Endosperm Development

Figure 1. Major tissues of the maize kernel

The endosperm represents the storage tissue of the cereal grain, providing nourishment to the growing seedling at germination, as well as food, feed and industrial feedstock for humans. Figure 1 shows a cartoon of a maize kernel in longitudinal section and histological sections highlighting 3 major cell types of the endosperm: the starchy endosperm (yellow), the basal transfer layer (blue) and the aleurone (red). The transfer layer functions in nutrient uptake from the mother plant during seed development. The starchy endosperm is the major storage site for starch and proteins. While performing some storage function, the primary role of the aleurone is as a digestive tissue. At germination, it secretes amylase into the starchy endosperm causing the breakdown of the stored starch, providing the growing seedlings with sugars for energy and growth. The apparent simplicity is deceptive; endosperm development is a highly specialized process with many unique features. For detailed information, see one of the following reviews: (Becraft and Gutierrez-Marcos, 2012; Olsen and Becraft, 2013).

My lab studies the genetic regulation of endosperm development: how genetic programs control the identity of the different cell types, and how each cell type acquires its specialized biological properties. We currently use 2 main approaches. 1. We employ mutant analysis to identify genes important for the differentiation of the aleurone layer. 2. We use genomics tools to decipher the gene regulatory networks that control the development of endosperm traits.

 

 

Aleurone Development

Normal kernels have one layer of aleurone cells.

dek1, a mutant that fails to produce aleurone.

nkd1; nkd2, a double mutant that impairs aleurone cell differentiation.

thk1, produces too many aleurone cells.

Figure 2. Examples of mutants that affect aleurone development.

 The aleurone layer is an attractive experimental system for genetic analyses. In the proper genotypes, the aleurone layer specifically accumulates purple anthocyanin pigment, allowing us to search for mutants that have disrupted aleurone development by screening for loss of pigment. These mutants define genes that are important for normal aleurone development (Becraft and Ascuncion-Crabb, 2000). Figure 2 shows what a normal aleurone looks like in microscopic section and some examples of aleurone mutants. Using molecular genetic techniques, we identify the genes that are disrupted in mutants such as these, and then determine how the normal version of each gene functions to control normal aleurone development. More information about our work on these particular mutants can be found in the following publications: (Becraft and Asuncion-Crabb, 2000; Becraft et al., 2002; Gontarek et al., 2016; Yi et al., 2011; Yi et al., 2015)


 

 

Genomic Analysis of Endosperm Development 

Figure 3. Examples of “pathways” regulated by the NKD transcription factors in developing endosperm.

The properties of a given cell or tissue are determined by the genes that are expressed there. Modern transcriptomic technologies permit the detection of essentially all the 10s of thousands of genes that are expressed in a typical tissue. Comparing the transcriptomes of tissues at different developmental stages, under different environmental conditions, or carrying different genetic mutations allows genes to be parsed into expression modules that be associated with the regulation of particular traits. Figure 3 shows examples of results obtained in a recent study where transcriptomes were analyzed from normal endosperm and nkd1; nkd2 mutant endosperm (Yi et al., 2015; Gontarek et al, 2016). The nkd genes encode transcription factors (TFs) and therefore, mRNAs that are present at different levels in normal vs. mutant identify genes that are regulated, directly or indirectly, by NKD TFs. In the figure, each blue cell represents a gene that is expressed at lower levels in the mutant and is therefore positively regulated by normal NKD, whereas red cells represent genes that are expressed at higher levels in the mutant and therefore are negatively regulated by NKD. Differences in genes associated with the ABA hormone signaling pathway were consistent with the mutant phenotype. Differences in the expression of many other TF genes suggest NKD functions as a central regulator. Decreased expression of genes involved in nutrient reservoir accumulation indicates the importance of NKD in promoting development of grain quality traits. Similar analyses are being applied to additional mutants and developmental stages to identify genes that are co-regulated, and establish the gene regulatory networks that direct endosperm development and impart grain quality traits.


 

 

 

References

 

Becraft PW, Asuncion-Crabb YT. (2000). Positional cues specify and maintain aleurone cell fate in maize endosperm development. Development 127: 4039-4048.

 

Becraft, PW, and Gutierrez-Marcos, J. (2012). Endosperm development: dynamic processes and cellular innovations underlying sibling altruism. WIREs Developmental Biology doi: 10.1002/wdev.31.

Becraft PW, Li K, Dey N, Asuncion-Crabb YT. (2002). The maize dek1gene functions in embryonic pattern formation and in cell fate specification. Development 129: 5217-5225.

 

Gontarek BC, Neelakandan AK, Wu H, Becraft PW. (2016). NKD Transcription Factors Are Central Regulators of Maize Endosperm Development. Plant Cell 28: 2916-2936.

 

Kiesselbach, T. A. (1949). "The structure and reproduction of corn." Nebraska Agric. Exp. Stn. Res. Bull. 161: 1-96.

Olsen O-A, Becraft PW (2013) Endosperm Development. In PW Becraft, ed, “Seed Genomics”. Wiley-Blackwell, Ames, IA, pp 43-62

Yi G, Lauter AM, Scott MP, Becraft PW. (2011). The thick aleurone1 mutant defines a negative regulation of maize aleurone cell fate that functions downstream of dek1. Plant Physiol 156: 1826-1836.

 

Yi G, Neelakandan AK, Gontarek BC, Vollbrecht E, Becraft PW. (2015). The naked endosperm genes encode duplicate INDETERMINATE domain transcription factors required for maize endosperm cell patterning and differentiation. Plant Physiol 167: 443-456.