Soil compaction is an agricultural challenge with profound influence on the physical, chemical, and biological properties of the soil. It causes drastic changes by increasing mechanical impedance, reducing water infiltration, gaseous exchange, and biological activities. Soil compaction hinders root growth, limiting nutrient and water foraging abilities of plants. Recent research reveals that plant roots sense soil compaction due to higher ethylene accumulation in and around root tips. Ethylene orchestrates auxin and abscisic acid as downstream signals to regulate root adaptive responses to soil compaction. In this review, we describe the changes inflicted by soil compaction ranging from cell to organ scale and explore the latest research regarding plant root compaction sensing and response.

Summary: The root barrier to radial O₂ loss (ROL) is a key root trait preventing O₂ loss from roots to anoxic soils, thereby enabling root growth into anoxic, flooded soils. We hypothesized that the ROL barrier can also prevent intrusion of hydrogen sulphide (H₂S), a potent phytotoxin in flooded soils. Using H₂S‐ and O₂‐sensitive microsensors, we measured the apparent permeance to H₂S of rice roots, tested whether restricted H₂S intrusion reduced its adverse effects on root respiration, and whether H₂S could induce the formation of a ROL barrier. The ROL barrier reduced apparent permeance to H₂S by almost 99%, greatly restricting H₂S intrusion. The ROL barrier acted as a shield towards H₂S; O₂ consumption in roots with a ROL barrier remained unaffected at high H₂S concentration (500 μM), compared to a 67% decline in roots without a barrier. Importantly, low H₂S concentrations induced the formation of a ROL barrier. In conclusion, the ROL barrier plays a key role in protecting against H₂S intrusion, and H₂S can act as an environmental signalling molecule for the induction of the barrier. This study demonstrates the multiple functions of the suberized/lignified outer part of the rice root beyond that of restricting ROL.

Floods and droughts are becoming more frequent as a result of climate change and it is imperative to find ways to enhance the resilience of staple crops to abiotic stresses. This is crucial to sustain food production during unfavourable conditions. Here, we analyse the current knowledge about suberised and lignified outer apoplastic barriers, focusing on the functional roles of the barrier to radial O2 loss formed as a response to soil flooding and we discuss whether this trait also provides resilience to multiple abiotic stresses. The barrier is composed of suberin and lignin depositions in the exodermal and/or sclerenchyma cell walls. In addition to the important role during soil flooding, the barrier can also restrict radial water loss, prevent phytotoxin intrusion, salt intrusion and the main components of the barrier can impede invasion of pathogens in the root. However, more research is needed to fully unravel the induction pathway of the outer apoplastic barriers and to address potential trade-offs such as reduced nutrient or water uptake. Nevertheless, we suggest that the outer apoplastic barriers might act as a jack of all trades providing tolerance to multiple abiotic and/or biotic stressors.

Summary: A key trait conferring flood tolerance is the ability to grow adventitious roots as a response to submergence. The genetic traits of deepwater rice determining the development and characteristics of aquatic adventitious roots (AAR) had not been evaluated. We used near‐isogenic lines introgressed to test the hypothesis that the impressive shoot elongation ability of deepwater rice linked to quantitative trait loci 1 and 12 also promote the development of AAR. The deepwater rice genotype NIL‐12 possessed expanded regions at the stem nodes where numerous AAR developed as a response to submergence. Two types (AR1 and AR2) of roots with distinct timing of emergence and large differences in morphological and anatomical traits formed within 3 (AR1) to 7 (AR2) d of submergence. The mechanical impedance provided by the leaf sheath caused AR2 to emerge later promoting thicker roots, higher elongation capacity and higher desiccation tolerance. Upregulation of key genes suggests a joint contribution in activating the meristem in AAR enhancing the development of these in response to submergence. The morphological and anatomical traits suggested that AR2 is better adapted to long‐term flooding than AR1. We therefore propose that AR2 in deepwater rice functions as an evolutionary defence strategy to tackle periodic submergence.

Optimizing the root architecture of crops is an effective strategy for improving crop yields. Soil compaction is a serious global problem that limits crop productivity by restricting root growth, but the underlying molecular mechanisms are largely unclear. Here, we show that ethylene stimulates rice (Oryza sativa) crown root development in response to soil compaction. First, we demonstrate that compacted soil promotes ethylene production and the accumulation of ETHYLENE INSENSITIVE 3-LIKE 1 (OsEIL1) in rice roots, stimulating crown root primordia initiation and development, thereby increasing crown root number in lower stem nodes. Through transcriptome profiling and molecular analyses, we reveal that OsEIL1 directly activates the expression of WUSCHEL-RELATED HOMEOBOX 11 (OsWOX11), an activator of crown root emergence and growth, and that OsWOX11 mutations delay crown root development, thus impairing the plant’s response to ethylene and soil compaction. Genetic analysis demonstrates that OsWOX11 functions downstream of OsEIL1. In summary, our results demonstrate that the OsEIL1–OsWOX11 module regulates ethylene action during crown root development in response to soil compaction, providing a strategy for the genetic modification of crop root architecture and grain agronomic traits.