In botany, brace roots (roots developing from aerial stem nodes) are a type of adventitious root that develop from aboveground stem nodes in many monocots. Anchorage, water and nutrient acquisition are the most important functions of roots. Thus, plants develop roots that maximize these functions for productivity and survival. In cereals such as maize, brace roots are one of the roots that contribute to these important functions. Brace roots develop constitutively in whorls from stem nodes, with the lowest whorl being the first to develop, enter the soil, branch out, and contribute the most to anchorage. Subsequent whorls may enter the soil and contribute to anchorage and resource acquisition, but they may also remain aerial. While these aerial roots do not contribute as much to anchorage, they could contribute in other ways such as forming an association with nitrogen-fixing bacteria.
Brace roots may remain aerial or penetrate the soil as they perform root functions such as anchorage and resource acquisition. Although brace root development in soil or aerial environments influences function, a lot is still unknown about how their anatomy, architecture and development contributes to their function. The physiology of brace roots is directly linked to their anatomy, architecture, and development. The dynamic interplay of internal regulators such as transcription factors, miRNAs, and phytohormones, lay the foundation for brace root development. Once brace roots emerge from stem nodes, the influence of external factors such as the availability of water, nutrients, light and humidity become prominent. Therefore, a combination of internal and external factors determine the overall organization, shape, and size of individual roots (root system architecture) and, as a result, root function.
Overview
Roots may develop from the embryo (contained in a seed) or post-embryonically (after germination). In young plants, root functions such as anchorage and resource acquisition (finding and taking up water and nutrients) are carried out by embryonic roots. Embryonic roots include primary roots and in some plants, seminal roots. In eudicot species (plants that have their embryo enclosed in two seed leaves), older plants continue to rely on a primary tap root for root functions with contribution from post-embryonic lateral roots. In contrast, monocot root functions are mostly carried out by post-embryonic nodal roots. Nodal roots are adventitious roots (roots originating from non-root tissues) that develop from stem nodes below (called crown roots) or above (called brace roots) the soil. Although many adventitious roots develop in response to stress conditions such as flooding or wounding, some adventitious roots develop as a normal (i.e., constitutive) part of the plant life cycle. A specialized type of constitutive adventitious root that originates from aboveground nodes in monocots, such as maize, sorghum, setaria and sugar cane, is called a brace root.
The term "brace root" has been inconsistently used. In some contexts, the term is used for only aboveground nodal roots that remain aerial and could provide support after tipping. This notion dates back to the work of Martin and Hershey in 1935Martin, J. N.; Hershey, A. L. (1935). The ontogeny of the maize plant. The early differentiation of stem and root structures and their morphological relationships. Iowa State Coll, J. Sci, 9: 489–503. and was further expounded by Hoppe et al. 1986. However, over time, the term has evolved to encompass all aboveground nodal roots or sometimes only those that enter the soil.
Brace roots develop starting from the lowest stem node (node closest to the soil), where multiple roots emerge arranged in a whorl around the stem. Depending on the plant and the environment, brace root whorls may develop from two, three, or more nodes up the stem. Due to the subsequent nature of brace root development, the brace root whorls that develop from higher stem nodes may remain aerial throughout the plant lifespan and are referred to as aerial brace roots while the brace root whorls closest to the ground penetrate the soil and are referred to as soil brace roots.
Brace root anatomy In maize, aerial brace roots, soil brace roots, and crown roots exhibit distinctive phenotypic traits. Anatomical differences start as early as the primordium (immature organ), where the shape of the root cap within primordia differs between belowground crown roots and aboveground brace roots. The crown root primordia has a conical root cap similar to the primary root, whereas the brace root primordia has a flattened root cap that extends further along the primordia length. As brace roots penetrate the soil, the root cap gradually resembles that of crown roots.
In general, the aerial portion of brace roots is different from the soil portion of brace roots, with the soil portion more closely resembling the crown roots. For example, the aerial segments of brace roots are green or purple in color and become colorless when the roots penetrate the soil. In addition, the aerial segments of brace roots have epidermis (outermost cell layer) that is reported to die; and a thickened hypodermis (layer of cell beneath the epidermis) and outer cortex (tissue layer located between the epidermis and the vascular tissues). When brace roots penetrate the soil, these phenotypes again become similar to crown roots. Thus, suggesting that the aerial versus soil environment plays an important role in shaping brace root anatomy.
