Research progress in physiological and molecular biology mechanism of drought resistance in rice *

Rice is one of the most important crops, providing staple food for about half population of the world. Drought stress affects plant growth and development seriously. This article reviewed the research progress of the physiological and molecular biology mechanism including osmotic adjustment, scavenging oxidative radicals, endogenous hormones, drought-resistance genes and epigenetic modification, it may be afford interrelated reference for increasing rice drought resistance and breeding drought resistance rice varieties.


INTRODUCTION
Rice is one of the most important crops, providing staple food for about half population of the world [1].Rice production must be increased 60% so as to meet for the contention by the year 2025 [2].A growing population, urbanization, industrialization, pollution, and drought have compounded the shortage of water resources [3].Drought is one of the most important constraints in crop resulting in large yield losses and limiting the average yield increase.In fact, since the 1990s China's average annual drought-affected area was up to 26.67 million hectares decreasing food production by 70 -80 billion kg [4].Therefore, the development and production of drought-resistant rice varieties is be of great significance in ensuring food security, shortage of water resources, protecting environment and increasing income.The article reviewed the research progresses of the Physiological and molecular biology mechanism, which could supply interrelated reference for increasing rice drought resistance and breeding drought resistance rice varieties.

Osmoregulation Substance
Study has shown that cellular holds turgidity through inducing solute accumulation and decrease of osmotic potential [5].Osmoregulation substance could be classified two categories according to mechanism of action: One is inorganicions adjusting osmotic potential of vacuole, such as K + , Na + etc., the other is organic matter adjusting osmotic potential of cytoplasm, such as proline, betaine etc. [6].Cai et al. [7] argued that ability of leaf osmoregulation is greater than the root after subjecting to water stress at different growth stages; In addition, K + gave the greatest contribution to whether roots or leaves, secondly Ca 2+ .The sequence ordered based on content is K + > Ca 2+ > soluble sugar > Mg 2+ > free amino acid > proline, however, the sequence is proline > free amino acid > soluble sugar > K + > Mg 2+ > Ca 2+ according to extent of increase.(Cai et al., 2008).Cabuslay et al. [8] revealed that ability of leaf osmoregulation is also greater than the root after subjecting to water stress.The results of experiment in wheat also showed that K + gave the greatest contribution in drought stress, yet the sequence (K + > soluble sugar > free amino acid > Ca 2+ > Mg 2+ > Pro) is different from that in rice [9].The difference of Osmoregulation in contribution rate could be concerned with plant varieties, growth stages, intensity, environment, and time in stress.In the last years, studies have showed small organic molecules play an important role such as proline, glycine, betaine, trehalose, mannitol, fructosan, etc. [10,11].Studies indicated that accumulation of proline and betaine could decrease extent of damage [12].Since 1954 accumulation of free Proline was discovered, the relation of accumulation of free proline and water stress was studied widely [13].Content of free praline would had a multiplication in crops such as sorghum, sorghum, rice etc. [14][15][16].Proline was synthesized through P5CS(△^1pyrrolin-5-carboxylate synthetase), but it was synthetized by ornithine in normal conditions (Delauney et al. [17] and.Sheela et al. [18] indicated that there was close relationship between free proline and drought resistance because of the strong hydratability to hold on water.Wu et al.'s [19] study showed that the gene for Na + /H + of reverse transport protein was expressed efficiently in rice and then activated biosynthesis of proline.Moreover, content of Na + and proline in transgenic were above them in contrast.Storey et al. [20] indicated that salinity and water result in accumulation of Glycine-beyaine in barley.

The ROS-Eliminating System
Production and elimination of ROS (reactive oxygen species) was in balance [21,22].The ROS could pose a hazard to plant cells in adversity.The plant eliminated the ROS by antioxidase system and antioxidant [23][24][25].Antioxidase system includes superoxide dismutase (SOD), peroxidase (POD)catalase (CAT) and ascorbate peroxidase (APX-POD).Generally, high and low of antioxidase activity is connected to stress resistance in plant.The stronger stress resistance, the higher antioxidase activity [26,27].There are diversities of antioxidase activity in different plant.SOD is in centre of antioxidase system and Widespread in the plant body [28,29].In addition, the POD and CAT are also major members of antioxidase system, playing a very important in stress drought.The research showed the protective enzymes of seedlings were increased significantly by pre-treating seeds with water and 10% -15% PEG and that effect of appropriate pre-treating with PEG is super to effect of pre-treating with water [30].Wu et al. [31] proved that drought stress could result in increasing of significantly.In further, the study showed Antioxidase activities in plant leaf were higher than them in plant root significantly.In the ROS-eliminating system, antioxidant also played a significant role such as ascorbic acid (ASA), glutathione (GSH), carotenoid, etc.These substance quenched reactive oxygen species by several ways directly or indirectly [32,33].

