Branching out in roots: uncovering form, function, and regulation

Abstract

Root branching is critical for plants to secure anchorage and ensure the supply of water, minerals, and nutrients. To date, research on root branching has focused on lateral root development in young seedlings. However, many other programs of postembryonic root organogenesis exist in angiosperms. In cereal crops, the majority of the mature root system is composed of several classes of adventitious roots that include crown roots and brace roots. In this Update, we initially describe the diversity of postembryonic root forms. Next, we review recent advances in our understanding of the genes, signals, and mechanisms regulating lateral root and adventitious root branching in the plant models Arabidopsis (Arabidopsis thaliana), maize (Zea mays), and rice (Oryza sativa). While many common signals, regulatory components, and mechanisms have been identified that control the initiation, morphogenesis, and emergence of new lateral and adventitious root organs, much more remains to be done. We conclude by discussing the challenges and opportunities facing root branching research.

Figure 1.

A to D, Schematics showing diversity in root system architecture at both seedling (left) and mature (right) stages in eudicots (A and C) and monocots (B and D). A, Arabidopsis root system. B, Maize root system. C, Tomato root system (for clarity, stem-derived adventitious roots are only shown in the labeled region). D, Wheat root system. E and F, Cross sections of emerging lateral root primordia in Arabidopsis (E) and rice (F). E and F are adapted from Péret et al. (2009b).

Figure 2.

Taproot (A) and fibrous (B) root systems. Root systems of tomato (A) and wheat (B) were imaged using x-ray microscale computed tomography. LR, Lateral root; LR2, second order lateral root; PR, primary root; SR, seminal root. Scans were taken at 12 d (A) and 3 weeks (B) after germination. Roots are false colored in white, and soil is false-colored in brown. Bars = 0.5 cm (A) and 1 cm (B).

Figure 3.

Gene regulatory networks regulating adventitious root (top) and lateral root (bottom) development in Arabidopsis. While knowledge of early steps in adventitious root prebranch site formation is very limited, initiation and emergence rely on gene networks regulated by auxin (IAA), jasmonic acid (JA), and strigolactones (top). GH3, GRETCHEN HAGEN3; HAE/HSL2, HAESA RECEPTOR-LIKE PROTEIN KINASE/HAESA-LIKE 2; IDA, INFLORESCENCE DEFICIENT IN ABSCISSION. In contrast, lateral root formation and development have been studied in more detail. Developing lateral root primordia have been formally divided into eight stages (I–VIII; Malamy and Benfey, 1997). Different auxin response gene networks regulate early steps of prebranch site formation, initiation, and emergence (bottom). Different colors denote different tissues.

Figure 4.

Hydropatterning in maize roots. Microscale computed tomography-generated images show maize seedlings grown through a continuous volume of soil (A) and a macropore of air (B). Root tissue is false colored in white, and soil is false-colored in brown. Bars = 5 mm.

Figure 5.

Gene regulatory networks regulating crown root (adventitious root) development in rice. Different colors denote different tissues. ARR, ARABIDOPSIS RESPONSE REGULATOR; NBD, 2,5-norbornadiene (ethylene inhibitor); PAC, paclobutrazol (GA inhibitor); WOX, WUSCHEL-related homeobox.