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Maize Kernel Development

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This is an authoritative book that acts as a guide to understanding maize kernel development. Written by a team of experts, it covers topics spanning pre- and post-fertilization events, embryo and endosperm development, grain filling and maturation, and factors influencing crop yield. It explores the significance of maize and other cereal grains, existing hypotheses and research, and important gaps in our knowledge and how we might fill them. This is a valuable resource for researchers of maize and other cereals, and anyone working on basic or applied science in the fields of seed development, plant genetics, and crop physiology.

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17 Chapters

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1: Kernel Evolution: From Teosinte to Maize


1  Kernel Evolution: From Teosinte to Maize

Sherry A. Flint-Garcia*

U.S. Department of Agriculture, Agricultural Research Service, Columbia, Missouri, USA

1.1 Introduction

1.2 Domestication

Maize is the most productive and highest value commodity crop in the U.S. and around the world: over 1 billion tons were produced each year in 2013 and 2014 (FAO,

2016). Together, maize, rice, and wheat comprise over 60% of the world’s caloric intake ( The importance of maize in terms of production and caloric intake is not a recent development.

In fact, Native Americans have relied on maize and its ancestor for more than 9000 years. The “Columbian exchange” allowed maize to spread around the world, to adapt to new environments and become a major crop that feeds large portions of the human population. Maize, and the kernel in particular, has undergone dramatic changes over the past 9000 years. The biology of maize seed size and its starch, protein, oil content, and food characteristics, are described in other chapters of this book. Here


2: Gametophyte Interactions Establishing Maize Kernel Development



Gametophyte Interactions Establishing

Maize Kernel Development

Erik Vollbrecht1 and Matthew M.S. Evans2,*

Department of Genetics, Development and Cell Biology, Iowa State University, USA;


Department of Plant Biology, Carnegie Institution for Science, Stanford, California, USA


2.1 Introduction

This chapter focuses on tissue- and cell-level interactions required to set in motion foundational processes that lead to and promote maize kernel development. After pollination, key cell biological, genetic, and epigenetic interactions occur, including those between the male gametophyte and the pistil, between the male and female gametophytes, and between the female gametophyte and the other seed tissues, ultimately leading to successful fertilization and initiation of kernel development (Fig. 2.1). The unicellular pollen tube germinates and grows through the transmitting tract of the silk until it reaches the ovule. It is guided by chemical cues to the ovule’s micropyle and the female gametophyte’s synergid cell. Upon interaction with the synergid, the pollen tube penetrates it and ruptures, releasing two sperm cells.


3: Endosperm Development and Cell Specialization




Endosperm Development and Cell


Junpeng Zhan1, Joanne M. Dannenhoffer2 and Ramin Yadegari1,*

School of Plant Sciences, University of Arizona, USA; 2Department of Biology,

Central Michigan University, Michigan, USA

3.1 Introduction

The endosperm of angiosperms is a seed structure that provides nutrients and signals for embryo development and seedling germination (Li and Berger, 2012; Olsen and

Becraft, 2013). In cereal crops, it occupies the largest portion of the mature grain, contains large amounts of storage compounds including primarily carbohydrates and storage proteins, and is an important source of biofuel (Lopes and Larkins, 1993; Sabelli and Larkins, 2009; FAO, 2015). Because of its value and relatively large size, maize endosperm has become a model system for studies of endosperm development.

Angiosperm seed development is initiated by a double fertilization during which one of two sperm cells fuses with the egg cell within the female gametophyte (embryo sac) to produce the diploid embryo (1 maternal:1 paternal) and the other fertilizes the central cell to form the triploid endosperm


4: What Can We Learn from Maize Kernel Mutants?



What Can We Learn from Maize

Kernel Mutants?

Donald R. McCarty*

Department of Horticultural Sciences, University of Florida, USA

4.1 Introduction

Maize kernel mutants have provided insight into the mechanisms of embryo and endosperm formation for more than a century

(Neuffer and Sheridan, 1980; Sheridan and

Neuffer, 1980; Clark and Sheridan 1991; Sheridan and Clark, 1993). Advances in genomics technologies revolutionized our ability to learn from them, and recent application of transposon mutagenesis enabled their genome-wide analysis (McCarty et al., 2005,

2013; Hunter et al., 2014). With current gene discovery and genome editing technologies, there is no longer a distinction between forward and reverse genetics approaches to linking genes and phenotypes. Moreover, genetic and phenotypic analyses can be integrated with other types of genomic data that place genes in networks, providing even deeper insight into their functions.


