The structure and function of maize scutellum during early stages of germination

Abstract

The embryo in grasses, at grain maturity, comprises the embryonic axis and the scutellum. The scutellum is supposed to be the single cotyledon in the monocotyledoneus embryos and is attached to the embryo axis in the scutelar node. The embryo has the highest concentration of lipid and lipid soluble vitamins in cereal grains. The embryo in grasses, at grain maturity, comprises the embryonic axis and the scutellum. The scutellum is supposed to be the single cotyledon in the monocotyledoneus embryos and is attached to the embryo axis in the scutelar node. The embryo has the highest concentration of lipid and lipid soluble vitamins in cereal grains.

The embryonic axis originates the root, leaves and stem of the new plant. In the mature seed, the embryo axis is formed by the primary root, protected by the coleorhiza, and the stem tip with five or six short internodes and leaf primordia which, as a whole, form the plumule that is surrounded by the coleoptile.

The name scutellum (small shield, in latin) derives from its shield-like shape and it liesbetween the embryonic axis and endosperm. Dissected scutellum constitutes 11% or the kernel mass, and about 90% of the embryo.

During germination, scutellar epithelial cells suffer an elongation that increases the contact surface between the endosperm and the scutellum and facilitates the transport of the nutrients from the endosperm to the embryo. Scutellar cell elongation is inhibited by ABA and salicylic acid, basic and acid pH and high concentrations of sorbitol. Exogenous gibberellins stimulate elongation, but a reduction in gibberellin synthesis or perception does not inhibit it. Elongation is inhibited by sucrose, but not glucose. Transcription and translation inhibitors reduce scutellar cell elongation, indicating that transcription and translation are necessary for the elongation process.

Scutellar epithelium cells play specific roles during germination different to parenchymal cells. So, we expect some differences in the gene expression pattern of this tissue. That’s why we construct a cDNA library using RNA extracted from scutellar epithelial cells 1 day after imbibition and selected them using array hybridization comparing the mRNA accumulated in epithelial cells with the mRNA accumulated in the other scutellar tissues. We identified 30 genes up-regulated in the epithelium. A high proportion of these genes are involved in metabolic processes, the production of energy or in the transport of peptides into the embryo. The roles of 43% of these genes remains undetermined, 27% of them are involved in metabolic processes, 13% in protein synthesis or processing and 7% in cell structure.

One of the identified genes from the macroarrays is a peptide transporter: ZmPTR1. It encodes a non-characterized maize peptide transporter protein which has 587 amino acids with a calculated molecular mass of 64.52 kDa. This maize transporter is predominantly expressed in the scutellar epithelium during germination. ZmPTR1 is also expressed to a less extent in the radicle and the hypocotyl. ZmPTR1 is located in the tonoplast and has high sequence similarity with tonoplast di- and tripeptide transporters AtPTR2, AtPTR4 and AtPTR6 from Arabidopsis thaliana. ZmPTR1 is able to transport at least Ala-Ala dipeptide across the membrane and could have a role in the intracellular transport of di- and tripeptides.

Seed germination is a complex process that requires cell division, expansion and differentiation. It involves the activation of many metabolic pathways and signal transduction processes, which require a great quantity of energy and stored materials which are provided by seed reserves (oil, storage proteins and starch) and the synthesis and/or activation of many proteins. Plant seeds store triacylglycerols (TAGs) into oil bodies (OBs), specialized organelles which serve as an energy reserve during germination and post-germinative growth.

Protein composition analysis of oil bodies from maize embryos during germination identified, in addition to the previously characterized OB-associated proteins, other proteins of diverse function: an embryonic protein DC-8, a globulin 2, 4 proteins with enzymatic activity (protein disulfide isomerase, xylose isomerase, strictosidine synthase and precursor and ATP synthase beta chain), a protein similar to karyopherin- beta-3 (Kap) and a stress induced membrane pore protein involved in membrane transport.

Quantitative subproteomic analysis of germinating related changes in the oil bodies of maize scutellum between dry seeds and 2 dai seeds allowed the identification of new proteins interacting with oil bodies in dry seeds or in germinating seeds. In dry seeds: oleosins, cupins, disulfide isomerases, a nucleoside phosphate kinase, a class IV heat shock protein, an embryonic protein DC-8, a 60S acidic ribosomal protein P0 and a rubber elongation factor protein. In germinating seeds: oleosins, mitochondrial protein Tim17, prohibitin-2 and a manganese superoxide dismutase (Mn-SOD).

ESTRUCTURA Y FUNCIÓN DEL ESCUTELO DE MAÍZ DURANTE LAS PRIMERAS ETAPAS DE LA GERMINACIÓN

Durante la germinación, las células epiteliales del escutelo sufren una elongación que aumenta la superficie de contacto entre el endospermo y el embrión y facilita el transporte de los nutrientes del endospermo al embrión. La elongación de las células del escutelo es inhibida por ABA y ácido salicílico, pH básico y ácido y altas concentraciones de sorbitol. Las giberelinas exógenas estimulan la elongación, pero una reducción en la síntesis de giberelinas o percepción no la inhiben. La elongación es inhibida por la sacarosa, pero no la glucosa. Los inhibidores de la transcripción y traducción reducen el alargamiento de las células escutelares, lo que indica que la transcripción y la traducción son necesarias para el proceso de alargamiento. Se identificaron 30 genes que muestran una expresión diferencial significativa en las células epiteliales del escutelo de un día después de la imbibición. Las funciones de un 43% de estos genes no se conoce, el 27% de ellos están involucrados en los procesos metabólicos, el 13% en la síntesis o la transformación de proteínas y el 7% en la estructura celular. ZmPTR1 codifica un transportador de maíz péptido no ha sido previamente caracterizado, se expresa predominantemente en el epitelio escutelar durante la germinación. ZmPTR1 se expresa también en menor medida en la radícula y del hipocotilo. La proteína ZmPTR1 se localiza en el tonoplasto y es homóloga a las otras proteínas transportadoras de di-y tripéptidos AtPTR2, AtPTR4 y AtPTR6 de Arabidopsis thaliana. ZmPTR1 es capaz de transportar al menos el dipéptido Ala-Ala. El análisis proteómico de las proteínas asociadas a los cuerpos lipídicos en escutelo de maíz durante la germinación ha permitido la identificación de, además de proteínas previamente caracterizadas asociadas OB, otras proteínas de funciones diversas. El Análisis cuantitativo subproteómico de los cambios relacionados con los cuerpos lipídicos en el escutelo de maíz entre semillas secas y semillas 2 dias después de la germinación permitieron la identificación de nuevas proteínas que interactúan con los cuerpos lipídicos en las semillas secas o en la germinación de las semillas.