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Soybeans (Glycine max) develop newly differentiated aerenchymatous phellem (AP) in response to waterlogging stress. AP is formed in the hypocotyl and root, thus contributing to internal aeration and adaptation to waterlogging for several legumes. Extensive accumulation of triterpenoids - lupeol and betulinic acid - has been identified in AP. However, their physiological roles in plants remain unclarified. Lupeol is converted from 2,3-oxidosqualene by lupeol synthase (LUS) and oxidized to betulinic acid. Notably, soybeans have two LUS genes (GmLUS1 and GmLUS2). Functional analysis was performed to reveal the biological and physiological functions of triterpenoids in AP using lus mutants. The AP cells of lus1 mutant lacked triterpenoid accumulation and epicuticular wax. Lupeol and betulinic acid were the major components of epicuticular wax and contributed to tissue hydrophobicity and oxygen transport to the roots. Tissue porosity in AP was lower in the lus1 mutant than in the wild-type, which resulted in reduced oxygen transport to the roots via AP. This reduction in oxygen transport resulted in shallow root systems under waterlogged conditions. Triterpenoid accumulation in AP contributes to effective internal aeration and root development for adaptation to waterlogging, suggesting the significance of triterpenoids in improving waterlogging tolerance., (© 2023 The Authors. New Phytologist © 2023 New Phytologist Foundation.)

The root barrier to radial O 2 loss (ROL) is a key root trait preventing O 2 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 (H 2 S), a potent phytotoxin in flooded soils. Using H 2 S- and O 2 -sensitive microsensors, we measured the apparent permeance to H 2 S of rice roots, tested whether restricted H 2 S intrusion reduced its adverse effects on root respiration, and whether H 2 S could induce the formation of a ROL barrier. The ROL barrier reduced apparent permeance to H 2 S by almost 99%, greatly restricting H 2 S intrusion. The ROL barrier acted as a shield towards H 2 S; O 2 consumption in roots with a ROL barrier remained unaffected at high H 2 S concentration (500 μM), compared to a 67% decline in roots without a barrier. Importantly, low H 2 S concentrations induced the formation of a ROL barrier. In conclusion, the ROL barrier plays a key role in protecting against H 2 S intrusion, and H 2 S 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., (© 2023 The Authors New Phytologist © 2023 New Phytologist Foundation.)

Drought and flooding are contrasting abiotic stressors for plants. Evidence is accumulating for root anatomical traits being essential for the adaptation to drought or flooding. However, an integrated approach to comprehensively understand root anatomical traits has not yet been established. Here we analysed the root anatomical traits of 18 wild Poaceae species differing in adaptation to a range of soil water content. Regression model analyses revealed the optimal anatomical traits that were required by the plants to adapt to low or high soil water content. While the area and number of each root tissue (e.g. stele, cortex, xylem or aerenchyma) were not strongly correlated to the soil water content, the ratio of the root tissue areas (cortex to stele ratio (CSR), xylem to stele ratio (XSR) and aerenchyma to cortex ratio (ACR)) could fully explain the adaptations of the wild Poaceae species to the soil water gradients. Our results demonstrate that the optimal anatomical traits for the adaptations to soil water content can be determined by three indices (i.e. CSR, XSR and ACR), and thus we propose that these root anatomical indices can be used to improve the tolerance of crops to drought and flooding stresses., (© 2020 The Authors New Phytologist © 2020 New Phytologist Foundation.)

Flooding causes oxygen deprivation in soils. Plants adapt to low soil oxygen availability by changes in root morphology, anatomy, and architecture to maintain root system functioning. Essential traits include aerenchyma formation, a barrier to radial oxygen loss, and outgrowth of adventitious roots into the soil or the floodwater. We highlight recent findings of mechanisms of constitutive aerenchyma formation and of changes in root architecture. Moreover, we use modelling of internal aeration to demonstrate the beneficial effect of increasing cortex-to-stele ratio on sustaining root growth in waterlogged soils. We know the genes for some of the beneficial traits, and the next step is to manipulate these genes in breeding in order to enhance the flood tolerance of our crops., (© 2020 The Authors New Phytologist © 2020 New Phytologist Foundation.)

