Understanding Grass Domestication through Maize Mutants

Striking a growth-defense balance: Stress regulators that function in maize development

Maize (Zea mays) cultivation is strongly affected by both abiotic and biotic stress, leading to reduced growth and productivity. It has recently become clear that regulators of plant stress responses, including the phytohormones abscisic acid (ABA), ethylene (ET), and jasmonic acid (JA), together with reactive oxygen species (ROS), shape plant growth and development. Beyond their well established functions in stress responses, these molecules play crucial roles in balancing growth and defense, which must be finely tuned to achieve high yields in crops while maintaining some level of defense. In this review, we provide an in-depth analysis of recent research on the developmental functions of stress regulators, focusing specifically on maize. By unraveling the contributions of these regulators to maize development, we present new avenues for enhancing maize cultivation and growth while highlighting the potential risks associated with manipulating stress regulators to enhance grain yields in the face of environmental challenges.

Boosting Triticeae crop grain yield by manipulating molecular modules to regulate inflorescence architecture: insights and knowledge from other cereal crops

One of the challenges for global food security is to reliably and sustainably improve the grain yield of cereal crops. One solution is to modify the architecture of the grain-bearing inflorescence to optimize for grain number and size. Cereal inflorescences are complex structures, with determinacy, branching patterns, and spikelet/floret growth patterns that vary by species. Recent decades have witnessed rapid advancements in our understanding of the genetic regulation of inflorescence architecture in rice, maize, wheat, and barley. Here, we summarize current knowledge on key genetic factors underlying the different inflorescence morphologies of these crops and model plants (Arabidopsis and tomato), focusing particularly on the regulation of inflorescence meristem determinacy and spikelet meristem identity and determinacy. We also discuss strategies to identify and utilize these superior alleles to optimize inflorescence architecture and, ultimately, improve crop grain yield.

Increasing the Grain Yield and Grain Protein Content of Common Wheat (Triticum aestivum) by Introducing Missense Mutations in the Q Gene

Grain yield (GY) and grain protein content (GPC) are important traits for wheat breeding and production; however, they are usually negatively correlated. The Q gene is the most important domestication gene in cultivated wheat because it influences many traits, including GY and GPC. Allelic variations in the Q gene may positively affect both GY and GPC. Accordingly, we characterized two new Q alleles (Qs1 and Qc1-N8) obtained through ethyl methanesulfonate-induced mutagenesis. Compared with the wild-type Q allele, Qs1 contains a missense mutation in the sequence encoding the first AP2 domain, whereas Qc1-N8 has two missense mutations: one in the sequence encoding the second AP2 domain and the other in the microRNA172-binding site. The Qs1 allele did not significantly affect GPC or other processing quality parameters, but it adversely affected GY by decreasing the thousand kernel weight and grain number per spike. In contrast, Qc1-N8 positively affected GPC and GY by increasing the thousand kernel weight and grain number per spike. Thus, we generated novel germplasm relevant for wheat breeding. A specific molecular marker was developed to facilitate the use of the Qc1-N8 allele in breeding. Furthermore, our findings provide useful new information for enhancing cereal crops via non-transgenic approaches.

Molecular, cellular, and developmental foundations of grass diversity

Humans have cultivated grasses for food, feed, beverages, and construction materials for millennia. Grasses also dominate the landscape in vast parts of the world, where they have adapted morphologically and physiologically, diversifying to form ~12,000 species. Sequences of hundreds of grass genomes show that they are essentially collinear; nonetheless, not all species have the same complement of genes. Here, we focus on the molecular, cellular, and developmental bases of grain yield and dispersal-traits that are essential for domestication. Distinct genes, networks, and pathways were selected in different crop species, reflecting underlying genomic diversity. With increasing genomic resources becoming available in nondomesticated species, we anticipate advances in coming years that illuminate the ecological and economic success of the grasses.

