Lateral roots, in addition to adventitious roots, form a barrier to radial oxygen loss in Zea nicaraguensis and a chromosome segment introgression line in maize.

Plants typically respond to waterlogging by producing new adventitious roots with aerenchyma and many wetland plants form a root barrier to radial O2 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 O2 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 O2 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 O2 loss, which for some species now appears to be less than hitherto thought.

On the role of transposons in balancing drought tolerance and yield.

Since their discovery in the 1950s it has been speculated that transposable elements play a role in stabilizing the expression of cardinal genes. Recently, Sun et al. demonstrated that a transposable element-mediated inverted repeats-derived small RNA- and gene-regulatory network is a key player underlying the trade-off between drought tolerance and yield.

Asymmetric auxin distribution establishes a contrasting pattern of aerenchyma formation in the nodal roots of Zea nicaraguensis during gravistimulation.

Auxin distribution is essential for determining root developmental patterns. The formation of lateral roots and constitutive aerenchyma, which is a gas space developed through cell death, is regulated by auxin in rice (Oryza sativa). However, it is unclear whether the involvement of auxin in constitutive aerenchyma formation is conserved in other species. In this study, we found that constitutive aerenchyma formation was regulated by auxin in the nodal roots of Zea nicaraguensis, a wild relative of maize (Zea mays ssp. mays) grown naturally on frequently flooded coastal plains. Subsequent gravistimulation (root rotation) experiments showed opposite patterns of aerenchyma and lateral root formation. Lateral root formation on the convex side of rotated roots is known to be stimulated by a transient increase in auxin level in the pericycle. We found that aerenchyma formation was accelerated in the cortex on the concave side of the rotated nodal roots of Z. nicaraguensis. A cortex-specific expression analysis of auxin-responsive genes suggested that the auxin level was higher on the concave side than on the convex side. These results suggest that asymmetric auxin distribution underlies the regulation of aerenchyma and lateral root formation in the nodal roots of Z. nicaraguensis. As aerenchyma reduces the respiratory cost of the roots, constitutive aerenchyma on the concave side of the nodal root may balance resource allocation, thereby contributing to the uptake of water and nutrients by newly formed lateral roots. Our study provides insights into auxin-dependent asymmetric root patterning such as that of gravistimulation and hydropatterning response.

Modeling-based age-dependent analysis reveals the net patterns of ethylene-dependent and -independent aerenchyma formation in rice and maize roots.

Plants require oxygen for the functioning of roots, and thus the establishment of a long-distance diffusion path from above-water tissues to the submerged roots is essential to survive flooding. Rice (Oryza sativa) constitutively forms aerenchyma (gas spaces) under aerobic conditions, and induces its formation in response to low-oxygen conditions. Constitutive aerenchyma formation in rice roots is regulated by the phytohormone auxin, whereas ethylene stimulates inducible aerenchyma formation. However, the net patterns of the ethylene-dependent and -independent (auxin-dependent) aerenchyma formation remain unclear. In the present study, we used a modeling approach to determine age-dependent aerenchyma formation in the wild-type rice and reduced culm number 1 mutant, in which ethylene production is reduced, to reveal the net patterns of ethylene-dependent and -independent aerenchyma formation. Subsequent comparison of age-dependent aerenchyma formation between rice and maize roots suggested that more rapid induction of ethylene-dependent aerenchyma formation and more aerenchyma in rice roots are essential to achieve efficient oxygen diffusion under low-oxygen conditions. Moreover, rice roots showed rapid increase in the expression levels of ethylene biosynthesis and responsive genes, suggesting that the local ethylene production at an early time point after root-cell emergence contributes to the rapid induction of the ethylene-dependent aerenchyma formation in rice. DATA AVAILABILITY: All data included in this study are available upon request by contact with the corresponding author.