With advancement in human civilization and consequent progress in industrialization and space, geological and geo-hydrological technologies, water conservation/management technologies and increasing demand of nuclear and fossil fuel, the environmental conditions are changing which has bearing on the global climate change impact, and this is posing new problems to all living creatures including the plant kingdom. Deprivation of accepted resources is possibly one of the most lethal blunders that humanity has ever committed in its quest for progress and civilization. As a result, the domains of atmosphere, edaphic, biotic and abiotic impediments are expanding. All these components determine the distribution of natural and man-made vegetation, their survival and propagation. In agricultural systems, even if conditions tend to be optimized, the effect of abiotic and biotic stresses resulting from changes in the physico-chemical environment is reflected at quantitative and qualitative levels. Thus, there are multiple challenges which affect crop production, including various abiotic stresses (such as high irradiance, salt, drought, flooding, metal toxicity) and biotic stresses (weeds, plant pathogens, etc.). Impact of abiotic stresses on crop productivity is a serious issue in the majority of countries. The causal agents of biotic stress deprive the hosts of their nutrients leading to death of plants. Plants cope up these biotic stresses by eliciting defense responses. The presence of large number of resistance genes and species as well as pathogen specific defense mechanisms has been reported in a number of plant species.

In order to survive under such severe conditions, plants show stress tolerance through acclimation and adaptation mechanism. Understanding these stress responses at the cell, tissue, or whole organism level of food, feed, fibre, vegetables and ornamental crops using genomics, transcriptomics, metabolomics, lipidomics, proteomics, incorporating evidences from physiological, biochemical and molecular physiology—have all become very important. Several perception and signalling molecules/agents such as G-proteins, salicylic acid, jasmonic acid, brassinosteroids, nitric oxide, polyamines, etc. play key role in cross-talk between stress signalling pathways which serve as connecting link between sensing the stressful environment and creating sustainable physiological and biochemical responses, as part of stress tolerance management. The role of plant growth promoting microorganisms and signalling molecules in combating such stresses as well as enhancing nutrient use efficiency, thus leading to stress tolerance and high yield stability of agricultural crops, are other important areas of defense mechanism.

This special issue invited contributions reporting crop responses to different abiotic stresses and biotic stimuli during microbial infections/plant pathogen interaction. Original research articles and reviews on these aspects were invited. Over 50 articles were accepted after rigorous reviewing process out of many received. Some of the key findings reported in these accepted articles are highlighted here:

The importance of drought-induced changes to root and anatomical features for improvement of plant drought adaptations was elucidated through role of root hydraulic strategy and xylem transport mechanism at cellular and structural levels. In this context, role of root-shoot junction as a hydraulic safety valve to regulate water transport in radial as well as axial direction and tissue levels under drought stress is reported (https://springer.longhoe.net/article/10.1007/s00344-022-10807-x). Application of sodium nitroprusside mitigated water deficit effects in vetiver grass (Chrysopogan zizanioides) by increasing not only antioxidant capacity, proline, phenolic compounds and reducing electrolyte leakage, but also through enhanced level of vetiver’s essential oil yield (https://springer.longhoe.net/article/10.1007/s00344-022-10719-w). In wheat, photosynthetic performance and heat stress tolerance is reported through ethylene-mediated enhanced accumulation of proline and antioxidant defense system (https://springer.longhoe.net/article/10.1007/s00344-022-10737-8). Under combined salinity stress and K+ deficiency, it was shown in a halophyte Cakile maritima that it affected ion balance, growth and antioxidant as well as NADPH-generating system. A noteworthy finding was that the effect of both stresses was not additive while salinity improved C. maritima response to K+. (https://springer.longhoe.net/article/10.1007/s00344-022-10819-7). Strategies for safer rice production under arsenic contamination through biogeochemical features of agro-ecosystem, water management technologies and biotechnological approaches, thus alleviating As contamination has been reviewed (https://springer.longhoe.net/article/10.1007/s00344-022-10863-3).The effect of tropospheric ozone using open top chamber in tomato cultivars was demonstrated showing high oxidative stress under elevated O3 and reduced fruit biomass in sensitive cultivar, but the O3 tolerant cultivar showed less MDA content and ROS production along with enhanced enzymatic antioxidants (https://springer.longhoe.net/article/10.1007/s00344-022-10870-4).

