Maize (Zea mays) is a leading cereal crop in the world. The maize kernel is the storage organ and the harvest portion of this crop and is closely related to its yield and quality. The development of maize kernel is initiated by the double fertilization event, leading to the formation of a diploid embryo and a triploid endosperm. The embryo and endosperm are then undergone independent developmental programs, resulting in a mature maize kernel which is comprised of a persistent endosperm, a large embryo, and a maternal pericarp. Due to the well-characterized morphogenesis and powerful genetics, maize kernel has long been an excellent model for the study of cereal kernel development. In recent years, with the release of the maize reference genome and the development of new genomic technologies, there has been an explosive expansion of new knowledge for maize kernel development. In this review, we overviewed recent progress in the study of maize kernel development, with an emphasis on genetic mapping of kernel traits, transcriptome analysis during kernel development, functional gene cloning of kernel mutants, and genetic engineering of kernel traits.

Maize kernel development

With increasingly frequent extreme weather events, lodging has become an important limiting factor for crop yield and quality and for mechanical harvesting. Lodging resistance is a precondition for “super high yield” crops, and the question of how to achieve lodging resistance to guarantee high yield is an urgent scientific problem. Here, we summarize the anatomical results of lodging resistance stems and find that the lodging resistance of stems is closely related to stem components. Therefore, we focus on the roles of lignin, cellulose and hemicellulose, which provide stem rigidity and strength, in crop lodging resistance. By combing the synthetic regulatory molecular network of lignin, cellulose and hemicellulose, we find that only some of the genes involved in the biosynthesis and regulation of lignin, cellulose, and hemicellulose have been shown to significantly affect lodging resistance. However, many relevant genes remain to be studied in sufficient detail to determine whether they can be applied in breeding for lodging resistance. This work provides valuable information for future studies of lodging resistance.

/pdf/crop-lodging-and-the-roles-of-lignin-cellulose-and-hy5sxtx5.pdf

Crop Lodging and The Roles of Lignin, Cellulose, and Hemicellulose in Lodging Resistance

Ribosome biogenesis is essential for plants to successfully acclimate to low temperature. Without dedicated steps supervising the 60S large subunits (LSUs) maturation in the cytosol, e.g., Rei-like (REIL) factors, plants fail to accumulate dry weight and fail to grow at suboptimal low temperatures. Around REIL, the final 60S cytosolic maturation steps include proofreading and assembly of functional ribosomal centers such as the polypeptide exit tunnel and the P-Stalk, respectively. In consequence, these ribosomal substructures and their assembly, especially during low temperatures, might be changed and provoke the need for dedicated quality controls. To test this, we blocked ribosome maturation during cold acclimation using two independent reil double mutant genotypes and tested changes in their ribosomal proteomes. Additionally, we normalized our mutant datasets using as a blank the cold responsiveness of a wild-type Arabidopsis genotype. This allowed us to neglect any reil-specific effects that may happen due to the presence or absence of the factor during LSU cytosolic maturation, thus allowing us to test for cold-induced changes that happen in the early nucleolar biogenesis. As a result, we report that cold acclimation triggers a reprogramming in the structural ribosomal proteome. The reprogramming alters the abundance of specific RP families and/or paralogs in non-translational LSU and translational polysome fractions, a phenomenon known as substoichiometry. Next, we tested whether the cold-substoichiometry was spatially confined to specific regions of the complex. In terms of RP proteoforms, we report that remodeling of ribosomes after a cold stimulus is significantly constrained to the polypeptide exit tunnel (PET), i.e., REIL factor binding and functional site. In terms of RP transcripts, cold acclimation induces changes in RP families or paralogs that are significantly constrained to the P-Stalk and the ribosomal head. The three modulated substructures represent possible targets of mechanisms that may constrain translation by controlled ribosome heterogeneity. We propose that non-random ribosome heterogeneity controlled by specialized biogenesis mechanisms may contribute to a preferential or ultimately even rigorous selection of transcripts needed for rapid proteome shifts and successful acclimation.

https://www.mdpi.com/1422-0067/22/11/6160/pdf

Spatially Enriched Paralog Rearrangements Argue Functionally Diverse Ribosomes Arise during Cold Acclimation in Arabidopsis.

As the centerpiece of the biomass production process, ribosome activity is highly coordinated with environmental cues. Findings revealing ribosome subgroups responsive to adverse conditions suggest this tight coordination may be grounded in the induction of variant ribosome compositions and the differential translation outcomes they might produce. In this perspective, we go through the literature linking ribosome heterogeneity to plants’ abiotic stress response. Once unraveled, this crosstalk may serve as the foundation of novel strategies to custom cultivars tolerant to challenging environments without the yield penalty.

/pdf/engineering-ribosomes-to-alleviate-abiotic-stress-in-plants-16ub1ksc.pdf

Engineering Ribosomes to Alleviate Abiotic Stress in Plants: A Perspective

Global climate change has great impacts on plant growth and development, reducing crop productivity worldwide. Rice (Oryza sativa L.), one of the world's most important food crops, is susceptible to high-temperature stress from seedling stage to reproductive stage. In this review, we summarize recent advances in understanding the molecular mechanisms underlying heat stress responses in rice, including heat sensing and signalling, transcriptional regulation, transcript processing, protein translation, and post-translational regulation. We also highlight the irreversible effects of high temperature on reproduction and grain quality in rice. Finally, we discuss challenges and opportunities for future research on heat stress responses in rice.

The hot science in rice research: How rice plants cope with heat stress.

