Abstract
Stem parenchyma is a major cell type that serves key metabolic functions for the plant especially in large grasses, such as sugarcane and sweet sorghum, where it serves to store sucrose or other products of photosynthesis. It is therefore desirable to understand the metabolism of this cell type as well as the mechanisms by which it provides its function for the rest of the plant. Ultimately, this information can be used to selectively manipulate this cell type in a controlled manner to achieve crop improvement. In this study, we show that Brachypodium distachyon is a useful model system for stem pith parenchyma biology. Brachypodium can be grown under condition where it resembles the growth patterns of important crops in that it produces large amounts of stem material with the lower leaves senescing and with significant stores of photosynthate located in the stem parenchyma cell types. We further characterize stem plastid morphology as a function of tissue types, as this organelle is central for a number of metabolic pathways, and quantify gene expression for the four main classes of starch biosynthetic genes. Notably, we find several of these genes differentially regulated between stem and leaf. These studies show, consistent with other grasses, that the stem functions as a specialized storage compartment in Brachypodium.
Highlights
In many grasses, the stem parenchyma cells function as a heterotrophic tissue for the deposition of excess photosynthate for medium- to long-term storage [1,2,3]
We propose the use of Brachypodium distachyon as a model system for studying mediumto long-term carbon storage in heterotrophic stem pith parenchyma cells
Growth of Brachypodium distachyon Bd21 under both long and short day conditions result in a short phenotype with relatively little stem material, and so shows little resemblance to the growth phenotype of larger grasses such as sugarcane, sorghum and maize grown in conventional cropping systems, where a majority of the growth phase concerns stem formation and elongation
Summary
The stem parenchyma cells function as a heterotrophic tissue for the deposition of excess photosynthate for medium- to long-term storage [1,2,3]. This mechanism is in contrast to short-term energy reservoirs, such as ‘transitory starch’ in leaves, which allow respiration in the dark period and are adjusted for depletion at the end of the night [4]. The medium- to long-term storages become important under stress conditions These stores enable the plant to fuel growth processes during less favorable growth conditions where current levels of photosynthesis cannot meet current demand for energy and carbon [5]. Storage of starch and sucrose has been described in PLOS ONE | DOI:10.1371/journal.pone.0173095 March 1, 2017
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