Another study has elucidated the role of elicitors/signalling agent in combating abiotic stress. GA3—primed seeds of rapeseed showed improved germination rate and seedling growth in response to PEG-induced drought stress, thus overcoming high ROS production and causing higher activity of SOD, APX, CAT, etc. Thus, GA3 priming produced better antioxidant defense system and also contributed to osmotic adjustments through more production of soluble sugars, proline and soluble proteins, thereby protecting cell wall from degradation and preserving chloroplast from autophagy (https://springer.longhoe.net/article/10.1007/s00344-022-10718-x). Similarly, role of biopolymer chitosan in overcoming drought stress responses in mung bean plant was demonstrated using seed priming and foliar application in pot experiments (https://springer.longhoe.net/article/10.1007/s00344-022-10792-1). A comprehensive review focused on the role of ROS/RNS in advancing cellular and metabolic processes in plants, triggering respiratory burst oxidase homologs that influence spatio-temporal coordination with other signalling molecules and phytohormones, thereby exerting control of plant stress responses. Their usefulness in enhancing immune responses in plants, and modulating stress-responsive genes/transcription factors leading to plant’s growth under stress condition, in coordination with phytohormones, has been reported in various studies (https://springer.longhoe.net/article/10.1007/s00344-022-10804-0). Ameliorative response of glycine betaine, used as foliar spray on Andrographis paniculata, under water deficit was established in one of the papers (https://springer.longhoe.net/article/10.1007/s00344-022-10818-8). Role of exogenous use of melatonin, which is now shown to be an important plant growth regulator, on tomato plants resulted in conferring tolerance to combined stress of cadmium and NaCl. Melatonin imparted redox homeostasis through increased SOD, CAT, APX and glutathione reductase (https://springer.longhoe.net/article/10.1007/s00344-022-10843-7).

A comprehensive coverage on the role of potassium (K+) uptake system and mechanism available across life domains in relation to plants is described. With examples, the role of K+ in stress management as well as its uptake during pathogenicity and symbiotic association has been presented. Due attention was given to elucidate approaches aimed at optimizing intracellular K+ levels through modulation of influx/efflux mechanism and vacuole pools which support and sustain plant’s growth under stress conditions. This in particular becomes more relevant in case of salt tolerance where there is K+ retention, supporting cellular K+ homeostasis, thereby mitigating salt stress responses (https://springer.longhoe.net/article/10.1007/s00344-022-10782-3). Under the thematic area of biotechnological tools in develo** abiotic and biotic stress tolerance, several papers have been included. One of the classes of ubiquitin E3 ligases, the SKP1—like protein genes in chickpea genome were identified demonstrating their potential functional roles in hormone and abiotic stress signalling in the plant. This was evidenced through presence of hormones, plant growth and other stress—related cis-regulatory elements in chickpea SKP 1—like gene promoters, and such genes were shown to be differentially expressed under various abiotic stresses like drought, salt and oxidative stress (https://springer.longhoe.net/article/10.1007/s00344-022-10777-0). The significant role of heme oxygenase (HO) in conferring tolerance to cadmium accumulation in Eruca sativa was demonstrated through its improved activity at 125 µM Cd uptake by plants whereby it led to enhanced plant growth, chlorophyll, more BCF in leaves and roots along with increased translocation of Cd from roots to leaves, indicating dual role of HO in Eruca seedlings by improving phytoremediation efficiency and escalating stress tolerance upregulation of antioxidant defense machinery (https://springer.longhoe.net/article/10.1007/s00344-022-10825-9).

