Mini Review - Journal of Agricultural Science and Botany (2023) Volume 7, Issue 5
Role of RSL-like genes in drought tolerance in wheatMr. Asif Islam*
MSc Biotechnology, School of Agricultural Biotechnology, Punjab Agricultural University, Ludhiana, Punjab, India
- *Corresponding Author:
- Mr. Asif Islam
MSc Biotechnology, School of Agricultural Biotechnology
Punjab Agricultural University, Ludhiana, Punjab, India
Received: 28-july-2023, Manuscript No. AAASCB-23- 117231; Editor assigned: 31 - July -2023, Pre QC No. AAASCB -23-117231(PQ); Reviewed: 20-aug -2023, QC No. AAASCB-23- 117231; Revised: 13-sep-2023, Manuscript No. AAASCB-23- 117231 (R); Published: 12- Oct -2023, DOI: 10.35841/2591-7366-7.5.205
Citation: Islam A. Role of RSL-like genes in drought tolerance in wheat. J Agric Sci Bot. 2023; 7(5):205
VRZ (Vesicle Rich Zone), RHD (Root Hair Development), RSL (RHD SIX-LIKE)
Roots are multicellular, gravitropic organs that carry nutrients and water through vascular tissue. For plants to collect water and nutrients, anchor themselves in the soil, and communicate with symbiotic fungi, root structures are necessary. An intriguing subject that has significance for the evolution of roots and the Earth system is the origin and development of roots. Because roots evolved independently at least twice— once in the euphyllophytes (ferns and seed plants) and once in the lycophytes (clubmosses and relations), roots are not a monophyletic feature among vascular plants [1, 2]. Bryophytes lack real roots and rely on rhizoids or other structures for attachment, and they are exclusively found in vascular plants.Evolution of Rooting Structure
A significant issue in plant biology is the development and origin of roots. It is widely acknowledged that roots of various vascular plant lineages, including lycophytes and euphyllophytes (ferns, gymnosperms, and angiosperms), developed independently. The genetic processes underpinning root growth and the nature of the original root-like structure are still up for dispute. According to some studies, roots descended from the branches of rhizomes, which are horizontal stems that can develop underground or on the surface. Other studies contend that the protostele, a central cylinder of vascular tissue encircled by cortex, was the original root-like structure and that roots developed from protostele tips.
When it comes to vascular plants, lycopsids are among the earliest, dating to the Silurian era (approximately 440 million years ago). Additionally, they were the mainstays of a number of Paleozoic ecosystems, particularly during the Carboniferous epoch (between 360 and 300 million years ago), when they created enormous coal swamps and forests. The distinctive body plan of lycopsids features a dichotomously branching stem with microphylls, which are tiny leaves with a single vein, and sporangia that are carried in terminal cones or strobili. Rhizomorphs, a particular kind of rooting structure found only in lycopsids, are made up of several axes that come together to form a cylindrical organ 43. The branching pattern, architecture, growth, and physiology of rhizomorphs are all different from those of roots .
Simple rhizomorphs, consisting of a single axis or several fused axes, were present in the earliest lycopsids. According to , these rhizomorphs probably formed mycorrhizal symbioses with early terrestrial plants since they were frequently linked to fungal hyphae. Rhizomorphs of lycopsids grew more intricate and specialised as they diversified and expanded in size. One way some rhizomorphs increased their surface area for water and nutrient absorption was by developing lateral branches that resembled rootlets or root hairs. Other rhizomorphs underwent secondary growth, generating wood and bark that improved their water transport and mechanical strength. Some rhizomorphs even evolved specialised structures like tuberous organs that store starch or water or pneumatophores, which are air-filled organs that facilitate gas exchange in moist habitats .
Lycopsid rooting structures have evolved in a way that is both conservative and unequal. Conservatism is the longterm persistence of ancestor traits, whereas disparity is the development of fresh traits that broaden morphological diversity . Lycopsids exhibit conservatism in their rhizomorph architecture and fundamental body design, which have been preserved for hundreds of millions of years. Rhizomorphs have evolved into varied forms and functions as a result of lycopsids' disparate adaptability to various habitats and ecological niches. Insights about the evolutionary history of plants and their interactions with the biotic and abiotic processes that determined their diversity can be gained from the study of lycopsid rooting structures .
