Editorial: Root Adaptations to Multiple Stress Factors

Abstract

Editorial on the Research Topic Root Adaptations to Multiple Stress Factors The unfavorable soil (low supply of nutrients, high levels of toxic elements, salinity, compaction) and climatic (drought, waterlogging, high temperature, low temperature) conditions reduce plant and crop productivity (Pereira, 2016). Low fertility soils, and extreme weather events resulting from climate change, are a major threat to global food security (Evans, 2009). Plants have evolved sophisticated adaptive mechanisms to withstand the multiple abiotic stresses to which they are exposed (Lamers et al., 2020). Most studies on plant adaptation to abiotic stress conditions are undertaken by applying a single stress condition and analyzing the different physiological, biochemical and molecular aspects of plant acclimation (Araújo et al., 2015). This contrasts to the conditions that occur in nature where crops and other plants are routinely subjected to a combination of different abiotic stresses (Mittler, 2006). A good example of combined soil stress is the co-occurrence of aluminum (Al) toxicity and phosphorus (P) deficiency in acid soils, particularly in the tropics (Rao et al., 2016). An example of a combined climatic stress is the co-occurrence of drought and heat stresses during summer (Hammer et al., 2020). The effect of combined stress factors on crops and plants is not always additive due to the nature of interactions between the stress factors which dictate the final outcome (Mickelbart et al., 2015; Magalhaes et al., 2018). Plants depend on their root system responses for their survival in nature, and their yield and nutritional quality in agriculture (Gregory et al., 2009). Root systems are complex, and a variety of traits have been identified over the past decade that contribute to adaptation to multiple stress factors (Chen et al., 2019; Lynch, 2019). As an example of research on multiple stresses, recent studies now suggest that Al resistance can exert pleiotropic effects on P acquisition, potentially expanding the role of Al resistance on plant adaptation to acid soils (Magalhaes et al., 2018). Thus, pleiotropy could be a genetic linkage between Al resistance and low P tolerance. Understanding the mechanisms by which plants adapt to combined stress factors is critical for creating efficient genetic and agronomic strategies to develop cultivars for the sustainable intensification of production systems for meeting the growing demand for food. This e-book on the Research Topic of “Root Adaptations to Multiple Stress Factors” contains 11 articles that addressed the way root systems respond to individual and combined abiotic stress factors, including soil and climatic stress conditions. It includes studies focused on the adaptations occurring in roots from the molecular, biochemical, physiological, morphological to agroecological levels that contribute to plant performance and crop yield. On tropical, acidic soils, Al toxicity, low P availability and drought stress are the major limitations to yield stability. Molecular breeding based on a small suite of pleiotropic genes, particularly those with moderate to major phenotypic effects, could help circumvent the need for complex breeding designs and large population sizes aimed at selecting transgressive progeny accumulating favorable alleles controlling polygenic traits. The underlying question is two-fold: do common tolerance mechanisms to Al toxicity, P deficiency and drought exist? And if they do, will they be useful in a plant breeding program that targets stress-prone environments. Barros et al. critically reviewed the literature and found candidate signaling and/or regulatory proteins that may play a role in regulating plant adaptations to Al toxicity, P deficiency and drought stress. Using RNA-seq, Ojeda-Rivera et al. performed a transcriptional dissection of wild-type and stop1 root responses, individually or in combination, to toxic levels of Al³⁺, low P availability, low pH and iron (Fe) excess. They found that the level of STOP1 is post-transcriptionally and coordinately upregulated in the roots of seedlings exposed to single or combined stresses. The accumulation of STOP1 correlated with the transcriptional activation of stress-specific and common gene sets that are activated in the roots of wild-type seedlings but not in stop1 mutant. Results from this study suggested that perception of different environmental cues converges in at two levels via STOP1 signaling: post-translationally through the regulation of STOP1 turnover, and transcriptionally, via the activation of STOP1-dependent gene expression pathways that enables the root to better adapt to abiotic stress factors present in acidic soils. Al and proton rhizotoxicities are major stresses of acid soil syndrome that limit world food production. Although Al and proton rhizotoxicities are co-existing in acid soils, it remains unclear about the relationship between genetic architecture and their regulated molecular mechanisms for adapting to acid soil. Nakano et al. provided a new insight into the genetic architecture that is complex and distinct in regulation of Al and proton tolerance. They used integrated analyses of genome-wide association study (GWAS), genomic prediction (GP) and co-expression genes network analyses and successfully identified multiple loci controlling each tolerance. This study also showed that rare-allele mutations are more important for generating Al tolerance variation than for proton tolerance variation. Velinov et al. described the role of an undescribed homolog of the Aspergillus nidulans NudC gene, named NMig1 (for Nuclear Migration 1), in the root growth and multiple abiotic stress tolerance of Arabidopsis thaliana. Transgenic plants overexpressing NMig1 had enhanced root growth and branching, and accumulate less reactive oxygen species under heat...