Drought causes substantial reductions in crop yields worldwide. Therefore, we set out to identify new chemical and genetic factors that regulate drought resistance in Arabidopsis thaliana. Karrikins (KARs) are a class of butenolide compounds found in smoke that promote seed germination, and have been reported to improve seedling vigor under stressful growth conditions. Here, we discovered that mutations in KARRIKIN INSENSITIVE2 (KAI2), encoding the proposed karrikin receptor, result in hypersensitivity to water deprivation. We performed transcriptomic, physiological and biochemical analyses of kai2 plants to understand the basis for KAI2-regulated drought resistance. We found that kai2 mutants have increased rates of water loss and drought-induced cell membrane damage, enlarged stomatal apertures, and higher cuticular permeability. In addition, kai2 plants have reduced anthocyanin biosynthesis during drought, and are hyposensitive to abscisic acid (ABA) in stomatal closure and cotyledon opening assays. We identified genes that are likely associated with the observed physiological and biochemical changes through a genome-wide transcriptome analysis of kai2 under both well-watered and dehydration conditions. These data provide evidence for crosstalk between ABA- and KAI2-dependent signaling pathways in regulating plant responses to drought. A comparison of the strigolactone receptor mutant d14 (DWARF14) to kai2 indicated that strigolactones also contributes to plant drought adaptation, although not by affecting cuticle development. Our findings suggest that chemical or genetic manipulation of KAI2 and D14 signaling may provide novel ways to improve drought resistance. MORE AXILLARY GROWTH2 (MAX2) is a central regulator of both strigolactone and karrikin (KAR) signaling pathways, which control many aspects of plant growth and development. MAX2 promotes plant resistance to drought. Application of a strigolactone analog enhances plant adaptation to drought, implying that strigolactone signaling regulates this trait; however, the potential contributions of the KAR signaling pathway to plant drought responses have not been explored. In this study, the functions of the KAR receptor KARRIKIN INSENSITIVE2 (KAI2) were analyzed to elucidate the role of KAI2-mediated signaling in plant drought adaptation. Our results indicate that the KAI2 signaling positively regulates drought resistance of Arabidopsis plants by both drought avoidance and drought tolerance strategies, such as through enhancing cuticle formation, stomatal closure, cell membrane integrity and anthocyanin biosynthesis. Our findings provide potential for manipulation of KAR- and/or strigolactone-mediated signaling pathways for improvement of drought resistance.

SUMMARY: Functional analyses of various strigolactone‐deficient mutants have demonstrated that strigolactones enhance drought resistance; however, the mechanistic involvement of the strigolactone receptor DWARF14 (D14) in this trait remains elusive. In this study, loss‐of‐function analysis of the D14 gene in Arabidopsis thaliana revealed that d14 mutant plants were more drought‐susceptible than wild‐type plants, which was associated with their larger stomatal aperture, slower abscisic acid (ABA)‐mediated stomatal closure, lower anthocyanin content and delayed senescence under drought stress. Transcriptome analysis revealed a consistent alteration in the expression levels of many genes related to the observed physiological and biochemical changes in d14 plants when compared with the wild type under normal and dehydration conditions. A comparative drought resistance assay confirmed that D14 plays a less critical role in Arabidopsis drought resistance than its paralog karrikin receptor KARRIKIN INSENSITIVE 2 (KAI2). In‐depth comparative analyses of the single mutants d14 and kai2 and the double mutant d14 kai2, in relation to various drought resistance‐associated mechanisms, revealed that D14 and KAI2 exhibited a similar effect on stomatal closure. On the other hand, D14 had a lesser role in the maintenance of cell membrane integrity, leaf cuticle structure and ABA‐induced leaf senescence, but a greater role in drought‐induced anthocyanin biosynthesis, than KAI2. Interestingly, a possible additive relationship between D14 and KAI2 could be observed in regulating cell membrane integrity and leaf cuticle development. In addition, our findings also suggest the existence of a complex interaction between the D14 and ABA signaling pathways in the adaptation of Arabidopsis to drought.

Summary: Phosphate (Pi) deficiency is known to be a major limitation for symbiotic nitrogen fixation (SNF), and hence legume crop productivity globally. However, very little information is available on the adaptive mechanisms, particularly in the important legume crop chickpea (Cicer arietinum L.), which enable nodules to respond to low‐Pi availability. Thus, to elucidate these mechanisms in chickpea nodules at molecular level, we used an RNA sequencing approach to investigate transcriptomes of the nodules in Mesorhizobium mediterraneum SWRI9–(MmSWRI9)–chickpea and M. ciceri CP‐31–(McCP‐31)–chickpea associations under Pi‐sufficient and Pi‐deficient conditions, of which the McCP‐31–chickpea association has a better SNF capacity than the MmSWRI9–chickpea association during Pi starvation. Our investigation revealed that more genes showed altered expression patterns in MmSWRI9‐induced nodules than in McCP‐31‐induced nodules (540 vs. 225) under Pi deficiency, suggesting that the Pi‐starvation‐more‐sensitive MmSWRI9‐induced nodules required expression change in a larger number of genes to cope with low‐Pi stress than the Pi‐starvation‐less‐sensitive McCP‐31‐induced nodules. The functional classification of differentially expressed genes (DEGs) was examined to gain an understanding of how chickpea nodules respond to Pi starvation, caused by soil Pi deficiency. As a result, more DEGs involved in nodulation, detoxification, nutrient/ion transport, transcriptional factors, key metabolic pathways, Pi remobilization and signalling were found in Pi‐starved MmSWRI9‐induced nodules than in Pi‐starved McCP‐31‐induced nodules. Our findings have enabled the identification of molecular processes that play important roles in the acclimation of nodules to Pi deficiency, ultimately leading to the development of Pi‐efficient chickpea symbiotic associations suitable for Pi‐deficient soils.

In plants, the Nuclear Factor-Y (NF-Y) transcription factors (TFs), which include three distinct types of NF-YA, NF-YB, and NF-YC TFs, have been identified to play key roles in the regulation of various plant growth and developmental processes under both normal and environmental stress conditions. In this work, a total of 40 CaNF-Y-encoding genes, including eight CaNF-YAs, 21 CaNF-YBs, and 11 CaNF-YCs, were identified in chickpea, and their major gene and protein characteristics were subsequently obtained using various web-based tools. Of our interest, a phylogenetically-based analysis predicted 18 CaNF-Ys (eight CaNF-YAs, seven CaNF-YBs, and three CaNF-YCs) that potentially play roles in chickpea responses to dehydration according to their close relationship with the well-characterized GmNF-Ys in soybean. These results were in good agreement with the enrichment of drought-responsive cis-regulatory motifs and expression patterns obtained from in silico analyses using publically available transcriptome data. Most of the phylogenetically predicted drought-responsive CaNF-Y genes (15 of 18) were quantitatively validated to significantly respond to dehydration treatment in leaves and/or roots, further supporting the results of in silico analyses. Among these CaNF-Y genes, the transcript levels of CaNF-YA01 and CaNF-YC10 were the most highly accumulated in leaves (by approximately eight-fold) and roots (by approximately 18-fold), respectively, by dehydration. Furthermore, 12 of the 18 CaNF-Y genes were found to be responsive to the most well-known stress hormone, namely abscisic acid (ABA), in leaves and/or roots, suggesting that these genes may act in chickpea response to dehydration in ABA-dependent manner. Taken together, our study has provided a comprehensive and fundamental information for further functional analyses of selected CaNF-Y candidate genes, ultimately leading to the improvement of chickpea growth under water-limited conditions.