INTRODUCTION The high-quality seeds have significant contribution in increasing the production potential of agricultural crops. Quality seeds with enhanced vigour contributes to nearly 30 % of total production potential of crops(Ellis, 2004). Early seedling vigour is most important attribute of quality seeds which can be enhanced through various seed-based treatment technologies. A wide range of seed-based techniques are now used in crop production to improve seedling vigour, establishment and growth under the changing environmental constraints. Seed based treatment techniques may be differentiated into physical, physiological and biological seed enhancements. Under biological seed enhancements, various plant growth-promoting microbes have been used for many decades. Among various plant growth-promoting agents, plant endophytes are becoming more popular in agricultural research and have shown positive results in enhancing plant growth and development (Lin et al., 2013). These endophytes can be used as seed bio-priming agents because of their ability to colonize diverse plant host systems through symbiotic nature. The bio-priming technique integrates both biological and physiological aspects to protect the seed and promote growth (Afzal et al., 2016). Endophytes includes bacteria, fungi, and unicellular eukaryotes are a class of plant-associated microorganisms that have shown potential in agriculture (Murphy et al., 2013; Rodriguez et al., 2009). They live at least part of their life cycle inter- or intra-cellularly inside the plants, usually without inducing any pathogenic symptoms. Bacterial and fungal endophytes have shown promise as beneficial crop inoculants, and many are known to enhance abiotic and biotic stress tolerance in plants. In the present study, an attempt has been made to evaluate the role of endophytes in enhancing early seedling growth and vigour through the seed bio-priming technique in Rice (Oryza sativa L.) as the rice is the most important food crop grown around the world, due to the ever-growing population and climate change, the pressure on the production system with available resources has become a challenging task in agricultural science. The major rice production area is reliant on water availability. Extensive cultivation of rice under lowland conditions has posed secondary salinization problems and making soil saline. Due to the scarcity of water in agriculture, direct-seeded/aerobic rice cultivation is gaining momentum. In this context, the use of endophytes to make crop systems more tolerant specifically at early growth stage to abiotic stress has become one of the research interests in agricultural science in developing sustainable agricultural production technology. In this context, a study was conducted to assess the effect of endophyte bio-priming on early seedling growth and vigour in rice var. IR 64 under normal (without stress), drought and salinity stress condition. MATERIAL AND METHODS Endophyte isolates. Five fungal endophytes isolate were collected from the School of Ecology and Conservation, UAS, GKVK, Bengaluru and listed in Table 1. Seed material. Rice (Oryza sativa L.) var. IR 64 seeds were collected from Seed Unit, Zonal Agricultural Research Station (ZARS), Mandya. Preparation of endophyte inoculums. A single hyphal tip from the actively growing endophyte fungi was cultured aseptically on PDA. Five-day-old colony culture was used to prepare mycelial suspension (Dhingra and Sinclair 1993). The mycelial suspension was prepared by washing the mycelial mat with sterile distilled water using a camel hairbrush. Spores/colony-forming units in the inoculum were counted using a haemo-cytometer under the light microscope. Further, the suspension concentration was adjusted to 2×106 spore/mycelia ml-1 and used for bio-priming. Seed bio-priming protocol. The mycelial suspension(2×106 spore/mycelia ml-1) of the respective fungal isolate was used to bio-prime 48 h of pre-germinated seeds and stirred occasionally for 3 h (Zhang et al., 2014). After 3 h of bio-priming, seeds were washed in sterile distilled water. One set of pre-germinated seeds was soaked in sterile distilled water and used as a control treatment. Each treatment was maintained with 4 replications, each replication with 50 seedlings. The final germination percentage, root and shoot length, seedling length and seedling dry weight, seedling vigour index I and II were recorded at the end of the fourteenth day. Induction of salt and drought stress. Salt stress was induced by using 170 mM (LC50) NaCl salt solution by moistening paper towels and moisture was maintained for 14 days. The control and the paper towels were moistened regularly either with water or NaCl solution. Drought stress was induced by using 17 % (LC50) PEG 8000 solution by moistening paper towels and moisture was maintained for 14 days. The control and the paper towels were moistened regularly either with water or PEG-8000 solution. Statistical design and analysis. Complete randomised design (CRD) and DMRT analysis were done using R -software. RESULTS AND DISCUSSION The experiments were conducted using