Mechanisms involved in drought tolerance: –
The molecular mechanism in drought stress tolerance in cotton: To cope up with water deficient condition plants change some of its balanced mechanism so that it could adapt. Several genes in several modified pathways are involved in this process in response to drought stress.
ABA is sesquiterpenoid with 15 carbon ring 66. When imposed to salt, drought, cold and any other abiotic stress, plants response to them mainly by ABA and phytochrome 67. Several ABA genes loss of function mutation have been reported 68 which are susceptible to wilt and dry if stress persists.
As ABA is induced during various stresses, considered as the stress hormone in the plant 67, 69. It plays some vital role in the plant like seed dormancy, storage protein synthesis, leaf senescence delayed germination and combat pathogenic infection 67. Generally, ABA synthesis occurred in root and transported via vascular tissues 70 resulting the level of ABA increased under drought stress resulting several gene expression and stimulating the closure of stomata 71-73. Among ABA-induced genes, 54%, i.e., 133 genes are induced during drought stress 71 and ABA expression initiated under the presence of the ABA-responsive element (ABRE), a cis-acting DNA binding element 74, 75. When the extracellular stress signal perceived by membrane receptors, receptors like kinase (RLK), histidine kinase (HK) and then activates several pathways leads to the formation of ABA, ROS, and Ca+ 69. This Ca+ ion is a secondary messenger and mediates crosstalk between several signaling pathways 76, 77. ABA biosynthesis precursor ?-carotene gets activated by several drought-induced genes like zeaxanthin oxidase (ZEP for conversion of zeaxanthin to violaxanthin), 9-cis epoxycarotenoid dioxygenase (NCED for Neoxanthin to Xanthoxin), and ABA aldehyde Oxidase (AAO for the ABA synthesis from ABA aldehyde). Many of these genes are regulated by calcium-dependent phosphorylation 76-78.
Overexpression of PYR (Pyrabactin Resistant) an ABA receptor genes induced during drought confers tolerance to this stress 79. In ABA signaling pathway the main three components are PYL, PP2C (Protein Phosphatase 2C) and SnRK2 (Sucrose non fermenting related protein kinase 2) 80, 81. In Arabidopsis Genome, there are PYR, PYL or RCAR as ABA receptors 80, 82. Once PYR/ PYL/ RCAR bound with ABA, these receptors able to bind PP2C (type 2C protein phosphatase), e.g., ABI 1 and ABI2 (ABA insensitive 1, 2), a negative regulator of ABA. Inbound form, PP2C is not able to bind and dephosphorylate SnRK2 (sucrose nonfermenting kinase-1 Related protein kinase). This SnRK2 are a protein of serine-threonine kinase having a role in drought condition 83. Thus the SnRK2 in the phosphorylated state (unbound and activated form), phosphorylate ABFs (ABA-responsive element binding factors), which again binds to ABRE and induce the ABA-responsive signals 84-87. In upland cotton varieties, several genes have been identified which have drought tolerant response by ABA-dependent manners such as GhATAF1 6, GhMKK3 88, GhNAC2 89 and GhCBF3 90.
Under focus of water deficit condition for long time plant may evolve or adopt some signaling pathways to combat severe dehydration conditions. Mitogen-Activated Protein Kinase (MAPK) signaling is one of the major factors among them, they are conserved in all eukaryotes from plant to animals, from insects to fungi 91, 92. MAP Kinase pathway is a cascade signaling which composed of three different kinases viz. MAPKKK, MAPKK, MAPK 93, 94. During stress condition, another protein kinase, MAPKKKK (i.e., MAP4K) may be activated 95. After getting external stress stimuli, first, the MAPKKK activates by phosphorylation and then it phosphorylates the two Serine/Threonine (or both) residue at S/TX3-5S/T motif situated in the MAPKK activation loop. This activated MAPKK then continues the cascade by phosphorylating conserved T-X-T motif of MAPKs, which Phosphorylates several other transcription factors by their activation. Those TFs then mediates several gene expressions in stress condition 84, 92, 96, 97. MAPK signaling components are activated by ABA, cold, drought, pH 98. To shut the signaling pathway down MKPs (MAPK phosphatase) are there which provide a cut off connection during favorable condition.
In cotton, a MAPKK gene GhMKK3 isolated which shows efficient tolerance to drought stress by up-regulating root hair elongation gene and reduce the rate of water loss by stomatal closure in Nicotiana benthamiana. Silencing this gene shows a high level of water loss and thus wilting during drought 88. In G. raimondii genome, bioinformatics analysis helps to identify 28 MAPK genes 99.
Aquaporin is a type of transmembrane protein present in all form of life due to its pivotal role in water balance in the cell, belongs to MIP (Major Intrinsic Protein) superfamily. It acts by a phosphorylation-dephosphorylation mechanism 100. Aquaporin in plants consists of 5 subfamilies PIP (Plasma membrane Intrinsic Protein), SIP (Small Intrinsic Protein), TIP (Tonoplast Intrinsic Protein), XIP (X or Unrecognised Intrinsic Protein), NIP (NOD-26 like Intrinsic Protein) 101. Most of them are expressed continuously throughout the life, but some of them expressed only during imbalance of environmental factors like drought, salinity, temperature, cold, Ph, blue light and all that 102-104. There are total 71 aquaporin genes 105 found in cotton by in silico method among them twenty-eight are PIP, twenty-three TIP, twelve NIP, seven SIP and rest one is XIP. All them shows common level structural similarity that is six alpha-helical structure with five inter-helical loops and a common AEP (Alanine-Glutamine-Phenylalanine) motif in N terminus and two NPA (Asparagine-Proline-Alanine) motif 106. Among five loops A, C and E are extracellular whereas B and D are intracellular. These two play an essential role in forming of water channel 107. Aquaporin not only acts as water transporter channel but also a channel of transport for glycerol and urea in the cell. In recent studied, related that Nt-AQP1, an aquaporin of tobacco plasma membrane, transport water, glycerol, and urea 108-110, NtTIPa, a tonoplast aquaporin mainly transport urea 110.
The plasma membrane aquaporin (PIP) is the leading component concerned with water balance in the plant cell. Opening and closing of plant plasma membrane aquaporins are depended on phosphorylation (open) and dephosphorylation (close) of particular amino acid(s). Dephosphorylation of Ser115 in cytosolic loop B and Ser214 in C-terminus and of SOPIP2;1 of spinach leads to closure of this aquaporin 103 again Leu197 along with His99, Val104, Leu108 of cytosolic loop D makes a hydrophobic motif blocking the water to enter. The diameter becomes 1.4Å and further becomes narrower to 0.8Å near these residues, where the minimum diameter should be 2.1Å to pass water. In case of open structure for the SOPIP2;1 aquaporin the N terminus helix 5 extends itself into the cytoplasm, followed by displacing the amino acids blockage. As a result, the cytoplasmic loop D moves 16Å from the blockage site due to phosphorylation of Ser115 and Ser274 also creating a free space for water entrance 111.
Thus when water potential is high, the amino acids become phosphorylated followed by the opening of the gate and in case of lower water potential dephosphorylation occur resulted in the closing of the gate so that, no efflux of water from the cell takes place 112.
In case of drought stress in cotton, GhPIP2;7 gene expressed in leaves and cotyledons and shows a high range of tolerance than plants than those doesn’t have that gene 113. In G. hirsutum expression of GhPIP1;3 and GhPIP1;1 shows good result in drought tolerance 105.