Lodging is a major problem limiting maize yield worldwide. However, the mechanisms of lodging resistance remain incompletely understood for maize. Here, we evaluated 443 maize accessions for lodging resistance in the field. Five lodging-resistant accessions and five lodging-sensitive accessions were selected for further research. The leaf number, plant height, stem diameter, and rind penetrometer resistance were similar between lodging-resistant and -sensitive inbred lines. The average thickness of sclerenchymatous hypodermis layer was thicker and the vascular area was larger in the lodging-resistant lines compared with lodging-sensitive lines. Although total lignin content in stem tissue did not significantly differ between lodging-resistant and -sensitive lines, phloroglucinol staining revealed that the lignin content of the cell wall in the stem cortex and in the stem vascular tissue near the cortex was higher in the lodging-resistant lines than in the lodging-sensitive lines. Analysis of strand-specific RNA-seq transcriptome showed that a total of 793 genes were up-regulated and 713 genes were down-regulated in lodging-resistant lines relative to lodging-sensitive lines. The up-regulated genes in lodging-resistant lines were enriched in cell wall biogenesis. These results indicated that modification of cell wall biosynthesis would contribute to lodging resistance of maize.

Identification of traits and genes associated with lodging resistance in maize

Ribosome biogenesis is a fundamental process in all eukaryotic cells and is coupled with the processing and maturation of pre-rRNAs. Maize is a primary staple crop across the world, but little is known about the exact pre-rRNA processing sites and pathways in this species. In this study, we present a detailed model of the pathway by identifying the critical endonucleolytic cleavage sites and determining the pre-rRNA intermediates by circular reverse-transcription PCR and northern blot analysis. We demonstrate that two pathways coexist in maize to promote the processing of 35S pre-rRNA, and that the processing of 27SA pre-rRNA can proceed via two different pathways, which are distinguished based on the order of ITS1 removal and ITS2 cleavage. Compared with yeast and mammals, this new 27SA pre-rRNA processing mechanism is unique to maize and other higher plants. In addition, we demonstrate that maize can modulate pre-rRNA processing levels in response to chilling and heat stress, as indicated by a significant reduction of the P-A3 intermediate. Our study provides information that will facilitate future research on ribosome biogenesis and pre-rRNA processing in maize.

Pre-rRNA processing and its response to temperature stress in maize.

Recent findings have revealed the important roles of microRNAs (miRNAs) in the secondary responses to oxidative damage caused by iron (Fe) excess. However, the functional importance of miRNAs in plant responses to Fe-deficiency remains to be explored. Here, we show that the expression level of miR164 in Arabidopsis (Arabidopsis thaliana) roots was repressed by Fe deficiency. Primary root length, lateral root number, ferric reductase activity, and mRNA abundance of IRON-REGULATED TRANSPORTER1 (IRT1) and FERRIC REDUCTION OXIDASE2 (FRO2) were higher in the mir164b mutant than in the wild type (WT) under Fe-deficient conditions. Analysis of the Fe concentrations and ferric reductase activities in the roots of miR164 knockdown transgenic plants showed that members of the miR164 family had different functions in Fe-deficiency responses. Promoter::GUS analysis showed that NAM/ATAF/CUC (NAC) domain transcription factor5 (NAC5) is regulated at both transcriptional and post-transcriptional levels under Fe-deficient conditions. Transgenic Arabidopsis plants overexpressing NAC5 were more tolerant of Fe-deficiency than the WT. NAC5 has transactivation activity and directly transactivates the expression of Nuclear Factor Y, Subunit A8 (NFYA8), as demonstrated by chromatin immunoprecipitation followed by quantitative polymerase chain reaction (ChIP-qPCR), electrophoretic mobility shift assay (EMSA), and dual-luciferase reporter assay. Like overexpression of NAC5, overexpression of NFYA8 increases primary root length, lateral root number, ferric reductase activity, and mRNA abundance of IRT1 and FRO2 under Fe-deficient conditions. Thus, MIR164b is important for Fe-deficiency responses by its regulation of the NAC5-NFYA8 module.

/pdf/mir164b-represses-iron-uptake-by-regulating-the-nac-domain-3myip2q4.pdf

MIR164b represses iron uptake by regulating the NAC domain transcription factor5-Nuclear Factor Y, Subunit A8 module in Arabidopsis.

Although nitrogen (N) is known to affect mineral element homeostasis in plants, the molecular mechanisms of interactions between N and other nutrients remain largely unclear. We report here that N supply affects ion homeostasis in maize. Berberine hemisulfate staining and a propidium iodide penetration assay showed that N luxury significantly delayed Casparian strip (CS) formation in maize roots. We further demonstrated that N-mediated CS formation in maize was independent of RBOHF-activated H2O2 production. N luxury induced the expression of ZmmiR528 in whole roots and root tips. Knockdown and loss-of-function of ZmmiR528 promoted CS formation under both N-luxury and N-deficient conditions. Both ZmMIR528a and ZmMIR528b contribute to early CS formation under different N conditions. RNA-seq and real-time RT-PCR analysis demonstrated that ZmLAC3, but not ZmLAC5, responded to N treatments. Consistent with results obtained with ZmmiR528 TM transgenic maize and mir528a/b loss-of-function mutants, transgenic maize overexpressing ZmLAC3 displayed early CS formation under different N conditions. Under field conditions, K, Ca, Mn, Cu, Mg, and Zn concentrations were greater in the ear leaf of ZmLAC3-overexpressing transgenic maize than in the wild type. These results indicate that ZmmiR528 affects CS formation in maize by regulating the expression of ZmLAC3, and modification of CS formation has the potential to improve maize quality. 

Yu Guo

Papers

Identification of traits and genes associated with lodging resistance in maize

Pre-rRNA processing and its response to temperature stress in maize.

MIR164b represses iron uptake by regulating the NAC domain transcription factor5-Nuclear Factor Y, Subunit A8 module in Arabidopsis.

Nitrogen supply affects ion homeostasis by modifying root Casparian strip formation through the miR528-LAC3 module in maize.