five fungal endophytes under three different conditions viz., normal condition (without stress), NaCl (170 mM) induced salinity stress, and PEG-8000 (17 %) induced drought stress. Seedling growth assay was conducted under laboratory condition to assess the effect of endophyte bio-priming in enhancing early seedling growth and vigour. Observations on final germination percentage, early seedling vigour traits like, shoot length, root length, total seedling length, seedling dry weight, and seedling vigour index (SVI) I and II were recorded. Effect of endophyte bio-priming on early seedling growth and vigour under without stress condition. Under without stress condition, the endophyte strain SF 5 (Fusarium sp.) recorded the highest final germination of 92 % followed by endophyte strain PJ 9 and V4 J with 91 % germination. Control (without endophyte treatment) recorded the lowest final germination percentage of 85 % which was not statistically significant compared to better treatment. The endophyte strain V4 J (Botryosphariadothedia) recorded the highest seedling length of 42.2 cm which is statistically significant compared to the control which was recorded the lowest seedling length of 33.6 cm. the treatments V6 E, SF 5, and LAS 6 were recorded 40.0 cm, 38.7 cm and 38.4 cm seedling length and which were significantly on par with better treatment (Table 2). The results of present study were in agreement with previous studies where the increase in seedling growth was linked to the production of phytohormones by endophytes, namely gibberellic acids, auxins and cytokinins on rice growth. For instance, the fungus Cladosporium sphaerospermum produces gibberellins (GA7 and GA4), and the inoculation of this endophyte enhances rice biomass (Hamayun et al., 2009). Inoculation of plants with key growth regulators like indole acetic acid (IAA)-producing endophytic bacterium Burkholderia vietnamiensis improves rice growth and yield (Trân Van et al., 2000). IAA-producing endophytic fungal isolates from aromatic rice, positively regulate rice seed germination (Syamsia et al., 2015). Similarly, IAA-producing bacterial endophytes such as Micrococcus yunnanensis RWL-2, Micrococcus luteus RWL-3, Enterobacter soli RWL-4, Leclercia adecarboxylata RWL-5, Pantoea dispersa RWL-6, and Staphylococcus epidermidis RWL-7, were reported to promote rice shoot and root elongation, biomass production and chlorophyll content (Shahzad et al., 2017a). The endophyte strain V4 J recorded a maximum seedling dry weight of 12.06 mg which was statistically on par with other treatments Viz, PJ 9, SF 5, and V6 E with the value of 12.03 mg, 11.91 mg, and 11.68 mg respectively. While control recorded significantly lower seedling dry weight (10.95 mg) values compared to better treatment. The treatment V4 J had shown increased seedling vigour index I of 3841 which was significantly higher compared to control (2854) and on par with SF 5 (3559) and V6 E (3518). The endophytic strain V4 J had a maximum value for seedling vigour index II OF 1098 which was not significantly different from the control (930). Lalngaihawmi et al. (2018) reported similar results upon treatment with fungal endophyte resulted in increased per cent germination, shoot length and root length in rice compared to control. Rice seeds inoculated with fungal endophytes promoted the growth of rice seedlings in term of seed germination, plant height, root length and degree of root colonization (Kundar et al., 2018). Zhi-lin et al. (2007) demonstrated similar results with significantly increased numbers of tillers, plant height, chlorophyll content, photosynthetic rate between endophyte-infected and endophyte-free plants, especially at the germination and seedling stages. Effect of endophyte bio-priming on early seedling growth and vigour under NaCl (170 mM) induced salinity stress. To study the effect of seed bio-priming with endophytes under induced saline stress condition, the paper towels were moistened with 170 mM NaCl solution and bio-primed seeds were used to study germination and seedling growth parameters analysis (Table 3). There was no significant difference found in germination % between the treatments and control. However, the endophytic strains viz, V6 E, SF 5, PJ 9, and LAS 6 recorded the highest value of 85 % while, control recorded the lowest germination of 79 % (Table 1). The endophyte strain V6 E significantly increased the shoot length with the value of 13.1 cm compared to the control which had 7.9 cm. the treatment was on par with the endophyte SF 5 (12.8 cm) treatment. The endophytic strain SF 5 recorded the highest root length of 11.3 cm, which was statistically on par with the control (9.6 cm). Among the endophyte strains tested, SF 5 had recorded the highest seedling length of 24.0 cm and which was on par with the treatments V6 E (23.5 cm), LAS 6 (22.3 cm), and V4 J (21.7 cm) but, significantly higher than the control (17.5 cm). The GA-producing endophytic Bacillus amyloliquefaciens RWL-1 enhances growth, photosynthesis and biomass of rice seedlings subjected to salt stress by increasing salicylic acid (SA) and essential