Abstract
Starch is of fundamental importance for plant development and reproduction and its optimized molecular assembly is potentially necessary for correct starch metabolism. Re-structuring of starch granules in-planta can therefore potentially affect plant metabolism. Modulation of granule micro-structure was achieved by decreasing starch branching and increasing starch-bound phosphate content in the barley caryopsis starch by RNAi suppression of all three Starch Branching Enzyme (SBE) isoforms or overexpression of potato Glucan Water Dikinase (GWD). The resulting lines displayed Amylose-Only (AO) and Hyper-Phosphorylated (HP) starch chemotypes, respectively. We studied the influence of these alterations on primary metabolism, grain composition, starch structural features and starch granule morphology over caryopsis development at 10, 20 and 30 days after pollination (DAP) and at grain maturity. While HP showed relatively little effect, AO showed significant reduction in starch accumulation with re-direction to protein and β-glucan (BG) accumulation. Metabolite profiling indicated significantly higher sugar accumulation in AO, with re-partitioning of carbon to accumulate amino acids, and interestingly it also had high levels of some important stress-related metabolites and potentially protective metabolites, possibly to elude deleterious effects. Investigations on starch molecular structure revealed significant increase in starch phosphate and amylose content in HP and AO respectively with obvious differences in starch granule morphology at maturity. The results demonstrate that decreasing the storage starch branching resulted in metabolic adjustments and re-directions, tuning to evade deleterious effects on caryopsis physiology and plant performance while only little effect was evident by increasing starch-bound phosphate as a result of overexpressing GWD.
Highlights
During cereal caryopsis development, caryopses act as the main sink tissue while the mature leaves, stems or green parts of the ear are the main sites for photosynthesis
Different developmental time points were investigated from 10 days after pollination (DAP) to maturity at four developmental stages: 10, 20, 30 DAP and at the mature dry grain stage (MDG, when the grain water content has decreased beneath 10%), to study the grain development phase and the post-desiccation phase (S1 Fig)
Endosperm specific silencing of Starch Branching Enzyme (SBE) resulted in decreased starch accumulation and grain dry weight, while the Glucan Water Dikinase (GWD) over-expressor remained unaltered
Summary
During cereal caryopsis (grain) development, caryopses act as the main sink tissue while the mature leaves, stems or green parts of the ear are the main sites for photosynthesis. The barley caryopsis consists of filial tissues (endosperm and embryo) surrounded by maternal tissue, palea, lemma and the pericarp, the pericarp containing the vascular tissues is the main site for phloem unloading [1]. Transient starch is deposited in the pericarp for a short period from anthesis to 4 days after fertilization (DAF) [2]. It is subsequently re-mobilized after a few days following the disintegration of pericarp cells [3]. The endosperm in the caryopsis is the main site for storage reserve compounds, and starch is the major storage product. Storage starch accumulation usually begins in the wings of the endosperm starting at around 6 DAF and continuing throughout the main grain filling period [2]. Deposition of (1,3;1,4)-β-D— glucans (β-glucan (BG) or mixed linkage glucan) begins slightly earlier at about 5 DAP [5]
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