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

Nutrient and Water Availability Influence Rice Physiology, Root Architecture and Ionomic Balance via Auxin Signalling

Water and soil nutrients are the vital ingredients of crop production, and their efficient uptake is essentially dependent on root development, majorly regulated by auxin. For a water-loving crop like rice, how water availability regulates nutrient acquisition, additionally, how ambient nutrient level modulates water uptake, and the role of auxin therein is not well studied. While investigating the cross-talks among these components, we found water to be essential for auxin re-distribution in roots and shaping the root architecture. We also found that supplementing rice seedlings with moderate concentrations of mineral nutrients facilitated faster water uptake and greater nutrient enrichment in leaves compared to adequate nutrient supplementation. Additionally, moderate nutrient availability favoured greater stomatal density, stomatal conductance, photosynthesis, transpiration rate and water use efficiency when water was not limiting. Further, auxin supplementation enhanced root formation in rice, while affecting their water uptake ability, photosynthesis and transpiration causing differential mineral-specific uptake trends. The present study uncovers the existence of an intricate crosstalk among water, nutrients and auxin signalling the knowledge of which will enable optimizing the growth conditions for speed breeding of rice and harnessing the components of auxin signalling to improve water and nutrient use efficiency of rice.

CDPK5 and CDPK13 play key roles in acclimation to low oxygen through the control of RBOH-mediated ROS production in rice

CALCIUM-DEPENDENT PROTEIN KINASE (CDPK) stimulates reactive oxygen species (ROS)-dependent signaling by activating RESPIRATORY BURST OXIDASE HOMOLOG (RBOH). The lysigenous aerenchyma is a gas space created by cortical cell death that facilitates oxygen diffusion from the shoot to the root tips. Previously, we showed that RBOHH is indispensable for the induction of aerenchyma formation in rice (Oryza sativa) roots under low-oxygen conditions. Here, we showed that CDPK5 and CDPK13 localize to the plasma membrane where RBOHH functions. Mutation analysis of the serine at residues 92 and 107 of RBOHH revealed that these residues are required for CDPK5- and CDPK13-mediated activation of ROS production. The requirement of Ca2+ for CDPK5 and CDPK13 function was confirmed using in vitro kinase assays. CRISPR/Cas9-based mutagenesis of CDPK5 and/or CDPK13 revealed that the double knockout almost completely suppressed inducible aerenchyma formation, whereas the effects were limited in the single knockout of either CDPK5 or CDPK13. Interestingly, the double knockout almost suppressed the induction of adventitious root formation, which is widely conserved in vascular plants, under low-oxygen conditions. Our results suggest that CDPKs are essential for the acclimation of rice to low-oxygen conditions and also for many other plant species conserving CDPK-targeted phosphorylation sites in RBOH homologs.

Root anatomical plasticity contributes to the different adaptive responses of two Phragmites species to water-deficit and low-oxygen conditions

The runner reed (Phragmites japonica ) is the dominant species on riverbanks, whereas the common reed (Phragmites australis ) thrives in continuously flooded areas. Here, we aimed to identify the key root anatomical traits that determine the different adaptative responses of the two Phragmites species to water-deficit and low-oxygen conditions. Growth measurements revealed that P . japonica tolerated high osmotic conditions, whereas P . australis preferred low-oxygen conditions. Root anatomical analysis revealed that the ratios of the cortex to stele area and aerenchyma (gas space) to cortex area in both species increased under low-oxygen conditions. However, a higher ratio of cortex to stele area in P . australis resulted in a higher ratio of aerenchyma to stele, which includes xylem vessels that are essential for water and nutrient uptakes. In contrast, a lower ratio of cortex to stele area in P . japonica could be advantageous for efficient water uptake under high-osmotic conditions. In addition to the ratio of root tissue areas, rigid outer apoplastic barriers composed of a suberised exodermis may contribute to the adaptation of P . japonica and P . australis to water-deficit and low-oxygen conditions, respectively. Our results suggested that root anatomical plasticity is essential for plants to adapt and respond to different soil moisture levels.

Age-dependent analysis dissects the stepwise control of auxin-mediated lateral root development in rice

As root elongation rates are different among each individual root, the distance from the root apices does not always reflect the age of root cells. Thus, methods for correcting variations in elongation rates are needed to accurately evaluate the root developmental process. Here, we show that modeling-based age-dependent analysis is effective for dissecting stepwise lateral root (LR) development in rice (Oryza sativa). First, we measured the increases in LR and LR primordium (LRP) numbers, diameters, and lengths in wild type and an auxin-signaling-defective mutant, which has a faster main (crown) root elongation rate caused by the mutation in the gene encoding AUXIN/INDOLE-3-ACETIC ACID protein 13 (IAA13). The longitudinal patterns of these parameters were fitted by the appropriate models and the age-dependent patterns were identified using the root elongation rates. As a result, we found that LR and LRP numbers and lengths were reduced in iaa13. We also found that the duration of the increases in LR and LRP diameters were prolonged in iaa13. Subsequent age-dependent comparisons with gene expression patterns suggest that AUXIN RESPONSE FACTOR11 (ARF11), the homolog of MONOPTEROS (MP)/ARF5 in Arabidopsis (Arabidopsis thaliana), is involved in the initiation and growth of LR(P). Indeed, the arf11 mutant showed a reduction of LR and LRP numbers and lengths. Our results also suggest that PINOID-dependent rootward-to-shootward shift of auxin flux contributes to the increase in LR and LRP diameters. Together, we propose that modeling-based age-dependent analysis is useful for root developmental studies by enabling accurate evaluation of root traits' expression.

An overview of oxygen transport in plants: diffusion and convection

The movement of gases within plants is crucial for species that live in flood-prone areas with limited soil oxygen. These plants adapt to hypoxia/anoxia not by using oxygen more efficiently, but by ensuring a steady oxygen supply to their cells. Wetland plants typically form gas-filled spaces (aerenchyma) in their tissues, providing a low-resistance pathway for gas movement between shoots and roots, especially when the shoots are above water, and the roots are submerged. Oxygen movement in plant roots is mainly through diffusion. However, in certain species, such as emergent and floating-leaved plants, pressurized flows can also facilitate the movement of gases within their stems and rhizomes. Three types of pressurized (convective) flows have been identified: humidity-induced pressurization (positive pressure), thermal osmosis (positive pressure with air flow against the heat gradient), and venturi-induced suction (negative pressure) caused by wind passing over broken culms. A clear diel variation in pressurized flows exists, with higher pressures and flows during the day and negligible pressures and flows during the night. This article discusses some key aspects of these mechanisms for oxygen movement.