Branching out in new directions: the control of root architecture by lateral root formation

Abstract

SummaryPlant roots are required for the acquisition of water and nutrients, for responses to abiotic and biotic signals in the soil, and to anchor the plant in the ground. Controlling plant root architecture is a fundamental part of plant development and evolution, enabling a plant to respond to changing environmental conditions and allowing plants to survive in different ecological niches. Variations in the size, shape and surface area of plant root systems are brought about largely by variations in root branching. Much is known about how root branching is controlled both by intracellular signalling pathays and by environmental signals. Here, we will review this knowledge, with particular emphasis on recent advances in the field that open new and exciting areas of research. I. BackgroundA plant's root system is the site of water and nutrient uptake from the soil, a sensor of abiotic and biotic stresses, and a structural anchor to support the shoot. The root system communicates with the shoot, and the shoot in turn sends signals to the roots. A plant root system initially consists of a primary root (PR) formed during embryogenesis that has dividing cells in a meristem at its tip. As the seedling develops, certain other cells within the PR acquire the capability to *These authors contributed equally to this work.Key words: abiotic stress, biotic stress, lateral root development, nutrients, plant hormones, root system architecture, transcriptomics. (Fig. 1a). These branch out from the PR, greatly increasing the total surface area and mechanical strength of the root system and allow the plant to explore the soil environment. Ultimately, millions of higher-order root branches can form, resulting in hundreds of miles of root system in a small area of soil (Dittmer, 1937). New roots, called adventitious roots (AR), can also be formed postembryonically at the shoot-root junction, optimizing the exploration of the upper soil layers (Fig. 1a). In cereals such as rice and maize, root structure becomes more complex, with the formation of additional shoot-borne and postembryonic roots, which in turn undergo higher-order branching (Hochholdinger et al., 2004; Hochholdinger & Zimmermann, 2008; Fig. 1b). The root system architecture (RSA) of plants varies hugely between species and also shows extensive natural variation within species, reflecting the plethora of environments in which plants can grow (Cannon, 1949;Loudet et al., 2005;Osmont et al., 2007). Root system architecture manipulation is instrumental in the domestication and breeding of crop plants, because using water and nutrients from the soil in the most efficient manner affects a plant's ability to survive in stressful or poor soils. Changes in RSA can therefore have huge impacts on the final yield of a crop (reviewed in de Dorlodot et al., 2007). Of the factors that control total RSA, LR formation and growth is one of the most important.Many of the hormonal and environmental signals affecting LR development also affect other components that have a bearing on RSA...

Atlas of woody plant roots: morphology and anatomy with special emphasis on fine roots

“…Root system architecture varies hugely among species and also shows extensive natural variation within species (Nibau et al 2008). Ramification angle tends to be large, often between 60 and 90°.…”

“…Since lateral roots arise in the pericycle and must penetrate the cortex before emerging, it seems reasonable to expect them to take the shortest path, which would mean that they would normally emerge at a 90° ramification angle (Fitter 1987). Formation of lateral roots is controlled by endogeneous factors as well as many environmental parameters, such as nitrogen and phosphate availability, lack of sulphur and potassium, water and salt stress, light and biotic factors (Nibau et al 2008).…”

The Ectomycorrhizal FungusLaccaria bicolorStimulates Lateral Root Formation in Poplar and Arabidopsis through Auxin Transport and Signaling

“…Second, LRI itself, comprising cell cycle reactivation of the founder cells and subsequent further cell divisions, occurs in more proximal regions of the root and leads to the formation of a LR primordium (Malamy and Benfey, 1997). Finally, the new LR emerges after having grown through the cortex and epidermis of the parental root.The phytohormone auxin (indole-3-acetic acid [IAA]) is considered to be one of the main triggers regulating all of the different steps of LR formation (Bainbridge et al, 2008;Ditengou et al, 2008;Laskowski et al, 2008;Nibau et al, 2008). For instance, the acquisition of founder cell identity, cell cycle reactivation, and LR emergence all correlate with and require local auxin accumulation in specific cell types (Dubrovsky et al, 2008;Fukaki and Tasaka, 2009).…”

SHORT-ROOT Regulates Primary, Lateral, and Adventitious Root Development in Arabidopsis

“…The progression of a plant root system from a single meristem-containing primary root, formed during embryogenesis, to the extensively elaborated RSA of a mature plant involves numerous exogenous abiotic and biotic factors plus endogenous genetic factors. One of the most important factors determining total RSA is the postembryonic appearance of lateral root primordia (LRP) and ultimately lateral roots that branch off from the primary root (Nibau et al, 2008). Given the fact that lateral root development is considered to be a broad recapitulation of equivalent processes in the primary root, we investigated whether SHR plays a role in LRP development and, consequently, on global RSA.…”