Abstract:

Plants experience a remarkable range of interactions with the abiotic factors in their environments, both aboveground (light, temperature, mechanical stress) and belowground (soil moisture, nutrients, and mechanical properties). Plants’ abilities to sense diverse environmental parameters and initiate signaling pathways that activate precise responses have crucial implications for their survival. Plant abiotic interactions also remain a fascinating area of research, providing basic insight into plant biology and enabling efforts to improve crop plants.

Transcriptional and epigenetic regulation mediate many abiotic stress responses. For example, Ullah et al. (2021) reveal a role for histone deacetylases in modulating genes that respond to abscisic acid (ABA) and salt stress. Moreover, Liu et al. (2021) compared the pools of nascent RNAs and messenger RNAs during heat shock in Arabidopsis and identified effects that occurred at different steps in transcription, from recruitment of polymerase to termination. In this issue, Song et al. (2021) review the functions of chromatin‐remodeling factors in plant stress responses and Chang et al. (2020) provide a comprehensive overview of epigenetic regulation in plant abiotic stress responses.

Kinases have key functions in abiotic stress signaling (reviewed in Chen et al., 2021a) and emerging research shows that plants modulate kinase activities to rein in stress responses. For example, Yu et al. (2021a) explore the function of the Arabidopsis mitogen‐activated protein kinase kinase kinase MAPKKK18 in drought stress, finding that two RING finger ubiquitin ligases control MAPKKK18 protein levels, thus negatively regulating its role in drought stress responses. Moreover, Chen et al. (2021b) identify two Arabidopsis PLANT U‐BOX (PUB) ubiquitin ligases that also negatively modulate drought responses by ubiquitin‐mediated degradation of the receptor‐like protein kinases LEUCINE‐RICH REPEAT PROTEIN1 and KINASE7. Exploring the role of calcium‐dependent protein kinases, Zhao et al. (2021) find that maize (Zea mays) ZmCDPK7 translocates from the plasma membrane to the cytosol under heat stress, where it interacts with a heat shock protein and a Respiratory Burst Oxidase Homolog protein to regulate reactive oxygen species (ROS) in thermotolerance. Understanding how plant cells regulate kinase activities to optimize their stress responses will improve our ability to enhance plant stress tolerance.

In addition to kinases, other post‐translational factors are emerging as important, rapid mechanisms to regulate abiotic stress responses. In addition to modulating kinase levels, the ubiquitin‐mediated proteolysis system functions in drought stress tolerance: Yu et al. (2021b) identify a RING‐type E3 Ub ligase that localizes to the endoplasmic reticulum and participates in mitigating drought stress‐induced proteotoxic stress. Intriguingly, Chu et al. (2021) show that an Arabidopsis Type 2C protein phosphatase regulates the activity of a high‐affinity K⁺‐transporter to modulate sodium levels under salt stress conditions.

New findings are expanding our understanding of the mechanisms by which the “drought stress” hormone ABA acts (reviewed in Chen et al., 2020). For example, Ma et al. (2021) discovered that the ABA receptors increase the activity of the ABA‐conjugating enzyme UDP‐glucosyltransferase in a rapid mechanism to alter ABA levels. Qiao et al. (2021) explored the crosstalk between ABA and ethylene in ripening of common fig (Ficus carica). Ngoc et al. (2021) show that N4‐methylcytidine (m4C) modification of the 16S chloroplast ribosomal RNA participates in the Arabidopsis response to ABA. The identification of these diverse cellular mechanisms affecting ABA signaling give us more information on this key hormone.

Root architecture and root growth affect plant tolerance to multiple stresses, particularly drought and nutrient deficiency. An intriguing study by Wang et al. (2021) examines the function of salicylic acid (SA), a key hormone for plant immunity, in maintenance of the root quiescent center (QC), finding that SA promotes ROS production and cell division in the QC. Moreover, Su et al. (2021) reveal that SUCROSE NON‐FERMENTING kinases 2 function downstream of ABA to modulate the activity of the nitrate transceptor NITRATE TRANSPORTER1.1 and thus root growth under nitrogen deficiency, suggesting that ABA signaling has a cross‐talk with nitrate uptaking in plants.

Understanding the mechanisms that mediate abiotic stress tolerance has key implications for improving crops, as the sustainable crops of the future will require tolerance to multiple stresses, including more than one stress at a time. Translating these exciting results from controlled laboratory conditions to the field remains a major challenge. Moreover, approaches such as removing a negative regulator, or activating positive regulators, will likely have knock‐on effects on plant fitness and yield. The papers in this issue provide important segments of the broad, foundational knowledge that will be required to design the stress‐tolerant, high‐yield crops of the future.