How Strigolactone Shapes Shoot Architecture

Despite its central role in the control of plant architecture, strigolactone has been recognized as a phytohormone only 15 years ago. Together with auxin, it regulates shoot branching in response to genetically encoded programs, as well as environmental cues. A central determinant of shoot architecture is apical dominance, i.e., the tendency of the main shoot apex to inhibit the outgrowth of axillary buds. Hence, the execution of apical dominance requires long-distance communication between the shoot apex and all axillary meristems. While the role of strigolactone and auxin in apical dominance appears to be conserved among flowering plants, the mechanisms involved in bud activation may be more divergent, and include not only hormonal pathways but also sugar signaling. Here, we discuss how spatial aspects of SL biosynthesis, transport, and sensing may relate to apical dominance, and we consider the mechanisms acting locally in axillary buds during dormancy and bud activation.

The arches and spandrels of maize domestication, adaptation, and improvement

People living in the Balsas River basin in southwest México domesticated maize from the bushy grass teosinte. Nine thousand years later, in 2021, Ms. Deb Haaland - a member of the Pueblo of Laguna tribe of New Mexico - wore a dress adorned with a cornstalk when she was sworn in as the Secretary of Interior of the United States of America. This choice of garment highlights the importance of the coevolution of maize and the farmers who, through careful selection over thousands of years, domesticated maize and adapted the physiology and shoot architecture of maize to fit local environments and growth habits. Some traits such as tillering were directly selected on (arches), and others such as tassel size are the by-products (spandrels) of maize evolution. Here, we review current knowledge of the underlying cellular, developmental, physiological, and metabolic processes that were selected by farmers and breeders, which have positioned maize as a top global staple crop.

Developmental genetics of maize vegetative shoot architecture

More than 1.1 billion tonnes of maize grain were harvested across 197 million hectares in 2019 (FAOSTAT 2020). The vast global productivity of maize is largely driven by denser planting practices, higher yield potential per area of land, and increased yield potential per plant. Shoot architecture, the three-dimensional structural arrangement of the above-ground plant body, is critical to maize grain yield and biomass. Structure of the shoot is integral to all aspects of modern agronomic practices. Here, the developmental genetics of the maize vegetative shoot is reviewed. Plant architecture is ultimately determined by meristem activity, developmental patterning, and growth. The following topics are discussed: shoot apical meristem, leaf architecture, axillary meristem and shoot branching, and intercalary meristem and stem activity. Where possible, classical and current studies in maize developmental genetics, as well as recent advances leveraged by "-omics" analyses, are highlighted within these sections.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-021-01208-1.

Harnessing Knowledge from Maize and Rice Domestication for New Crop Breeding

Crop domestication has fundamentally altered the course of human history, causing a shift from hunter-gatherer to agricultural societies and stimulating the rise of modern civilization. A greater understanding of crop domestication would provide a theoretical basis for how we could improve current crops and develop new crops to deal with environmental challenges in a sustainable manner. Here, we provide a comprehensive summary of the similarities and differences in the domestication processes of maize and rice, two major staple food crops that feed the world. We propose that maize and rice might have evolved distinct genetic solutions toward domestication. Maize and rice domestication appears to be associated with distinct regulatory and evolutionary mechanisms. Rice domestication tended to select de novo, loss-of-function, coding variation, while maize domestication more frequently favored standing, gain-of-function, regulatory variation. At the gene network level, distinct genetic paths were used to acquire convergent phenotypes in maize and rice domestication, during which different central genes were utilized, orthologous genes played different evolutionary roles, and unique genes or regulatory modules were acquired for establishing new traits. Finally, we discuss how the knowledge gained from past domestication processes, together with emerging technologies, could be exploited to improve modern crop breeding and domesticate new crops to meet increasing human demands.

