A Possible Role of the Aleurone Expressed Gene HvMAN1 in the Hydrolysis of the Cell Wall Mannans of the Starchy Endosperm in Germinating Hordeum vulgare L. Seeds.

Raquel Iglesias-Fernández,Elena Pastor-Mora,Jesús Vicente-Carbajosa,Pilar Carbonero

doi:10.3389/fpls.2019.01706

Abstract

The barley endo-β-mannanase (MAN) gene family (HvMAN1-6) has been identified and the expression of its members analyzed throughout different plant organs, and upon grain development and germination. The HvMAN1 gene has been found to be highly expressed in developing and germinating grains. The MAN (EC 3.2.1.78) enzymatic activity gets a maximum in grains at 48 h of germination (post-germination event). Immunolocalization of mannan polymers in grains has revealed the presence of these polysaccharides in the endosperm cell walls (CWs). By mRNA in situ hybridization assays, the HvMAN1 transcripts have been localized to the aleurone layer, but not to the dead starchy endosperm cells. These data suggest that MAN1 is synthesized in the aleurone layer during early grain imbibition and moves potentially through the apoplast to the endosperm where the hydrolysis of the mannan polymers takes place after germination sensu stricto. Hence, mannans in the starchy endosperm CWs, besides their structural function, could be used as reserve compounds upon barley post-germination.

Highlights

The monospermic fruit of the Poaceae is made up by the seed and the pericarp
Many other hydrolytic enzymes induced by GA and repressed by abscisic acid (ABA) have been described as involved in reserve mobilization upon monocotyledonous, and dicotyledonous seed germination and post-germination, such as cathepsin b-like cysteine proteases, that are endopeptidases mainly associated to the hydrolysis of seed storage proteins (SSPs; Mena et al, 2002; Isabel-Lamoneda et al, 2003; Martínez et al, 2003; MorenoRisueno et al, 2007; Tan-Wilson and Wilson, 2012; IglesiasFernández et al, 2014; Díaz-Mendoza et al, 2019)
The MAN protein sequences from Brachypodium (BdMAN1-6), Arabidopsis (AtMAN1-7) together with those from Hordeum vulgare were used to construct an unrooted dendrogram by using the neighbor-joining algorithm

Summary

Introduction

The monospermic fruit of the Poaceae (grain) is made up by the seed and the pericarp (fruit tissue proper). It has been proposed that starch and proteins and other structural polysaccharides, such as bmannans and b-glucans present in the endosperm cell walls (CWs), could be hydrolyzed and remobilized as reserve compounds to the growing embryo (Barrero et al, 2009; Guillon et al, 2012; González-Calle et al, 2015). This remobilization process needs the synthesis and secretion of appropriate hydrolytic enzymes (mannanases, glucanases, etc.). Many other hydrolytic enzymes induced by GA and repressed by abscisic acid (ABA) have been described as involved in reserve mobilization upon monocotyledonous, and dicotyledonous seed germination and post-germination, such as cathepsin b-like cysteine proteases, that are endopeptidases mainly associated to the hydrolysis of seed storage proteins (SSPs; Mena et al, 2002; Isabel-Lamoneda et al, 2003; Martínez et al, 2003; MorenoRisueno et al, 2007; Tan-Wilson and Wilson, 2012; IglesiasFernández et al, 2014; Díaz-Mendoza et al, 2019)

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