Microbial Involvement in Growth of Paddy

This study reviews the role of Plant Growth Promoting Rhizobacteria (PGPR), Plant Growth Promoting Fungi (PGPF) and Mycorrhizal Fungi in the enhancement of rice growth and yield. A total 70 papers spanning from 1970 to 2013 was screened and noted its conclusion to enhance rice growth and yield. The mechanisms employed by microbes in enhancing rice growth and yield includes; the production of growth regulating substances, phosphate solubilization, N-fixation, cellulose degradation and side rophore production. Some microbes are also involved in cell regulation and signaling in rice plants. We can conclude that there are significant influences on the growth and yield of rice plant by microbes.


INTRODUCTION
Currently growth of paddy uses a lot of chemical fertilizer and chemical pesticide. Over the years this has led to serious environmental problems such as depletion of soil quality and health, emergence of resistant pathogens and elimination of soil microbes involvement in growth of paddy. The increase of production of paddy must be achieved through improvement in agricultural productivity. Microbes considered as beneficial are key factor in maintaining soil quality and paddy production. As such decades, Interest in beneficial microorganism in rice has increased due to their potential use as plant growth regulator (Diem et al., 1978;Barraquio et al., 1986;Mehnaz et al., 2000;Jha and Subramanian, 2012).
Previous studies have reviewed the microbial producers of plant growth stimulators and their practical use (Tsavkelova et al., 2005), use of plant growth-promoting bacteria for biocontrol of plant diseases (Compant et al., 2005), plant-microbes interactions in enhanced fertilizer-use efficiency (Adesemoye and Kloepper, 2009), bacterial and fungal contributions to carbon sequestration in agroecosystems (Six et al., 2006), biological nitrogen fixation (Kumar and Rao, 2012) and impact of root exudates and plant defense signaling on bacterial communities in the rhizosphere (Doornbos et al., 2012).
This review discusses the ability of Plant Growth Promoting Rhizobacteria (PGPR) to enhance rice growth and yield, the ability of Plant Growth Promoting Fungi (PGPF) to enhance rice growth and yield, the ability of mycorrhizal fungi to enhance rice growth and yield and rice plant and microorganism molecular interaction.

RESULTS AND DISCUSSION
Plant growth promoting rhizobacteria: PGPR (Plant Growth Promoting Rhizobacteria) are the rhizosphere bacteria which may grow in, on or around plant tissues and have ability to improve plant growth by several mechanisms namely Phosphatesolubilizing, siderophore-producting, nitrogen-fixaxing, phytohormone-producing and exhibiting antifungal activity. (Bhattacharyya and Jha, 2012;Vessey, 2003). The mechanism of PGPR to improve plant growth is highlighted on Table 1.
Reports on the application of PGPR to improve rice growth and yield have been published by many researchers. Carreres et al. (1996) stated that application of cyanobacteria to the rice plant significantly increased the uptake of N by the rice plant. Further, Malik et al. (1997) stated that five strains of bacteria, namely, Azospirillum lipoferum N-4, A. brasilense Wb-3, Pseudomonas 96-51, Zoogloea Ky-1 and Azoarcus K-1 were able to increase biomass and nitrogen content of the rice tissue. Ashrafuzzaman et al.
isolated 10 PGPR strains from Mymensingh soil to improve rice growth. Most of isolates resulted in a  GibberellinsIAAN-Fixation Cassan et al. (2001), Mehnaz and Lazarovits (2006) and Tien et al. (1976 (2010) and Gujar et al. (2013) significant increase in plant height, root length and dry matter production of shoot and root of rice seedlings. Mia et al. (2012) has inoculated 4 PGPR strains to rice seedlings growth. The results stated that inoculation those PGPR strains have been increasing the seedling emergence, seedling vigor and root growth. The ability of PGPR to enhance rice yield has been reported in several studies. Sakthivel and Gnanamanickam (1987) reported that application of Pseudomonas fluorescens to rice plant significantly improve plant height, number of tillers and grain yields.
demonstrated that Azotobacter, Azorhizobioum and Azospirilium were able to enhance rice yield components. Vahed et al. (2012) found that Phosphate Solubilizing Bacteria (PSB) had a significant influence on grain yield and biological yield of rice. Gopalakrishnan et al. (2012) reported that four PGPR isolates from rhizosphere of a SRI field significantly enhanced rice tiller numbers, stover and grain yields, total dry matter, root length, volume and dry weight. Ruíz-Sánchez et al. (2011) examined the response of rice plants to inoculation with A. brasilense. Result showed that A. brasilense is able to enhance ascorbate content on rice plants. Pedraza et al. (2009) assessed the Azospirillum inoculation on effect on grain yield in a rice rainfed crop. The results suggested that Azospirilium significantly increased rice yield. Further, Banayo et al. (2012) evaluated three different biofertilizers. The result showed significant yield increases for all biofertilizers in some seasons but the most consistent results were achieved by the Azospirillum-based biofertilizer.

