The aleurone layer of cereal grains: Development, genetic regulation, and breeding applications

Abstract

Cereal aleurone cells are differentiated from triploid endosperm cells and exhibit distinct cytological, physiological, and biochemical characteristics that distinguish them from the starchy endosperm cells of cereals. Aleurone cells maintain viability throughout seed development, whereas starchy endosperm cells undergo programmed cell death during maturation. Despite variations in aleurone-related traits among cereal species, the aleurone layer plays a crucial role in regulating many aspects of seed development, including the accumulation of storage reserves, the acquisition of dormancy, and germination. Given that many nutrients-such as lipids, dietary fibers, vitamins, and minerals like iron and zinc-are predominantly accumulated in the aleurone cells of cereal grains, this layer has attracted considerable attention aimed at improving the nutritional value of cereals. This review provides a comprehensive overview of the developmental, genetic, and molecular basis of aleurone cell differentiation and proliferation. It focuses on the improvement of aleurone-related traits informed by knowledge of the molecular networks governing aleurone development and presents a detailed discussion on the challenges and potential solutions associated with cereal improvement through the manipulation of aleurone-related traits.

Keywords: aleurone layer; breeding application; cell differentiation; cereal; molecular regulation; nutritional quality.

Figure 1

Aleurone layer in different cereals. (A) A microscopic section of maize endosperm. Starchy endosperm cells are filled with starch granules stained pink with periodic acid-Schiff (PAS) reagent. Al, aleurone layer; En, starchy endosperm. Scale bar corresponds to 50 μm. Image reproduced and modified from Becraft and Yi (2011). (B and C) Autofluorescence images showing the aleurone layer in wheat (B) and barley (C). Images were collected using UV excitation (330–380 nm) and emission > 420 nm. Scale bars correspond to 100 μm. Images were duplicated and modified from Jääskeläinen et al. (2013). (D and E) Overview of rice aleurone layers on the ventral (D) and dorsal (E) side of rice caryopsis. Carbohydrates are magenta, stained by PAS. Proteins are blue, stained by Coomassie brilliant blue (CBB). Scale bars correspond to 50 μm. Images were duplicated and modified from Yu et al. (2021).

Figure 2

Biological significance of the aleurone layer. This diagram illustrates the roles of aleurone cells during development to ensure seed maturation and germination. During the filling stage, the aleurone cells store many nutrients, including lipids, proteins, minerals, and vitamins. During germination, the aleurone layer acts as a primary determinant of seed dormancy due to the accumulation of the phytohormone abscisic acid (ABA) in the cells. The aleurone cells form a defensive barrier against pathogens throughout seed development. During germination, aleurone cells secrete hydrolases (such as amylase and lipase) induced by gibberellin (GA) to digest the reserves in aleurone and endosperm cells to promote seed germination and seedling growth.

Figure 3

Environmental and genetic regulation of aleurone cell development in cereals. (A) Differentiation of aleurone cells is coordinately determined by endosperm-derived positional cues, maternal signals, and environmental factors such as temperature, photoperiod, and atmospheric CO₂ concentration. (B) Genetic regulation of aleurone differentiation in maize (left) and rice (right). In maize, DEK1, CR4, and ZmYSL2 positively regulate aleurone differentiation, whereas THK1, SAL1, and ZmDOF3 negatively regulate it. THK1 is downstream of DEK1 and NKD1/2 is downstream of ZmDOF3. In rice, OsCR4 and OsDEK1 promote aleurone differentiation, whereas RPBF, RISBZ1, TA1, and TA2 inhibit it. OsCR4 and OsGA20ox1 are downstream targets of the rice PRC2 complex in the aleurone. Arrows and T-bars indicate positive and negative regulation of aleurone differentiation, respectively.

Figure 4

Molecular regulatory mechanisms of aleurone cell differentiation in maize and rice. (A) Maize CR4, a plasma membrane-localized receptor-like kinase, and DEK1, a plasma membrane-targeted protein with cytoplasmic calpain protease activity, potentially perceive positional cues released from endosperm cells to trigger downstream signaling. When activated, CR4 may phosphorylate an unidentified downstream target through its kinase activity to promote aleurone differentiation. Upon perception of the positional cue, the protease activity of DEK1 may be activated, leading to degradation of a yet-to-be-identified substrate to promote aleurone differentiation. The vacuolar sorting protein SAL1 regulates the concentration of DEK1 and CR4 in the plasma membrane through endosome-mediated degradation. THK1 acts downstream of DEK1 as a CCR4-NOT scaffold protein. ZmDOF3, a DOF family transcription factor, regulates the expression of NKD1 and NKD2, which encode IDD family transcription factors that influence aleurone differentiation through transcriptional regulation. The YELLOW STRIPE-LIKE oligopeptide metal transporter ZmYSL2 is also involved in aleurone differentiation in maize. (B) OsCR4 and OsDEK1 play conserved roles in promoting aleurone differentiation in rice. The secreted protein OsCIP1 interacts with OsCR4 to stabilize it on the membrane. OsCR4 expression is negatively regulated by the rice PRC2 complex through H3K27me3 modification. OsGA20ox1, another target of rice PRC2, positively regulates aleurone differentiation through GA biosynthesis in seeds. The DOF family transcription factor RPBF represses rice aleurone differentiation through transcriptional regulation. DNA at RPBF and RISBZ1 loci is methylated, and the rice demethylase OsROS1a promotes the expression of these genes by removing methylation marks. In addition, the single-stranded DNA-binding protein OsmtSSB1 cooperates with the DNA recombinase RECA3 and the DNA helicase TWINKLE to repress aleurone differentiation in rice by maintaining the integrity of the mitochondrial genome. Similarly, the mitochondrion-localized factor OsGCD1 is involved in aleurone development in rice.

Figure 5

Utilization of the aleurone-related traits. Aleurone-related traits have potential to be widely utilized for enhancing the nutritional value, milling and malting characteristics, seed storability, and yield-related traits of cereal crops. In addition, the aleurone can be used to enhance the production of bran oil, a valuable source of nutritionally beneficial compounds.

Figure 6

Strategies to enhance aleurone-related traits in cereals. Implementing high-throughput phenotyping will streamline the process for screening varieties or mutants with specific aleurone characteristics. The underlying causal genes can be cloned using linkage or association populations. Subsequently, gene editing, transgenic breeding, or traditional gene pyramiding strategies may be employed to facilitate the improvement of aleurone-related traits. Traits such as reduced aleurone cells or increased aleurone cells can be leveraged for crop improvement; the former may enhance shelf life and milling yield, while the latter may increase the nutritional compounds in grains. These traits may be combined with improved appearance, taste, and cooking qualities through traditional or modern breeding technologies, including marker-assisted selection, gene editing, and transgenic breeding. Moreover, enhancing the efficiency of aleurone cell isolation from bran could be beneficial for food biofortification efforts that utilize these isolated aleurone cells.