Spatiotemporal Profiling of Starch Biosynthesis and Degradation in the Developing Barley Grain  

Abstract

Barley (Hordeum vulgare) grains synthesize starch as the main storage compound. However, some starch is degraded already during caryopsis development. We studied temporal and spatial expression patterns of genes coding for enzymes of starch synthesis and degradation. These profiles coupled with measurements of selected enzyme activities and metabolites have allowed us to propose a role for starch degradation in maternal and filial tissues of developing grains. Early maternal pericarp functions as a major short-term starch storage tissue, possibly ensuring sink strength of the young caryopsis. Gene expression patterns and enzyme activities suggest two different pathways for starch degradation in maternal tissues. One pathway possibly occurs via a-amylases 1 and 4 and b-amylase 1 in pericarp, nucellus, and nucellar projection, tissues that undergo programmed cell death. Another pathway is deducted for living pericarp and chlorenchyma cells, where transient starch breakdown correlates with expression of chloroplast-localized b-amylases 5, 6, and 7, glucan, water dikinase 1, phosphoglucan, water dikinase, isoamylase 3, and disproportionating enzyme. The suite of genes involved in starch synthesis in filial starchy endosperm is much more complex than in pericarp and involves several endosperm-specific genes. Transient starch turnover occurs in transfer cells, ensuring the maintenance of sink strength in filial tissues and the reallocation of sugars into more proximal regions of the starchy endosperm. Starch is temporally accumulated also in aleurone cells, where it is degraded during the seed filling period, to be replaced by storage proteins and lipids.

Significance of Light, Sugar, and Amino Acid Supply for Diurnal Gene Regulation in Developing Barley Caryopses

“…7). As reported earlier (Sun et al, 1998;Burton et al, 1999;Radchuk et al, 2009), the transcripts of HvSBEIIb and HvLD were mainly found in the endosperm and are thus likely to be involved in the biosynthesis of storage starch (Table I; Fig. 8).…”

“…9; Supplemental Table S7). This observation lends support to a role of HvBAM5 in the degradation of transitory starch during the night with granular starch and/or linear maltodextrins as substrate (Fulton et al, 2008;Radchuk et al, 2009). Moreover, activation of HvBAM5 by Suc starvation at the end of the night contrasted to the activity of genes involved in the initial metabolism of Suc, such as HvCWINV1, HvSUT2, and HvSTP1 (Fig.…”

“…The combined analysis with the data set of Sreenivasulu et al (2008) revealed only a few genes that were unambiguously located to the pericarp and endosperm (Table I). However, the comparison with recent published studies on the spatial distribution of barley transcripts (Radchuk et al, 2009) and suborgan localization using qPCR (Fig. 8) allowed us to identify the predominant locations of several of these genes and to shed light on several diurnal regulatory aspects occurring in the developing caryopsis.…”

“…The expression of these genes peaked during the middle to late light phase, when sugar supply was at its maximum. The HvSSIIIb gene (Radchuk et al, 2009), which is preferentially expressed in the endosperm, was strongly induced in anticipation of the light rather than being correlated to high sugar levels (Figs. 2 and 7).…”

“…Metabolic and regulatory changes throughout the development of the barley (Hordeum vulgare) caryopsis, from early flowering to late seed maturation and germination processes, have been studied to a great extent (Sreenivasulu et al, 2006Radchuk et al, 2009). However, little is known about diurnal adaptations in the cereal caryopses, and several aspects pose intriguing questions regarding the regulation of these processes.…”