Droughts

Drought is one of the most severe abiotic stresses limiting agricultural productivity and threatening global food security. As the central organelle responsible for photosynthesis and stress perception, the chloroplast is highly sensitive to drought, and its structural and functional stability directly determines plant adaptability. Recent studies have revealed that chloroplasts undergo pronounced ultrastructural alterations under drought stress, including thylakoid membrane shrinkage, disorganization of grana stacks, and accumulation of reactive oxygen species (ROS). Excessive ROS production causes oxidative damage to lipids, proteins, and nucleic acids, whereas moderate ROS levels act as retrograde signals to regulate nuclear gene expression. In parallel, calcium (Ca2+) oscillations and retrograde signaling pathways-such as those mediated by GENOMES UNCOUPLED PROTEIN1 (GUN), 3'-phosphoadenosine-5'-phosphate (PAP), and Methylerythritol cyclodiphosphate (MecPP)-integrate chloroplast-derived stress cues with nuclear responses. To counteract drought-induced damage, plants activate a series of antioxidant systems-both enzymatic (Superoxide Dismutase (SOD), Ascorbate Peroxidase (APX), Catalase (CAT)) and non-enzymatic (Ascorbic Acid (ASA), (Glutathione) GSH, tocopherols, carotenoids)-along with protective proteins such as fibrillins (FBNs) and WHIRLYs that stabilize thylakoid and membrane structures. In addition, autophagy and plastid degradation pathways selectively remove severely damaged chloroplasts to maintain cellular homeostasis. Exogenous substances, including melatonin, 5-aminolevulinic acid (ALA), and Zinc oxide (ZnO) nanoparticles, have also been shown to enhance chloroplast stability and antioxidant capacity under drought stress. In this review, we discuss the structural and functional changes in chloroplasts, signaling networks, and protective repair mechanisms under drought stress. Furthermore, we highlight future research prospects for enhancing plant stress resilience through multi-omics integration, application of functional regulators, and molecular design breeding.

Drought alters the soil microbiota by selecting for functional traits that preserve fitness in dry conditions. Legacy effects or ecological memory refers to how past stress exposure influences microbiota responses to future environmental challenges. How precipitation legacy effects impact soil microorganisms and plants is unclear, especially in the context of subsequent drought. Here we characterized the metagenomes of six prairie soils spanning a precipitation gradient in Kansas, United States. A microbial precipitation legacy, which persisted over a 5-month-long experimental drought, mitigated the negative physiological effects of acute drought for a native wild grass species, but not for the domesticated crop species maize. RNA sequencing of roots revealed that soil microbiota with a low precipitation legacy altered expression of plant genes that mediate transpiration and intrinsic water-use efficiency during drought. Our results show how historical exposure to water stress alters soil microbiota, with consequences for future drought responses of some plant species.

Drought can impact terrestrial ecosystems concurrently but also lagged in time, that is, through legacy effects. Drought legacy effects have been identified in parameters such as tree radial growth or satellite-based greenness. Evidence in ecosystem-scale fluxes, for example, gross primary productivity (GPP), is emerging, but still limited to individual sites or specific regions. Based on GPP data at 76 long-term (≥ 7 years) eddy-covariance sites across climates and biomes, we found drought legacy effects for 26 out of 52 droughts, with magnitudes comparable to the drought concurrent effects and lasting typically up to 1 year following drought. These effects diverged in direction: 19 events led to reduced GPP in the following years (negative legacy effects), while 7 showed increased GPP (positive legacy effects). Forests experienced more negative legacy effects than non-forest ecosystems. Legacy effects were more pronounced in forests with higher hydraulic vulnerability. Our findings demonstrate the global relevance of drought legacy effects on GPP and the need for their integration into drought impact assessments.

The limiting factors of tree recovery from drought, particularly the coordination between carbon sources and sinks, remain poorly understood. In this study, juvenile Douglas fir (Pseudotsuga menziesii) were exposed to 28 d of mild or severe drought, followed by 35 d of recovery. We continuously monitored CO₂ and H₂O fluxes in shoots and roots to derive gas exchange and carbon accumulation, while measuring basal area to estimate stem growth and sapwood development. To identify underlying mechanisms of drought recovery, we periodically measured nonstructural carbohydrates (NSC), midday water potential (Ψmd), and foliar abscisic acid (ABA). We found no evidence that ABA or Ψmd limited gas exchange recovery, with stomatal conductance recovery instead related to drought-induced reductions in sapwood development. While carbon accumulation ultimately recovered to control levels following mild stress, severe stress caused persistent impairments, ultimately reducing carbon accumulation by 51%, with stem growth similarly affected. We found no evidence of growth being limited by NSC, which remained abundant. However, we suggest that drought-induced limitations to stem development govern this pattern. This became clear when considering the diurnal growth cycle, where daytime growth was largely absent in trees after exposure to severe drought despite accounting for up to 30% of total growth in control trees. Daytime growth appeared to depend on sufficient sapwood area, which likely buffered xylem tension to support growth conditions. Our findings suggest drought-induced reductions of stem hydraulic development constrain the recovery of gas exchange and growth. Further, altered diurnal growth patterns may explain prolonged productivity declines in forests following drought.