Signal Molecule-Hormone
Plant hormone the generic terms of growth regulation with trace by synthesizing in plant.Although simpleness in chemical structure, it has the complicated physiological effect [34].The growth and development of every stages was regulated by the hormone [35].During the reaction process to water stress, the plant hormone perform a key role in the signal transduction.At present, the research on this field is one of the most pop content in life science [36][37][38][39][40]. ABA (abscisic acid) was mostly researched in plant hormone [41].Most research showed that ABA was the major signal substance, especially for root.In the early 1960s, Wright and Hiron [42] had proved that osmosis stress could induce synthesis of ABA in cellular and synthesis of ABA had great relationship with stress drought.At the same time, ABA could be used for evaluation index in authenticating stress drought.Research showed that ABA could adjust stomatal behavior of plant to enhance stress drought [41].Root is earlier than leaf in the synthesis of ABA [43].Chen et al. [44] proved that content of cytokine in the rice organs (overground part) change unconspicuously, yet ABA changed remarkably in content.Studies showed that hormonal balance was effected in water stress.Content of cytokinin and ethylene decreased, yet content of ABA increased significantly [45,46].In addition, Yang et al. [47] argued that antagonism of ABA and ethylene regulated the development of spikelet, furthermore Ratio of ABA and ethylene is a physiological feature coping with water stress.
In recent years, researches about SA (salicylic acid), BR (brassinolide) [48,49], JA (jasmonic acid) [50-54,] were studied in water stress.In addition, some secondary metabolites such as peptide, NO, SL (strigol-actones), etc, play a part like hormone in growth and development [55].Wu et al. [56] argued that a new regulatory factor, protein (14-3-3), was in signal transduction pathway of brassinolide.In addition, they also reveal a new regulating mechanism for protein OsBZR1.This Would supply a new way and means for plant in stress drought.

RESEARCH ADVANC OF DROUGHT RESISTANCE INMOLECULAR BIOLOGY
During adapting to the drought stress, the plant has some reactions in molecular level.The reactions were regulated by several genes forming response systems to the drought [57].Drought genes were divided into two classes according to the way of action [58].The first is function genes with protecting directly in drought resisting; the second is regulation genes, productions which encold could regulate function genes in signal transduction and gene expression.

Function Gene with Drought-Resistance
Research showed that there were several significant differences between transgenic rice with gene for GST and CAT1 and normal rice in growth, photosynthesis, reduction extent of RWC (relative water content), accumulation of H 2 O 2 and MDA (malon dialdehyde) and so on.These results implied that the GST and CAT1 transgene mitigated oxidative damage from water stress [59].Excessive expression of the coli trehalose synthesis A (otsA) and coli trehalose synthesis B (otsB) transgene rice, accumulation of trehalose increased and oxidative damage caused by photooxidation decreased [60].Jang et al. [61] revealed that the bifunctional fusion of the TPS and TPP (TPSP) transgene enhanced resistance to drought, salty and cold.By activating gene GH3.13 of rice and adapting the content of IAA in leaf, stem and tuber, adaptability to the drought was enhanced [62].

Regulator Gene with Drought Resistance
Hou et al. [63]

Epigenetics Mechanism
Epigenetics refers to the heritable changes in gene expression without any alteration in DNA sequence, including DNA methylation, histone modification and chromatin conformation [65,66].DNA methylation is one of the important modifications in eukaryotic genome.It regulates genetic information from the epigenetic level including regulation of gene expression, growth and development, genomic imprinting and so on.In the recent years, researches have showed that DNA methylation would be effected in stress such as heat, drought, etc. [67], and the genes with DNA methylation were connected with stress [68].That is to say that DNA methylation involved the expression and regulation progress of gene in stress.Pan et al. [69] argued that the level of DNA methylation increased significantly in water stress, the level and state of DNA methylation under drought was temporal-spatial specific and variety specific, and the change of DNA methylation was related to drought responsiveness.

CONCLUSION
It is important issue of water resource shortage which is harm to agriculture development of the world.Efficient use of water resources is an urgent problem around the world [70].However, drought-resistance is a complicated problem and is regulated by several pathways involved with many genes [71].Thus, there are several problems to resolve in the signal perceiving, the relations between several factors in signal transduction, epigenetics mechanism, transgene silence etc.Therefore, these researches would help us understand molecular mechanism in drought-resistance in depth.In addition, we should further study by integrating molecular level, cell, organ, and the whole etc.The studies would improve resistance of crops and water utilization efficiency, and establish basement for cultivating the varieties of resistance.
[64]rated a gene named osSKIPa.Studies implied that the gene osSKIPa, an upstream gene for regulating, would mobilize the other gene of drought resistance, promote the cell vitality, increased viability in water stress.The mechanism similar to chain reaction was never founded before.Huang et al.[64]acquired the rice mutant named drought and salt tolerance (DST) through screening from mutant library large-scaly.Further; they cloned the gene DST with stress-resistance.They argued that DST down-regulated expression of the gene related to metabolization of H 2 O 2 , and the ability with eliminating H 2 O 2 declined.This resulted in accumulation of H 2 O 2 , and shut of stoma in guard cells，and then improved the ability of drought-resistance by decreasing evaporation of water.