Brace roots
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.
Depending on the plant and the environment, brace root whorls may develop from two, three, or more nodes up the stem.
Thus, suggesting that the aerial versus soil environment plays an important role in shaping brace root anatomy.
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.
These different environments impact the function of brace roots for anchorage, water, and nutrient acquisition.
Anchorage failure (termed root lodging in agricultural contexts) hinders plant growth, development, and productivity.
[18] The aerial brace roots do not directly contribute to anchorage, but typically prevent lodged plants from remaining on the ground.
[7][10][18] 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.
[9][19] Aerial brace roots on the other hand, are rigid, unbranched, and covered by a gelatinous substance called mucilage, which prevents dehydration.
[18] According to a study on the ancient Sierra Mixe maize variety, this mucilage can also harbor nitrogen-fixing microbes that contribute to nitrogen acquisition.
The relationship between juvenile-to-adult transition and brace root development means that the two phenotypes are closely linked.
Brace root development can be summarized into four main stages based on anatomy and/or gene expression.
In addition, external application of an ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), to stem nodes induces the outgrowth of brace roots.
Both YUC2 and YUC4 are preferentially expressed in brace root tips, and their proteins are localized in the cytoplasm and endoplasmic reticulum respectively.
RTCL interacts with a stress-responsive protein (STR) exclusively in the cytosol suggesting its involvement in brace root stress response.
These include the overexpression of mir156, which reduces squamosa promoter binding protein (SBP) transcription factor expression,[25] mutants of teopod1, teopod2 and teopod3 (TP1, TP2 and TP3),[38][39] co, constans, co-like and timing of cab1 (CCT10),[40] dwarf1, dwarf3 and dwarf5 (D1, D3 and D5),[41] anther ear1 (AN1),[41] teosinte glume architecture1 (TGA1),[42] and vivaparous8 (VP8).
Indeed, transcriptome profiling of early brace root development identified 307 up-regulated and 372 down-regulated genes,[44] the majority of which have yet to be further investigated.
As previously highlighted, brace root development is determined by a combination of internal genetic components and external environmental factors.
The response of nodal root development to withholding water has been assessed in maize, sorghum, setaria, switchgrass, brachypodium, and teosinte.
[45] Withholding water resulted in nodal (crown) root arrest after emergence and inhibition of entry into the elongation stage.
Similar to water availability, nitrogen stress can have adverse effects on nodal root development.
In a separate study, nitrogen deficiency was shown to induce steeper brace root angles,[47] which is an outcome of altering the gravitropic response in the elongation stage.
[48] This may be due to increased humidity and decreased light availability, which promotes ethylene production and retention.
[50] This consistency in the crown position is determined by a change in the red to far-red light ratio near the soil surface as the seedling emerges.
When the coleoptile reaches near the soil surface, the change in light ratio alters hormone supply, halting mesocotyl elongation.
Whether for anchorage or for water and nutrient uptake, the anatomy, architecture, and function of brace roots is environmentally influenced.
Thus, more studies are required to utilize environmental cue perception and response in brace root development to maximize their function in plant survival and fitness.
This article was adapted from the following source under a CC BY 4.0 license (2024) (reviewer reports): Thanduanlung Kamei; Irene Ikiriko; Susan Abernathy; Amanda Rasmussen; Erin E Sparks (13 October 2024).