Mapping Regulatory Determinants in Plants

The domestication and improvement of many plant species have frequently involved modulation of transcriptional outputs and continue to offer much promise for targeted trait engineering. The cis-regulatory elements (CREs) controlling these trait-associated transcriptional variants however reside within non-coding regions that are currently poorly annotated in most plant species. This is particularly true in large crop genomes where regulatory regions constitute only a small fraction of the total genomic space. Furthermore, relatively little is known about how CREs function to modulate transcription in plants. Therefore understanding where regulatory regions are located within a genome, what genes they control, and how they are structured are important factors that could be used to guide both traditional and synthetic plant breeding efforts. Here, we describe classic examples of regulatory instances as well as recent advances in plant regulatory genomics. We highlight valuable molecular tools that are enabling large-scale identification of CREs and offering unprecedented insight into how genes are regulated in diverse plant species. We focus on chromatin environment, transcription factor (TF) binding, the role of transposable elements, and the association between regulatory regions and target genes.

TB1: from domestication gene to tool for many trades

This article comments on: Dixon LE, Pasquariello M, Boden SA. 2020. TEOSINTE BRANCHED1 regulates height and stem internode length in bread wheat. Journal of Experimental Botany 71, 4742–4750.

New Food Crop Domestication in the Age of Gene Editing: Genetic, Agronomic and Cultural Change Remain Co-evolutionarily Entangled

The classic domestication scenario for grains and fruits has been portrayed as the lucky fixation of major-effect "domestication genes." Characterization of these genes plus recent improvements in generating novel alleles (e.g., by gene editing) have created great interest in de novo domestication of new crops from wild species. While new gene editing technologies may accelerate some genetic aspects of domestication, we caution that de novo domestication should be understood as an iterative process rather than a singular event. Changes in human social preferences and relationships and ongoing agronomic innovation, along with broad genetic changes, may be foundational. Allele frequency changes at many loci controlling quantitative traits not normally included in the domestication syndrome may be required to achieve sufficient yield, quality, defense, and broad adaptation. The environments, practices and tools developed and maintained by farmers and researchers over generations contribute to crop yield and success, yet those may not be appropriate for new crops without a history of agronomy. New crops must compete with crops that benefit from long-standing participation in human cultural evolution; adoption of new crops may require accelerating the evolution of new crops' culinary and cultural significance, the emergence of markets and trade, and the formation and support of agricultural and scholarly institutions. We provide a practical framework that highlights and integrates these genetic, agronomic, and cultural drivers of change to conceptualize de novo domestication for communities of new crop domesticators, growers and consumers. Major gene-focused domestication may be valuable in creating allele variants that are critical to domestication but will not alone result in widespread and ongoing cultivation of new crops. Gene editing does not bypass or diminish the need for classical breeding, ethnobotanical and horticultural knowledge, local agronomy and crop protection research and extension, farmer participation, and social and cultural research and outreach. To realize the ecological and social benefits that a new era of de novo domestication could offer, we call on funding agencies, proposal reviewers and authors, and research communities to value and support these disciplines and approaches as essential to the success of the breakthroughs that are expected from gene editing techniques.

Molecular characterization of teosinte branched1 gene governing branching architecture in cultivated maize and wild relatives

We sequenced the entire tb1 gene in six maize inbreds and its wild relatives (parviglumis, mexicana, perennis and luxurians) to characterize it at molecular level. Hopscotch and Tourist transposable elements were observed in the upstream of tb1 in all maize inbreds, while they were absent in wild relatives. In maize, tb1 consisted of 431-443 bp 5'UTR, 1101 bp coding sequence and 211-219 bp 3'UTR. In promoter region, mutations in the light response element in mexicana (~ 35 bp and ~ 55 bp upstream of TSS) and perennis (at ~ 35 bp upstream of TSS) were found. A 6 bp insertion at 420 bp downstream of the polyA signal site was present among teosinte accessions, while it was not observed in maize. A codominant marker flanking the 6 bp InDel was developed, and it differentiated the teosintes from maize. In Tb1 protein, alanine (12.7-14.6%) was the most abundant amino acid with tryptophan as the rarest (0.5-0.9%). The molecular weight of Tb1 protein was 38757.15 g/mol except 'Palomero Toluqueno' and HKI1128. R and TCP motifs in Tb1 protein were highly conserved across maize, teosinte and orthologues, while TCP domain differed for tb1 paralogue. Tb1 possessed important role in light-, auxin-, stress-response and meristem identity maintenance. Presence of molecular signal suggested its localization in mitochondria, nucleus and nucleolus. Parviglumis and mexicana shared closer relationship with maize than perennis and luxurians. A highly conserved 59-60 amino acids long bHLH region was observed across genotypes. Information generated here assumes significance in evolution of tb1 gene and breeding for enhancement of prolificacy in maize.