One of the papers presents evidence on the miRNAs involvement in extending adaptability to Pokkali rice to excess salt through genetic regulation at the transcriptional or post translational levels in a sequence—specific manner was demonstrated. This was achieved through comparison of small RNA and transcriptome datasets of roots and leaves of plants growing in excess salt. Focused upon miRNA nodes involved in root-leaf synergism, it was found that osa-miR167a and osa-miR399d were important part of synergistically interacting nodes that regulate growth of pokkali plants in presence of excess salt (https://springer.longhoe.net/article/10.1007/s00344-022-10801-3). Using omics tools, molecular basis for quality evaluation and ecological adaptations to salinity stress was demonstrated in a salt—tolerant medicinal halophyte, Apocyin Veneti Folium (AVF). Specific genes related to flavonoid glucoside synthesis and salt tolerance regulation were upregulated by low level of salt (https://springer.longhoe.net/article/10.1007/s00344-023-10908-1). Another paper presents a genetic strategy to improve NUE of rice plants by ectopic expression of OsGS1;1 and OsGS2 encoding genes to increase selective glutamine synthetase isoforms, thus improving nitrogen use economy by re-assimilating internally released NH3. Such an increase in assimilated nitrogen in transgenic rice plants positively partitioned into production of photosynthetic enzymes, chlorophyll pigment accumulation, improving net photosynthetic efficiency with a significant gain in N and C compared to WT control plants (https://springer.longhoe.net/article/10.1007/s00344-023-10988-z).

One review focused on the role of plant protease inhibitor (PPI) as a means for plant defense mechanism against herbivores. Protease gene regulation in herbivore insects could make use of PPI for develo** effective pest management approaches (https://springer.longhoe.net/article/10.1007/s00344-022-10767-2). Recruitment of Trichoderma asperellum T42 with pea seeds increased total N uptake efficiency in pea promoting root traits, total biomass and yield. The T42 interaction with plant roots increased nitric oxide (NO) generation. This NO generation in nitrogen nutrient confirmed that NO generation was via nitrate reductase dependent pathway. Higher expression of nitrate transporter (NRT) genes in response to Trichoderma recruitment in NO3−—fed condition and suppressive expression effect of ammonia transporter (AMT 2.1) suggested that NRTs had more affinity for NO3− acquisition through pea roots. This finding is favourable in condition where there is fluctuating nitrate availability in soil (https://springer.longhoe.net/article/10.1007/s00344-022-10861-5). Plant growth promoting nodule endophyte alleviated salt stress in groundnut. Rhizobial isolates showed plant growth promoting traits and also produced highest number of nodule per plant in saline soil (https://springer.longhoe.net/article/10.1007/s00344-023-10919-y). An elaborate review for climate resistant crops for sustainable agriculture focused on the role of molecular tool—kit of RNAi pathway that include RNAi, miRNA, atasi RNA/syn-tasiRNA for design of crop plants to confer abiotic and biotic stress resistance. It portrays various challenges that have to be addressed in this regard including efficiency of RNAi, degradation of silencing signal, environment risk assessments, so as to make RNAi technology as effective tool for plant stress management (https://springer.longhoe.net/article/10.1007/s00344-022-10880-2). Under the area of role of PGPM in overcoming stress responses caused by pathogenic infection as well as abiotic stresses in crop plants, a study involving threat posed by increasing soil salinity to Fusarium wilt severity in chickpea is reported. Plants challenged with F. oxysporum f.sp. ciceris (Foc-49) in salinity stress (FocNaCl) showed better antioxidant activity; it was found that G-protein mediated defense signalling compared to salinity without Foc and control. The defense genes in chickpea were upregulated by 2–3 folds in NaCl compared to FocNaCl indicating suppression of defense responses by Foc-49 (https://springer.longhoe.net/article/10.1007/s00344-022-10697-z). An elaborate review on role of microbes in rhizosphere to tackle heavy metal contamination in soil is reported. Using genetic engineering and transgenics approaches, an understanding of physico-biochemical and molecular mechanism associated with plant–microbe interaction during phytoremediation has been presented (https://springer.longhoe.net/article/10.1007/s00344-022-10879-9). Similarly, Sclerotinia stem and leaf rot resistant accessions were identified  from different Brassicaceae species, and the study reported generating resistant cultivars against Sclerotinia sclerotiorum (https://springer.longhoe.net/article/10.1007/s00344-022-10759-2). A review is presented that describes the mechanism—based evidences to establish the role of halophytic plants and associated microbial communities in establishing a strong base for their application in bio-saline agriculture. In this endeavour, cutting edge omics tools have facilitated our understanding of interaction of halophytes, associated microbiomes and soil rhizospheric habitats (https://springer.longhoe.net/article/10.1007/s00344-023-10912-5).

