Genetic control of cell wall invertases in developing endosperm of maize

Abstract

We show here that the total invertase activity in developing seeds of maize is due to two cell wall invertase (CWI) genes, Incw1 and Incw2 (Mn1). Our previous results have shown that loss-of-function mutations at the Mn1 locus lead to the miniature-1 (mn1) seed phenotype, marked by a loss of >70% of seed weight at maturity. The mn1 seed mutant is, however, non-lethal presumably because it retains a residual low level, approximately 1%, of the total CWI activity relative to the Mn1 endosperm throughout seed development. Evidence here shows that the residual activity in the mn1 mutant is encoded by the Incw1 gene. RNA level analyses, especially quantitative real-time PCR studies, showed significant spatial and temporal heterogeneity in the expression of the two CWI genes in the developing endosperm. The Mn1-encoded Incw2 transcripts were seen at the highest levels in the basal region (the sugar unloading zone) during the early phase of cell division and elongation in the endosperm. In contrast, the highest levels of Incw1 transcripts were seen in the storage phase in both the upper (storage cells) and the lower parts of the endosperm. Protein and enzyme level analyses, however, appeared to show a lack of concordance with the RNA level of expression in both the Mn1 and mn1 endosperms, indicating a possibility of post-transcriptional control in the expression of these two genes. Collectively, the data suggest an important role for apoplastic cleavage of sucrose throughout the duration of seed development; and, of the two isoforms, the INCW2 appears to control metabolic flux of sugar utilization in the developing endosperm.

Cytokinins and Their Possible Role in Seed Size and Seed Mass Determination in Maize

“…The mn1-89 allele encodes normal or higher levels of Incw2 RNA compared to the wild type, but exceedingly low levels, ~ 6% of the Mn1, of the INCW2 protein and enzyme activity (Cheng et al, 1996). More importantly for the studies here, genetic analyses showed that the mn1-89 allele is semi-dominant in seed phenotype (Cheng et al, 1996) when crossed to the standard null allele, mn1-1, which has approximately ~1% of the wild type levels of invertase activity encoded by the non-allelic gene, Inc1, a paralog of the Mn1 locus (Chourey et al, 2006). The reciprocal hybrids from such crosses yield gene-dose dependent invertase activity levels of ~4 and 2% encoded by two and one mn1-89 alleles, respectively, present in the triploid endosperms.…”

“…3). It is noteworthy that there is a discrepancy between protein and enzyme activity levels and Incw2 transcript levels in Mn1 and mn1 endosperms (Chourey et al, 2006), since high RNA level in the homozygous mn1-89 mutant results in only ~6 % of the Mn1 invertase activity (Cheng et al, 1996). It has been suggested that a large proportion of INCW2 is dispensable without a significant change in seed phenotype (Cheng et al, 1996).…”

“…The mn1 seed mutation is associated with a conspicuous loss of seed mass at maturity and small seed size, related to the decreased cell number and size in developing endosperm (Vilhar et al, 2002). Several lines of correlative evidence from various plant species are available to show that cell wall invertase plays a major role in controlling sink strength through source-to-sink unloading of sucrose (Chourey et al, 2006). As expected from the invertase-deficiency, the mn1 mutation also exhibits much altered sugar metabolism relative to Mn1 (LeClere et al, 2010).…”

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).…”

Influence of sugars and hormones on the genes involved in sucrose metabolism in maize endosperms

“…In addition, Ivr2 was found to be specifically induced under water stress (Kim et al, 2000a;Trouverie et al, 2003). Incw1 and Incw2 expression showed significant spatial and temporal heterogeneity in the developing endosperm (Chourey et al, 2006). Three genes encode maize sucrose synthase, including Sh1, Sus1, and Sus2 (Werr et al, 1985;Gupta et al, 1988;Carlson et al, 2002).…”