NAKED ENDOSPERM1 (NKD1) and NKD2 are duplicate INDETERMINATE DOMAIN (IDD) transcription factors important for maize (Zea mays) endosperm development. RNA-seq analysis of the nkd1 nkd2 mutant endosperm revealed that NKD1 and NKD2 influence 6.4% of the transcriptome in developing aleurone and 6.7% in starchy endosperm. Processes regulated by NKD1 and NKD2 include gene expression, epigenetic functions, cell growth and division, hormone pathways, and resource reserve deposition. The NKD1 and NKD2 proteins bind a consensus DNA sequence of TTGTCGT with slightly different properties. This motif was enriched in the promoters of gene transcripts differentially expressed (DE) in mutant endosperm. DE genes with a NKD binding motif in the 5′ promoter region were considered as likely direct targets of NKD1 and NKD2 regulation, and these putative direct target genes were notably enriched for storage proteins. Transcription assays demonstrate that NKD1 and NKD2 can directly regulate gene transcription, including activation of opaque2 and viviparous1 promoters. NKD2 functions as a negative regulator of nkd1 transcription, consistent with previously reported feedback regulation. NKD1 and NKD2 can homo-and heterodimerize through their ID domains. These analyses implicate NKD1 and NKD2 as central regulators of gene expression in developing maize endosperm. Disciplines Agronomy and Crop Sciences | Genetics | Plant Biology | Plant Breeding and Genetics CommentsThis article is from The Plant Cell 28 (2016)

The aleurone is the outermost layer of cereal endosperm and functions to digest storage products accumulated in starchy endosperm cells as well as to confer important dietary health benefits. Whereas normal maize (Zea mays [Zm]) has a single aleurone layer, naked endosperm (nkd) mutants produce multiple outer cell layers of partially differentiated cells that show sporadic expression of aleurone identity markers such as a viviparous1 promoter-b-glucuronidase transgene. The 15:1 F2 segregation ratio suggested that two recessive genes were involved, and map-based cloning identified two homologous genes in duplicated regions of the genome. The nkd1 and nkd2 genes encode the INDETERMINATE1 domain (IDD) containing transcription factors ZmIDDveg9 and ZmIDD9 on chromosomes 2 and 10, respectively. Independent mutant alleles of nkd1 and nkd2, as well as nkd2-RNA interference lines in which both nkd genes were knocked down, also showed the nkd mutant phenotype, confirming the gene identities. In wild-type kernels, the nkd transcripts were most abundant around 11 to 16 d after pollination. The NKD proteins have putative nuclear localization signals, and green fluorescent protein fusion proteins showed nuclear localization. The mutant phenotype and gene identities suggest that NKD controls a gene regulatory network involved in aleurone cell fate specification and cell differentiation.

A mutation that causes multiple aleurone layers disrupts the scaffolding subunit of a regulatory complex and dysregulates genes involved in cell division, signaling, differentiation and metabolism.

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. This chapter discusses the biological functions, practical properties, ontogeny and differentiation, and development regulation of aleurone. Despite recent advances in understanding aleurone development, many questions of both fundamental and practical importance remain: (1) What are the positional cues that specify aleurone cell fate?, (2) What determines aleurone competency?, (3) What is the significance of endosperm patterning reflected by mosaic aleurone mutants and how does it relate to aleurone development?, (4) What is the genetic programme that confers aleurone identity? Is aleurone homologous to epidermis?, (5) Is aleurone patterning related to the radial patterning mechanism in Arabidopsis thaliana roots?, (6) How are hormones involved in aleurone development?, (7) Is there a functional link between aleurone development and carotenoid biosynthesis?; (8) How can we manipulate metabolic pathways in the aleurone to improve biological functions, grain quality traits, and dietary health benefits?

reserves in cereal grains. These reserves consist primarily of starch granules and protein bodies which provide the nutrition to fuel the growth of the seedling during seed germination. Starchy endosperm cells undergo endoreduplication and programmed cell death at maturity (Edgar et al., 2014; Sabelli et al., 2014). The basal transfer cells occupy the basal end of the endosperm adjacent to the maternal pedicel tissue. Basal transfer cells are characterized by thick cell wall ingrowths which function in the uptake and transport of nutrients from the maternal tissue into the endosperm which fuels the development of both the embryo and endosperm (Offler et al., 2003; Becraft and Gutierrez-Marcos, 2012). Basal transfer cells are dead at maturity.