5: The Basal Endosperm Transfer Layer (BETL): Gateway to the Maize Kernel


5   The Basal Endosperm Transfer Layer

(BETL): Gateway to the Maize Kernel


Prem S. Chourey1 and Gregorio Hueros2,*

U.S. Department of Agriculture, Agricultural Research Service, and

University of Florida, USA; 2Departamento de Biomedicina y

Biotecnología, Universidad de Alcalá, Madrid, Spain

5.1 Introduction

The maize basal endosperm transfer layer

(BETL), with its unique location at the juncture of maternal and filial tissues (Fig. 5.1), plays a critical role in grain-filling and defense. Symplastic discontinuity between the mother plant and BETL is elaborated through programmed cell death (PCD) in the placenta– chalaza (P–C) region. Early in development, cells in the BETL undergo structural modification through development of wall ingrowths (WIGs), which facilitate transport of sugars, nutrients, and water into the kernel. WIG development is an evolutionarily conserved trait, as it occurs in other cereal and plant species, including the maize precursor, teosinte. The BETL partitions the current and subsequent plant generation and creates an antimicrobial barrier between them with cytotoxic peptides. Our insight into the structure, function, and signaling roles of the BETL will foster future research into the development and function of this important seed tissue.


6: Aleurone


6 Aleurone

Bryan C. Gontarek and Philip W. Becraft*

Department of Genetics, Development and Cell Biology,

Iowa State University, Iowa, USA

6.1 Introduction

6.2  Biological Functions of Aleurone

The aleurone cell layer forms at the surface of the endosperm and is present in seeds of most flowering plants. It has epidermal-like characteristics, except that it is not directly exposed to the atmosphere; rather, it is covered by maternally derived testa and pericarp. Maize aleurone has a rich history, being instrumental in fundamental discoveries by pioneering geneticists, including

Barbara McClintock. Anthocyanin pigmentation of aleurone provides a convenient genetic marker that has led to the discovery of genes that regulate anthocyanin biosynthesis and endosperm development. Anthocyanin pigmentation in the aleurone has also been utilized to study the inheritance patterns and behaviors of genes. Transposable elements, imprinting and paramutation are among the significant discoveries facilitated by anthocyanin in the aleurone (McClintock, 1950; Brink, 1956; Kermicle, 1970). More recently, attention has focused on the aleurone per se, due to its important biological functions, implications for agronomic performance and industrial applications, and healthful properties.


7: Embryo Development



Embryo Development

William F. Sheridan* and Janice K. Clark

Department of Biology, University of North Dakota, North Dakota, USA

7.1 Introduction

The maize embryo develops over a 40–50-day period from a single-celled zygote into a miniature plant consisting of five or six leaf primordia and a single primary root. The first detailed description of the development of the maize embryo and caryopsis, wherein it is formed, was by Randolph (1936). This was followed by an extensive report on the structure and reproduction of corn by Kiesselbach (1949). In both publications the authors utilized ink drawings and photographic images to illustrate embryo morphogenesis throughout its development. Genetic analysis of this process began early in the 20th century with the reports of Jones (1920), Demerec (1923), Mangelsdorf (1923, 1926), and Wentz (1930). Following the iconic publications of Randolph (1936) and Kiesselbach

(1949), a third descriptive paper was published by Abbe and Stein (1954). These authors introduced the terminology currently used to describe the stages of embryo development. In this chapter we describe the process of maize embryo morphogenesis and mutations that have been shown to disrupt this process.


8: Embryo–Endosperm–Sporophyte Interactions in Maize Seeds


8 Embryo–Endosperm–Sporophyte

Interactions in Maize Seeds

Thomas Widiez1,*, Gwyneth C. Ingram1 and José F. Gutiérrez-Marcos2

Laboratoire Reproduction et Développement des Plantes, Université de Lyon,

ENS de Lyon, France; 2School of Life Sciences, University of Warwick, Coventry, UK


8.1 Introduction

Maize seeds, like those of all other angiosperms, are highly complex biological systems. This complexity is a consequence of the fact that the angiosperm seed is composed of tissues that evolved from three genetically distinct organisms: the mother plant

(maternal sporophyte—specifically the nucellus, integuments, and in the case of maize and other cereals, other floral organs that fuse with the integuments to form the pericarp); the developing embryo (zygotic sporophyte); and the endosperm (arising through fertilization-­ dependent proliferation of a

­second fertilization competent cell of the female gametophyte). These tissues are organized one inside the other like Russian dolls.


9: Aneuploidy and Ploidy in the Endosperm: Dosage, Imprinting, and Maternal Effects on Development



Aneuploidy and Ploidy in the Endosperm:

Dosage, Imprinting, and Maternal Effects on


James A. Birchler* and Adam F. Johnson

Division of Biological Sciences, University of Missouri, Columbia, Missouri, USA

9.1 Introduction

For nearly 100 years it has been recognized that changes in the dosage of parts of plant genomes (aneuploidy) impact their development, stature, and vigor, even more than changes in copies of the entire genome

(polyploidy). The first studies of aneuploidy and polyploidy, using Datura stramonium, were conducted by Blakeslee and colleagues

(Blakeslee et al., 1920; Blakeslee, 1934), who made an extensive set of changes in each chromosome as well as a dosage series of various ploidies. The effects of aneuploidy were manifested in all aspects of the life cycle. The question we address in this chapter is how these effects, which appear to be manifested somewhat differently, apply to endosperm development. We propose that the stoichiometry of regulatory complexes for carrying out the maternally contributed program for endosperm development and the primary endosperm nucleus dosage contributions affect endosperm development at a critical early stage.