The rhizobacterium Stenotrophomonas sp. cv 83 has a plant growth promoting effect alleviating adverse impact of drought in chick pea plant, by modulating activities of defense related antioxidant enzymes—SOD, APX, peroxidase, lipoxygenase, and increasing proline, photosynthetic pigments, sugar and protein contents (https://springer.longhoe.net/article/10.1007/s00344-023-11010-2). A tobacco seed endophyte Bacillus amyloliquefaciens YN 201732 was able to enhance the ability of tobacco plants against powdery mildew, mediated through JA/ET disease resistance. Upregulation of chitinase gene and synthesis of lignin and flavonoids increased in diseased plants upon endophyte treatment, and this made the plants to resist pathogenic infection (https://springer.longhoe.net/article/10.1007/s00344-023-10922-3). Defensins are antimicrobial peptides that play a significant role in providing innate immunity to various biotic stresses in plants. In a study, identification and characterization of 22 defensin and defensin—like genes was done in chickpea based on their structural expression, chromosomal localization, conserved motifs and cis-regulatory elements. Studies showed that genotype-specific responses of many defensin genes in fungal bioassay are involved in defense against fungal pathogens like F. oxysporum f.sp. ciceris and dry root rot causing R. bataticola (https://springer.longhoe.net/article/10.1007/s00344-022-10811-1). A study signifying functional characterization of GARP gene family under biotic stress conditions that would help develop stress tolerant legumes with improved yield was highlighted (https://springer.longhoe.net/article/10.1007/s00344-022-10746-7). Helopeltis theivora—tea mosquito bug (TMB) is most devastating pests of tea plant. The transcriptomic reprogramming related to TMB infestation uncovered the long non—coding RNAs as possible regulators of tea metabolism as is reported in one of the papers. This led to modulation of genes during TMB infestation affecting metabolism in tea plant (https://springer.longhoe.net/article/10.1007/s00344-022-10893-x).

In the area of use of nanoparticles in tackling abiotic stress and pathogenesis, protein corona—the nanoparticle when enters biological system and environment is described in a review that determined behaviour of NPs such as their uptake, transport and localization. Its role in phyto-nanotechnology and safety of living organisms has been addressed considering its importance in processes like energy synthesis, pathogenesis and salt stress responses in plants (https://springer.longhoe.net/article/10.1007/s00344-022-10857-1).

The guest editors are thankful to all the authors who have contributed their research work in the form of original research papers and reviews, and to the editorial office bearers including Ms. Rhonda Peavy and Priya Gopalakrishnan. We sincerely thank Dr. Matthew Cheng and the Editor in-Chief Prof. Jutta Ludwig-Mueller for extending their support and co-operation throughout this journey. Last but not least, we extend our sincere gratitude to the esteemed reviewers who took pains to go through manuscript rigorously for quality improvement by giving judicious decisions.

Guest Editors

Padmanabh Dwivedi

Sudhir K Sopory

Dated: 4th August 2023