Plant growth promoting fungi: Plant Growth
Promoting Fungi (PGPF) is a class of beneficial fungi that have ability to enhance plant growth. The PGPF are including; hormone production, substrate degradation (mineralization) and suppression of deleterious microorganism (Hyakumachi, 1994;Nenwani et al., 2010). The mechanism of plant growth promotion by PGPF is listed in Table 2.
The ability of PGPF to enhance growth and yield of rice has been reported by many researchers. Four fungal (Gliocladium virens, Trichoderma virens, T. harzianum and Aspergillus niger) were examined for their effect on germination, root and shoot length and seedling weight of rice. The results suggested that seed root length, shoot height and fresh weight of rice seedlings were significant increased (Mishra and Sinha, 2000). Further, Al-Taweil et al. (2009) has applied T. viride to rice seedling and the results reported that T. viride is effectively increasing rice seedlings growth. Khan et al. (2005) examined activity of T. harzianum on rice seed germination and seeding vigor. The result showed that T. harzianum has ability to increase seedling emergence, root and shoot length, fresh and dry weight of root of rice seedlings. The same result of the ability of T. harzianum on improving rice seedling growth was also reported by Shukla et al. (2012). Choi et al. (2010) reported that imidazole-4-carboxamide (ICA), a plant-growth regulating compound that was isolated from a fairy ring forming fungus, Lepista sordida, in a greenhouse experiment, this compound increasedricegrain yield by 26% compared with control. Amprayn et al. (2012) investigated the ability of CandidatropicalisHY(CtHY) to stimulate rice seedlings growth. The application of CtHY on germinated seedlings resulted in better rice plant root growth and the colonization of CtHY was confirmed to persist on plant roots at least for 3 weeks. Furthermore, Banaay et al. (2012) reported that T. ghanense has ability to promote seedlings growth of aerobic rice variety.
Mycorrhizal fungi: Arbuscular mycorrhizal (AM) fungi are group of beneficial fungi that symbioses with the roots of around 80% of vascular plants and often increasing Phosphate (P) uptake and growth (Smith et al., 2003). Hajiboland et al. (2009) stated that Mycorrhizal colonization on rice significantly contributes to the uptake of P and K in rice plants. The ability of AM to enhance rice growth and yield has been reported by Secilia and Bagyaraj (1992), Secilia and Bagyraj (1994), Solaiman and Hirata (1997), Li et al. (2011) and Zhang et al. (2012). Yeasmin et al. (2007) stated that Mycorrhizal fungi significantly improved rice plants growth by increasing soil nutrients such as nitrogen and phosphorus. Isahak et al. (2012) examined the influence of AM to influence rice seedlings growth, as the result showed that AM is significantly enhancing plant height. Zhang et al. (2005) investigated the ability of AM to increase upland rice growth under combined soil contamination. The results showed that Mycorrhizal colonization on upland rice had a large influence on rice growth by increasing the shoot and root biomass. The inoculation of AM also gave the protective effects on upland rice under the combined soil contamination Further, Xu et al. (2013) stated that AM is able on enhancing rice production when growing in As-contaminated soils.

Paddy and microorganism molecular interaction:
Plant-microbe interactions may occur at several core spaces, specific spaces and sub-spaces within the specific space of choice and inter-spaces. These may include amongst others; Phyllosphere, Endosphere and Rhizosphere as core spaces. Phyllosphere is related with the aerial domains of the plants and endosphere being related with the internal cellular systems and its attendant transport systems. Rhizosphere can be defined as any volume of soil specially and specifically influenced by the plant roots or in association with the roots and plant-produced material. Rhizosphere, the term, can be defined as any volume of soil specially influenced by the plant roots or in association with the roots and plant-produced material. Bhattacharyya and Jha (2012) also mentioned in relation to the existence of the secorespaces. Plantexudes some phenolic compounds into the rhizosphere as signal to attract microorganism (Peters and Verma, 1990;Bais et al., 2004). Bacilio-Jiminez et al. (2003) identified rice root exudates which fall into two separate groups, amino acids (histidine, proline, valine, alanine, glycine, aspartic, arginine, tyrosine and methionine) and carbohydrate (mannose, galatose, glucose and glucuronic acid).
The abundance of rice root exudates might attract microorganism to colonize rice root that penetrate to root tissue. Reinhold- Hurek and Hurek (1997) examined the ability of Azoarcus to colonize rice roots endophytically. The result showed that bacteria invade the roots in the zone of elongation and differentiation, colonize the cortex intra-and inter-cellularly and penetrate deeply into the vascular system, entering xylem vessels, allowing systemic spreading into the rice shoot. Blilou et al. (2000) stated that the expression of a Lipid Transfer Protein (LTP) gene is regulated in Oryza sativa roots in response to colonization by the mycorrhizal fungus Glomus mosseae. And then, Transcript levels increased when the fungus forms appressoria and penetrate the root epidermis and decreased at the onset of the intercellular colonization of the root cortex. Further, Rediers et al. (2003) examined Pseudomonas stutzeri A15 genes that are switched on during rice root colonization and are switched off during free-living growth on synthetic medium. This strain is able to promote rice growth. Further, Guimil et al. (2005) stated 12 genes has transcribed only when the mycorrhizal fungus Glomus intraradices colonized rice root and those 12 genes were not detected in the absence of symbiosis.

CONCLUSION
This review has shown that there are significant influences of microbes to the growth and yield of rice plant. Many studies investigate the role of microbes to enhance rice growth and yield including producing of growth regulating substances, phosphate-solubilizing, N-fixation, cellulose degrading and siderophore producing. Some microbes are also involving in cell regulation and signaling in rice plants.  Bot., 51: 1969Bot., 51: -1977