10: Cell Cycle and Cell Size Regulation during Maize Seed Development: Current Understanding and Challenging Questions



Cell Cycle and Cell Size Regulation during Maize Seed Development: Current

Understanding and Challenging Questions

Paolo A. Sabelli*

School of Plant Sciences, University of Arizona, Tucson, Arizona, USA

10.1 Introduction

Formation of the maize seed and that of related cereals occurs through coordination of different biological processes, including cell proliferation, cell fate specification, endoreduplication, cell differentiation, accumulation of storage metabolites, and programmed cell death (PCD). Development of the three genetically distinct seed compartments, the sporophyte (i.e. the embryo), the triploid endosperm, and the maternal pericarp, involves extensive crosstalk and tight regulation between and within maternal and filial structures, with genetic, epigenetic, and environmental factors playing important roles. The objective of this chapter is to provide a perspective on the roles of cell cycle and cell size regulation during maize seed development, with an emphasis on what is not yet understood about these processes.


11: Central Metabolism and Its Spatial Heterogeneity in Maize Endosperm


11  Central Metabolism and Its Spatial

Heterogeneity in Maize Endosperm

Hardy Rolletschek1, Ljudmilla Borisjuk1, Tracie A. Hennen-Bierwagen2 and Alan M. Myers2,*


Leibniz Institute of Plant Genetics and Crop Plant Research (IPK), Department of

Molecular Genetics, Gatersleben, Germany; 2Roy J. Carver Department of

Biochemistry, Biophysics, and Molecular Biology, Iowa State University, Ames, Iowa, USA

11.1 Introduction

This chapter addresses bioenergetic considerations of the metabolic processes in maize kernels by which sugars are converted to starch, protein, and other metabolites. Central metabolism in this context is divided into modules: (i) hexose and sucrose supply;

(ii) hexose phosphorylation; (iii) ATP production; (iv) starch biosynthesis; (v) amino acid biosynthesis; (vi) storage protein synthesis; and (vii) lipid biosynthesis. The components of each node are a group of enzymes and the genes encoding them, so queries of RNA transcripts and quantitative proteomics reflect these metabolic pathways. Flux analysis is relatively well developed for maize endosperm and provides information about rates of metabolic interconversions in particular nodes. Connected metabolic pathways can be proposed based on these considerations and models tested by perturbing them.


12: Starch Biosynthesis in Maize Endosperm



Starch Biosynthesis in Maize Endosperm

L. Curtis Hannah* and Susan Boehlein

Program in Plant Molecular and Cellular Biology and Horticultural Sciences,

University of Florida, Gainesville, Florida, USA

12.1 Introduction

12.2  Maize Endosperm Starch

Starch constitutes approximately 70% of the maize kernel and provides energy for the germinating seed, allowing embryo growth and development until the seedling is photosynthetically active. Starch provides approximately 50% of calories for humans and other animals and is used to manufacture many products, including biofuels. With incipient climate change and adverse environmental conditions, plant scientists are challenged to find ways to enhance starch synthesis in order to meet the needs of a growing human population.

Because of the importance of starch and the availability of seminal mutants affecting its biosynthesis, our knowledge of this process is robust. Here we describe our current understanding of this process in maize endosperm, the primary storage site for starch in the kernel. Our understanding of this process is incomplete, and questions are identified that serve as a guide for those interested in investigating starch synthesis.


13: Maize Kernel Oil Content



Maize Kernel Oil Content

Bo Shen* and Keith Roesler

Agricultural Biotechnology, DuPont-Pioneer, Johnston, Iowa, USA

13.1 Introduction

Although maize is a staple food crop that contributes a large percentage of calories to human diets in a few countries, its major use worldwide is for animal feed. One approach to improve the metabolizable energy of maize for feed applications involves increasing kernel oil content, because oil has the highest energy density. The average

­kernel oil content in commodity maize is

~4.5% on a dry weight basis; each kilogram of oil contains 9400 calories, which is 2.25 times greater than that of starch on a weight basis. Several feeding trials using high-oil maize for poultry, hogs, and dairy cattle have shown increased growth rates and feed efficiency (Perry, 1988). In addition to direct use of maize grain in feed, distillers dried grains with solubles (DDGS), a co-product of ethanol production that contains 10% oil, is an economically valuable feed ingredient for livestock.