Anatomical differences in brace roots have also been used to predict their function. For example, the number and size of differentiated late metaxylem vessels, which are utilized in water and nutrient transport, are much larger compared to those in the primary root. Indeed, brace roots from whorls high on the stem contain up to 41 times more metaxylem vessels than primary roots. Another anatomical feature that influences brace root resource acquisition is the presence of root cortical aerenchyma. Root cortical aerenchyma are enlarged air spaces in root cortices that enhance oxygen transport, which is essential to nutrient uptake during respiration. Although these air spaces do not occur in the aerial portion of brace roots, root cortical aerenchyma are observed in brace roots penetrating the soil.
Brace root architecture and function
The function of roots is partially determined by organization, shape, and size of individual roots, which is collectively called root system architecture. However, this term generally considers only the roots within the soil. Brace roots have a unique architecture that expands beyond the soil-based definition of root system architecture to include aerial environments. These different environments impact the function of brace roots for anchorage, water, and nutrient acquisition.
Brace roots were historically named for their perceived role in anchorage. Anchorage failure (termed root lodging in agricultural contexts) hinders plant growth, development, and productivity. In Zea mays (maize or corn), soil brace roots limit root lodging by stabilizing the stem, with more brace root whorls in the soil and greater brace root density within whorls correlating with better anchorage. Each whorl, however, contributes differently to anchorage with the lowest whorl (closest to the soil) contributing the most and subsequent whorls contributing less. Soil brace roots may generate a branched architecture by developing lateral roots, which theoretically increases anchorage. The aerial brace roots do not directly contribute to anchorage, but typically prevent lodged plants from remaining on the ground.
The branched architecture of soil brace roots that is advantageous for anchorage also increases surface area, which in turn impacts the efficiency of water and nutrient acquisition. Aerial brace roots on the other hand, are rigid, unbranched, and covered by a gelatinous substance called mucilage, which prevents dehydration. According to a study on the ancient Sierra Mixe maize variety, this mucilage can also harbor nitrogen-fixing microbes that contribute to nitrogen acquisition. When considering modern maize lines, one study revealed that while mucilage secretion is common, only a few lines have retained nitrogen-fixing traits similar to that of ancient maize. Moreover, genetic mapping studies identified subtilin3 (SBT3) as a negative regulator of mucilage secretion in maize. Indeed, knockout of SBT3 in a low-mucilage producing line increased mucilage secretion without impacting the number of brace root whorls, the number of brace roots per whorl, or the diameter of the brace roots. Thus, highlighting the future of engineering mucilage production to facilitate association with nitrogen-fixing bacteria.
In addition to nitrogen acquisition, brace roots that enter the soil during tasseling (the stage at which maize plants develop the male reproductive structure called tassel) have been shown to take up phosphorus. It remains unknown if this is specific to the tasseling stage or if brace roots provide an important role in phosphorus acquisition at other stages as well.
Furthermore, characterization of root architectural traits within and among maize genotypes showed that node-position impacts the growth patterns and characteristics of nodal roots; with size-related traits (e.g., stem width, number of roots per whorl, and nodal root diameter) showing significant sensitivity to node position. In contrast, traits such as root growth angle showed little variation across whorls or genotypes. However, both the root growth angle and the number of roots per whorl are impacted by the availability of soil nitrogen, suggesting that root traits are not purely allometric (related to plant size) but also environmentally dependent.
There may be other ways brace root anatomy and architecture influence root function, including how and when these features develop, thus, a clear understanding of brace root development is required to fully grasp the function of these specialized roots. This understanding will prove vital in maximizing brace root function through selective breeding.
Brace root development
Brace root development has been proposed to be a juvenile trait because brace root emergence is halted once the plant reaches maturity. As plants transition from juvenile to adult, the adult nodes favor development of reproductive structures like ears, over brace roots. The relationship between juvenile-to-adult transition and brace root development means that the two phenotypes are closely linked. This has made it difficult to separate genes directly involved in juvenile-to-adult transition from those involved in brace root development.
Signals that influence the development of brace roots are both internal and external. Internal signals include transcription factors, phytohormones, and small RNAs; external environmental signals include the availability of water, nutrients, light and humidity. Although environmental factors can influence the outcome of brace root development, it is, however, the internal genetic and cellular molecular regulation that determines the cell fate (in our context), to form brace roots.
Internal genetic and molecular regulation of brace root development Brace root development can be summarized into four main stages based on anatomy and/or gene expression. These stages have been best defined in maize and are summarized below.
Stage 1: Induction