Significant similarities between the genes used in roots and their expression patterns during development have been found in genome-wide comparative analysis of the molecular programmes used by roots in many species. As a result, the fundamental molecular mechanism involved in root creation appears to be conserved. This shows that roots developed in the two main vascular plant lineages either through simultaneous recruitment of a developmental programme that was basically the same or through the development of a root programme that was already present in the vascular plants' common ancestor.
The non-vascular plant group known as bryophytes includes mosses, liverworts, and hornworts. They are among the oldest diverging land plant groups, but it is still unclear what their evolutionary connections are .
Vascular plants are those that have specialised xylem and phloem tissues for transporting water and nutrients. Lycophytes, ferns, gymnosperms, and angiosperms are some of them. In their life cycle, bryophytes and vascular plants rotate between haploid gametophyte and diploid sporophyte generations. The dominant and photosynthetic phase in vascular plants is the sporophyte, whereas the gametophyte is the main and photosynthetic phase in bryophytes.
According to several studies, a conserved genetic network that includes transcription factors including Radicleless1, Scarecrow, Shortroot, and Wox5 regulates root development. Scarecrow (SCR) and short root (SHR), two GRAStranscription factor family members, are necessary for the development of the endodermal layers in both roots and shoots. SCR might also regulate cell division in the QC (quiescent centre), which is where it is expressed. SCR is also expressed in flowering plants' Shoot Apical Meristem L1 layer, though mutant study has not yet shown its role there, probably due to redundancy. Together with a variety of additional transcription regulators, such as the myb-like transcription factors Werewolf (in roots) and its functionally redundant paralog, G labra2 (GL2) homeodomain-leucine zipper transcription factors have been identified. Trichoblasts (hair cells) and atrichoblasts must be differentiated in both roots and shoots, and GL1 is necessary for this process. GL2 plays different roles in the roots and the shoots; it encourages the production of trichoblasts in the shoots but suppresses it in the roots. It results in quantitative predictions that are consistent with observations on the frequency of long-term duplicate gene preservation and with observations that show that members of duplicate gene pairs share the fate of having the ancestral gene's expression patterns partitioned into different tissues. Other research suggests that lineage-specific genetic pathways involving various transcription factors, including LBD9 in lycophytes, LBD18 in ferns, and LBD16 and LBD29 in angiosperms, govern root growth .
Although bryophytes lack roots, plants do have other structures that, in some ways, mimic roots. Some liverworts, for instance, contain scales or pegs that pierce the substrate and absorb nutrients and water. Caulonemata, filamentous cells that arise from protonemata (the initial stage of gametophyte development), are seen in several mosses. These cells eventually form rhizoid-like structures. To hold onto water and nutrients, some hornworts have mucilage chambers that release polysaccharides .
It is also up for question how bryophytes' root-like structures evolved and where they came from. According to certain studies, these structures are descended from ancestors who shared traits with the ancestor of all terrestrial plants . According to other research , these structures represent convergent adaptations to terrestrial conditions that separately developed in various bryophyte lineages.Morphology of Rooting Structures
The root's fundamental tissues are all produced by the root's root apical meristem (RAM), which is an area of rapidly proliferating cells near the tip of the root [7,9,12,13]. According to Root Apical Meristem, Biology Teach, Apical Meristem Britannica, the RAM is in charge of the root's length growth and lateral root production.
The distal tip of the root's distal tip is protected by a layer of cells called the Root Cap (RC), which also makes it easier for the root to move through the soil. Additionally, the RC secretes mucilage, which lubricates the root and aids in water and nutrient absorption. The RC can also sense gravity, which causes it to steer the root's growth in the direction of the earth's centre (Root Development.exe).
Different cell lineages in the root are where the RAM and RC are produced from. The Quiescent Centre (QC), a collection of largely dormant cells that preserve their stem cell identity, is where the RAM is derived from. The protoderm, the outermost primary meristem, is where the calyptrogen layer of cells, which make up the RC, arises (Root Apical Meristem. Biology Teach.).
The RAM and RC are involved in a variety of physiological and developmental processes in the root, including gravitropism, nutrient uptake, cell elongation, cell differentiation, tissue maturation, and response to environmental stimuli (Biology Teach, Root Development.exe, Root Apical Meristem).