14: Maize Seed Storage Proteins



Maize Seed Storage Proteins

Brian A. Larkins1,*, Yongrui Wu2, Rentao Song3 and Joachim Messing4

Department of Agronomy and Horticulture, University of Nebraska-Lincoln, USA;


Institute of Plant Physiology & Ecology, Shanghai Institutes for Biological Sciences, Chinese

Academy of Sciences, China; 3National Maize Improvement Center of China, China

Agricultural University, China; 4Waksman Institute of Microbiology, Rutgers University, USA


14.1 Introduction

Maize kernels contain several types of storage proteins. By far the most abundant are prolamins, zeins, a unique storage protein found only in cereals. Because of their abundance, zeins have a profound influence on human and livestock nutrition. These proteins also appear to influence the mechanical strength of the kernel, which is important for harvesting and storage, and they affect the functional properties of food products made from corn. While a great deal of research has been devoted to the characterization of genes encoding zeins and the mechanisms by which the proteins are synthesized and stored in endosperm cells, many important questions remain regarding their structure, the regulation of the genes encoding them, and how they influence the formation of the hard, vitreous regions of the mature kernel. This chapter reviews what is known about maize storage proteins, and describes important questions that remain to be answered about their synthesis and functions in the grain. It also considers technical approaches for altering the storage protein content of maize kernels to increase the level of lysine, the most limiting essential amino acid for monogastric animals.


15: Determinants of Kernel Sink Strength



Determinants of Kernel Sink Strength

Karen E. Koch* and Fangfang Ma

Department of Horticultural Science, University of Florida, Gainesville, Florida, USA

15.1 Introduction

As a large, C4-photosynthetic plant, maize provides an abundant amount of photosynthate for sinks, compared to the more

“source-limited” small-grain species (rice, wheat, barley) with C3 photosynthesis. In a general sense, “sink strength” refers to factors responsible for transport of metabolites from one plant part to another. This is typically phloem-borne sucrose from leaves, but other nutrients, plant organs, and non-phloem paths can be involved, e.g. endosperm-to-embryo transfer. Sinks acquire a spectrum of assimilates containing C, N, S, as well as other vital resources. By the time the maize kernel reaches maturity, its cumulative sink strength accounts for its composition. Consequently, yield is highly dependent on sink strength.

Some determinants of sink strength in maize kernels are common among other plant species, but differences exist. While we have learned much about how sink strength is generated, many aspects of this process are poorly understood. Our overall understanding is as follows: (i) Sucrose moves down a turgor gradient through phloem tissue toward the kernel, which has diverse mechanisms, hence


16: Natural Variations in Maize Kernel Size: A Resource for Discovering Biological Mechanisms



Natural Variations in Maize Kernel Size:

A Resource for Discovering Biological



Xia Zhang1 and Shawn K. Kaeppler2,*

Biotechnology Research Institute, Chinese Academy of Agricultural Sciences, Beijing,

China; 2Department of Agronomy and Great Lakes Bioenergy Research Center,

University of Wisconsin, Madison, Wisconsin, USA

16.1 Introduction

Seed size is a trait that has been selected during the domestication and improvement of multiple crop species. In maize, the caryopsis or kernel is a fruit composed of the maternal pericarp surrounding the zygotic seed tissues. Kernel size increased dramatically during the domestication of maize (Zea mays ssp. mays) from its wild progenitor, teosinte (Zea mays ssp. parviglumis) (Doebley and Stec, 1993). Today, maize is one of the most important crops worldwide, providing food for human consumption, feed for livestock, and raw materials for industrial products. Given the increasing size of the human population and concomitant demand for food and renewable resources, crop scientists are striving to increase the productivity and sustainability of maize and other primary agricultural crops. Yield components, such as kernel size, are among targets to increase yield potential in maize. In industrialized countries, maize kernel size and shape are of great consequence to growers because of their relevance to mechanized cultivation, harvesting, and processing.


17: Effects of Drought Stress on Maize Kernel Set



Effects of Drought Stress on Maize

Kernel Set

Jeffrey E. Habben* and Jeffrey R. Schussler

Research and Development, DuPont-Pioneer, Johnston, Iowa, USA

17.1 Introduction

For centuries, humans have depended on maize for sustenance, and thus have endeavored to increase its productivity. During most of this time grain yield was determined by the maize grower, who selected the most “attractive” ears after each harvest as a source of seed for the following year

(Crabb, 1947). Unfortunately, even though this process provided a ready source of seed, yields were not significantly improved

(Duvick, 2005). In the early 1900s, this scheme changed when hybrid maize was developed by professional geneticists and plant breeders; thereafter, grain yields began to increase steadily. Selection for improved reproductive resilience, combined with better agronomics, resulted in maize germplasm with higher yield potential and greater yield stability. In 2014, for the first time in recorded history, the winner of the U.S. National



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