate partitioning via an influence on the sucrose synthase/sucrose-phosphate synthase pathway. The developmentally regulated degradation of the thin-walled parenchyma expressing the invertase apparently initiates the storage phase. This is characterized by a switch to a low sucrose/hexoses ratio. Feeding hexoses to storage-phase cotyledons in vitro increases the sucrose-phosphate synthase/sucrose synthase ratio and changes carbohydrate partitioning in favor of sucrose. Concomitantly, the transcript level of the major storage product legumin B is downregulated.

Sucrolytic Enzyme Activities in Cotyledons of the Faba Bean (Developmental Changes and Purification of Alkaline Invertase)

“…The present study was extended to seed maturity (80 DAA) and was based on plants grown in the field rather than in growth chambers. However, in agreement with Weber et al (1995), we have demonstrated that acid invertase activity is extremely low in the reserve cotyledon tissue throughout development. Heim et al (1993) proposed that storage product synthesis is dependent on the Suc synthase pathway and a low hexose to SUC ratio.…”

“…However, no comparative data have been presented on invertase and SUC synthase enzyme activities in the seed coats. The work of Weber et al (1995) concentrates primarily on the prestorage phase of seed development, and, although Heim et al (1993) present results for the storage phase of seed development, they do so only as far as 50 d after flowering.…”

“…The recent manuscript of Weber et al (1995) attributes a key role for seed coat, cell wall-bound acid invertase activity in the generation of sink strength during early seed development in V. faba. Transcripts encoding Suc synthase are also detectable in seed coats at a similar stage in development, corresponding with the deposition of transient starch (Heim et al, 1993).…”

New Insights into Roles of Cell Wall Invertase in Early Seed Development Revealed by Comprehensive Spatial and Temporal Expression Patterns of GhCWIN1 in Cotton

“…Therefore, CWINs are central to phloem unloading and carbon partitioning, especially in cellular sites lacking symplasmic (plasmodesmal) connections (Patrick and Offler, 2001;Weschke et al, 2003). A positive correlation between CWIN activity and seed development has been observed in maize (Zea mays; Cheng et al, 1996;Vilhar et al, 2002;Chourey et al, 2006), faba bean (Vicia faba; Weber et al, 1995Weber et al, , 1996, barley (Hordeum vulgare; Weschke et al, 2003;Sreenivasulu et al, 2004), rice (Oryza sativa; Hirose et al, 2002;Wang et al, 2008), and tomato (Solanum lycopersicum; Zanor et al, 2009). With an exception in tobacco (Nicotiana tabacum; Tomlinson et al, 2004), high and low Glc-to-Suc ratios generally favor cell division or promote cell differentiation, respectively, during seed development (for review, see Weber et al, 2005).…”

Invertases. Primary Structures, Functions, and Roles in Plant Development and Sucrose Partitioning

“…The authors reported that Suc synthase had consistently higher activity than invertase throughout endosperm development, and that invertase activity did not change appreciably. Weber et al (1995) studied the molecular physiology of photosynthate unloading and partitioning during seed development of fava bean and reached the opposite conclusion. During the prestorage phase, they found that high levels of hexoses in the cotyledons and the apoplastic endospermal space were correlated with high levels of cell wall-bound invertase in the seed coat.…”