The regulatory landscape of a core maize domestication module controlling bud dormancy and growth repression

Many domesticated crop plants have been bred for increased apical dominance, displaying greatly reduced axillary branching compared to their wild ancestors. In maize, this was achieved through selection for a gain-of-function allele of the TCP transcription factor teosinte branched1 (tb1). The mechanism for how a dominant Tb1 allele increased apical dominance, is unknown. Through ChIP seq, RNA seq, hormone and sugar measurements on 1 mm axillary bud tissue, we identify the genetic pathways putatively regulated by TB1. These include pathways regulating phytohormones such as gibberellins, abscisic acid and jasmonic acid, but surprisingly, not auxin. In addition, metabolites involved in sugar sensing such as trehalose 6-phosphate were increased. This suggests that TB1 induces bud suppression through the production of inhibitory phytohormones and by reducing sugar levels and energy balance. Interestingly, TB1 also putatively targets several other domestication loci, including teosinte glume architecture1, prol1.1/grassy tillers1, as well as itself. This places tb1 on top of the domestication hierarchy, demonstrating its critical importance during the domestication of maize from teosinte.

The TB1 transcription factor was selected for the increased apical dominance of maize compared to its ancestor teosinte. A metabolic and genomic analysis of domesticated axillary buds suggest that TB1 achieved this by regulating phytohormone signaling, sugar metabolism and other domestication genes.

CsBRC1 inhibits axillary bud outgrowth by directly repressing the auxin efflux carrier CsPIN3 in cucumber

Shoot branching is an important agronomic trait that directly determines plant architecture and affects crop productivity. To promote crop yield and quality, axillary branches need to be manually removed during cucumber production for fresh market and thus are undesirable. Auxin is well known as the primary signal imposing for apical dominance and acts as a repressor for lateral bud outgrowth indirectly. The TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) family gene BRANCHED1 (BRC1) has been shown to be the central integrator for multiple environmental and developmental factors that functions locally to inhibit shoot branching. However, the direct molecular link between auxin and BRC1 remains elusive. Here we find that cucumber BRANCHED1 (CsBRC1) is expressed in axillary buds and displays a higher expression level in cultivated cucumber than in its wild ancestor. Knockdown of CsBRC1 by RNAi leads to increased bud outgrowth and reduced auxin accumulation in buds. We further show that CsBRC1 directly binds to the auxin efflux carrier PIN-FORMED (CsPIN3) and negatively regulates its expression in vitro and in vivo. Elevated expression of CsPIN3 driven by the CsBRC1 promoter results in highly branched cucumber with decreased auxin levels in lateral buds. Therefore, our data suggest that CsBRC1 inhibits lateral bud outgrowth by direct suppression of CsPIN3 functioning and thus auxin accumulation in axillary buds in cucumber, providing a strategy to breed for cultivars with varying degrees of shoot branching grown in different cucumber production systems.

BRANCHED1: A Key Hub of Shoot Branching

Shoot branching is a key process for plant growth and fitness. Newly produced axes result from axillary bud outgrowth, which is at least partly mediated through the regulation of BRANCHED1 gene expression (BRC1/TB1/FC1). BRC1 encodes a pivotal bud-outgrowth-inhibiting transcription factor belonging to the TCP family. As the regulation of BRC1 expression is a hub for many shoot-branching-related mechanisms, it is influenced by endogenous (phytohormones and nutrients) and exogenous (light) inputs, which involve so-far only partly identified molecular networks. This review highlights the central role of BRC1 in shoot branching and its responsiveness to different stimuli, and emphasizes the different knowledge gaps that should be addressed in the near future.