amino acid levels resulted in improvement in rice growth (Shahzad et al., 2017b). these findings indicate that the reduction of endogenous stress-responsive hormones, such as the senescence promoting ABA (Song et al., 2016) and the growth-inhibiting JAs (Pérez-Salamó et al., 2019; Wang et al., 2020), represents a crucial mechanism employed by phytohormone-producing endophytes to mitigate different stress responses in rice. The Seedling dry weight was increased in seedlings treated with V4 J endophyte (9.68 mg) and which was on par with other treatments except for control (7.93 mg). The endophytic strain SF 5 treated seedlings showed increased seedling vigour index (SVI) I (2042) and seedling vigour index II (807) and it was significantly higher than the control which had SVI I of 1383 and SVI II of 626. Fungal endophytes protect crops against abiotic stresses under laboratory conditions, as shown for salt (Baltruschat et al., 2008; Manasa et al., 2020). Megha et al., 2020, demonstrated that a salt-tolerant endophyte isolated from salt-adapted Pokkali rice, a Fusarium sp., colonizes the salt-sensitive rice variety IR-64, promotes its growth under salt stress and confers salinity stress tolerance to its host. The GA-producing endophytic Bacillus amyloliquefaciens RWL-1 enhances growth, photosynthesis and biomass of rice seedlings subjected to Cu stress and ameliorates the plant stress response by regulating Cu uptake, carbohydrate, and amino acid levels, and antioxidation (Shahzad et al., 2019). Endophyte and early seedling growth and vigour under PEG-8000 (17 %) induced drought stress. To study the effect of seed bio-priming with endophytes under drought stress condition, the paper towels were moistened with 17 % of PEG-8000 solution and bio-primed seeds were used to study germination and seedling growth parameters analysis (Table 4). The data on germination percentage was found non-significant due to treatments. However, LAS 6 endophyte recorded the highest germination percentage of 87 %, and the control recorded 81 %. The endophytic strain V4 J had a maximum shoot length of 14.6 cm which was on par with endophyte V6 E (13.6 cm), while the control recorded a significantly reduced shoot length of 11.9 cm. The root length was significantly increased in the seedlings treated with endophyte V4 J (23.7 cm) which was on par with SF 5 (21.2 cm) but significantly higher than the control (14.3 cm). As for as seedling length is concerned, endophyte V4 J recorded a significantly higher seedling length of 38.3 cm compared to all other treatments and the control recorded a seedling length of 26.2 cm. Similar results were reported by earlier researchers are in agreement with our present study where they showed that, reduction in stress-induced membrane damage in endophyte-inoculated rice, mirrored by lower malondialdehyde (MDA) content, has been reported (Li et al., 2012; Kakar et al., 2016; Jaemsaeng et al., 2018; Qin et al., 2019b; Shahzad et al., 2019; Sun et al., 2020; Tsai et al., 2020). High ABA levels have been associated with reduced water-deficit in endophyte- inoculated rice. ABA-producing Salicaceae endophytes reduce stomatal conductance, density and leaf water potential, enhancing water use efficiency (WUE) under drought conditions (Rho et al., 2018). The endophyte strain SF 5 has recorded a significantly higher seedling dry weight of 8.79 mg which was statistically on par with V4 J (8.67mg) and V6 E (8.63 mg) and the control recorded a lower seedling dry weight of 7.47 mg. Endophytic strain V4 J had recorded higher seedling vigour index I (3213) and it was on par with SF 5 (2967) while, the control had a significantly lower value of 2123. The seedling vigour index II was found significantly higher in seedlings treated with SF 5 endophyte (756) while control recorded lower SVI II of 605. Comparable results were reported by earlier findings, where, fungal endophytes protect crops against abiotic stresses under laboratory conditions, as shown for heat and drought (Redman et al., 2002; Bailey et al., 2006; Hubbard et al., 2014; Ali et al., 2018) stresses.Similar results were reported using P. indica, fungus has shown its multifarious functions in various fields like hardening of tissue culture plants, seedling germination, vegetative growth, early flowering, nutrient acquisition, increase yield, biotic stress tolerance and abiotic stress tolerance like drought, salinity, stress, heavy metal stress through various mechanisms (Singh et al., 2003; Sahay and Varma 1999; Waller et al., 2005; Sherameti et al., 2005; Yadav et al., 2010; Kumar et al., 2011; Jogawat et al., 2013; Das et al., 2012; Ansari et al., 2014; Rabiey et al., 2015; Ye et al., 2014; Hui et al., 2015; Sharma et al., 2015). Similarly, Sangamesh et al. (2018) evaluated thermo-tolerance of the isolates by culturing the fungi at 40 °C and 45 °C and showed that, LAS-6 (Chaetomium sp.) conferred high-temperature tolerance and other three OTUs, namely, LAS-4 (Aspergillus sp.), SAP-3 (Aspergillus sp.) and SAP-6 conferred drought tolerance in ricecultivar, IR-64, at the early seedling stage under drought stress.