Plants typically respond to waterlogging by producing new adventitious roots with aerenchyma and many wetland plants form a root barrier to radial O 2 loss (ROL), but it was not known if this was also the case for lateral roots. We tested the hypothesis that lateral roots arising from adventitious roots can form a ROL barrier, using root-sleeving electrodes and O 2 microsensors to assess ROL of Zea nicaraguensis, the maize (Zea mays ssp. mays) introgression line with a locus for ROL barrier formation (introgression line (IL) #468) from Z. nicaraguensis and a maize inbred line (Mi29). Lateral roots of Z. nicaraguensis and IL #468 both formed a ROL barrier under stagnant, deoxygenated conditions, whereas Mi29 did not. Lateral roots of Z. nicaraguensis had higher tissue O 2 status than for IL #468 and Mi29. The ROL barrier was visible as suberin in the root hypodermis/exodermis. Modelling showed that laterals roots can grow to a maximum length of 74 mm with a ROL barrier, but only to 33 mm without a barrier. Presence of a ROL barrier in lateral roots requires reconsideration of the role of these roots as sites of O 2 loss, which for some species now appears to be less than hitherto thought., (© 2020 The Authors New Phytologist © 2020 New Phytologist Foundation.)

Although the genes involved in metal homeostasis have been investigated over the past few decades, many genes related to metal homeostasis remain uncharacterized, and a comprehensive analysis of the expression of these genes is required. In the present study, we investigated the spatial gene expression profile of iron (Fe)-deficient and cadmium (Cd)-stressed Oryza sativa (rice) using laser microdissection and microarray analysis. Roots of Fe-deficient and Cd-stressed rice were separated into the vascular bundle, cortex, and epidermis plus exodermis. In addition, vascular bundles from new and old leaves at the lowest node, which are important for metal distribution, were analyzed separately. The spatial expression patterns were distinct in each tissue type. Fe deficiency and Cd stress also had significant effects on the transcriptomes, although these were less pronounced than the spatial effects. Genes encoding transporters involved in metal homeostasis, proteins associated with heavy metal detoxification, and phytohormone-related proteins were comprehensively investigated. Additionally, cis motifs involved in the regulation of these diverse expression changes in various tissue types were predicted. The spatial transcriptomes presented here provide novel insight into the molecular mechanisms of metal homeostasis., (© 2013 The Authors. New Phytologist © 2013 New Phytologist Trust.)

• To adapt to waterlogging in soil, some gramineous plants, such as maize (Zea mays), form lysigenous aerenchyma in the root cortex. Ethylene, which is accumulated during waterlogging, promotes aerenchyma formation. However, the molecular mechanism of aerenchyma formation is not understood. • The aim of this study was to identify aerenchyma formation-associated genes expressed in maize roots as a basis for understanding the molecular mechanism of aerenchyma formation. Maize plants were grown under waterlogged conditions, with or without pretreatment with an ethylene perception inhibitor 1-methylcyclopropene (1-MCP), or under aerobic conditions. Cortical cells were isolated by laser microdissection and their mRNA levels were examined with a microarray. • The microarray analysis revealed 575 genes in the cortical cells, whose expression was either up-regulated or down-regulated under waterlogged conditions and whose induction or repression was suppressed by pretreatment with 1-MCP. • The differentially expressed genes included genes related to the generation or scavenging of reactive oxygen species, Ca(2+) signaling, and cell wall loosening and degradation. The results of this study should lead to a better understanding of the mechanism of root lysigenous aerenchyma formation., (© 2010 The Authors. New Phytologist © 2010 New Phytologist Trust.)