Basic helix-loop-helix transcription factors known as RSL like genes are involved in the production of root hair in plants. Root hairs are filamentous protrusions from the root epidermis that improve water and nutrient absorption. Root hair-forming cells contain RSL-like genes, which positively control their differentiation and lengthening. Land plants including Arabidopsis, rice, moss, and hornwort all have RSL-like genes .
According to , rizoids are filamentous outgrowths that serve as the plant's anchor to the substrate and take up nutrients from the surrounding soil. According to , they are present in early diverging groups of terrestrial plants like mosses, liverworts, and hornworts. Both unicellular and multicellular gametophytic rhizoids arise in liverworts and mosses under the regulation of RSL class I genes. This demonstrates that RSL class I genes have been retained since these plants last shared a common ancestor and that, by controlling the emergence of soil anchoring systems, they were essential for the formation of the first continental vegetation.RSL Genes and its Implications
In rice, another cereal crop, RSL-like genes are also involved in the production of root hair. In the cells that will become root hairs, the three RSL class I genes produced in rice are active. Shorter root hairs are produced when the activity of these genes is reduced, while ectopic root hairs are produced when they are overexpressed. According to this, rice has RSL class I genes that are adequate for root hair production, whereas Arabidopsis does not .
A group of RSL genes known as RSL class I genes play a conserved role in the growth of root hair in land plants. AtRHD6 and AtRSL1, two RSL class I genes, are expressed in the cells that will give rise to root hairs in Arabidopsis. Since root hairs fail to differentiate in the double mutant of these genes, these genes are required for root hair development [16, 17].
Rhizoid development requires only the expression of RSL class I genes, which are present in the cells that will give rise to rhizoids. This suggests that RSL class I genes have a long history of influencing terrestrial plants' soil anchoring mechanisms.
Another subfamily of RSL genes called RSL class II genes also contributes to the emergence of root hairs in plants. In rice, there are seven RSL class II genes, and these genes are expressed in the cells that will eventually give rise to root hairs. As their function is decreased, root hairs get shorter, whereas when they are overexpressed, they become longer and denser. This indicates that these genes positively influence the growth of root hair. Additionally, basic helix-loop-helix transcription factors are encoded by RSL class II genes and interact with the root hair development protein RHL1. RHL1's nuclear localization, which is essential for its functionality, is regulated by RSL class II genes .
RSL genes play a role in other aspects of plant growth and stress reactions in addition to root hair development. For instance, several RSL genes control the size and activity of the shoot apical meristem by being expressed there. Some RSL genes are also activated by the stress of drought and may contribute to drought tolerance. Auxin, a plant hormone that controls a variety of developmental processes, including the growth of root hair, is also responsive to several RSL genes. According to ,different plant species and evolutionary lineages may interact differently with auxin and RSL genes.
Different plant species, including rice and Brachypodium distachyon, can generate ectopic root hair cells as a result of ectopic overexpression of RSL class I genes. Since these plants' last common ancestor, it appears that RSL class I genes have been preserved. Members of the Poaceae and Brassicaceae have different strategies for suppressing RSL class I gene activity, though.Role of RSL Like Genes in Drought Mitigation
The spatial configuration of roots in the soil, known as root system architecture (RSA), has an impact on how well plants can absorb water and nutrients under various environmental conditions. Complex genetic and environmental factors, as well as their interconnections, all have an impact on RSA. One of the main environmental elements that impacts RSA and plant development and productivity is drought stress. Numerous genes and quantitative trait loci (QTLs) in agricultural plants control the quantitative trait of drought tolerance. The production of wheat, a significant cereal crop and a major food source worldwide, is endangered by drought stress as a result of climate change. The genetic underpinnings of RSA and drought tolerance in wheat must therefore be understood in order to improve wheat breeding and food security.
Deep roots, ideal root length density, optimum xylem diameter, and improved root surface area are characteristics of crop roots that aid in drought tolerance. Under drought stress, these characteristics can increase water intake effectiveness and decrease water loss. Root hairs are filamentous protrusions from the root epidermis that widen the soil contact area and improve nutrient and water uptake. One of the important genes controlling plant root development is the RSL-like gene. Because roots are important for absorbing water and nutrients from the soil, root development is essential for drought tolerance. Deep roots, optimum root length density, xylem diameter, and enhanced root surface area are root characteristics linked to drought resistance. Under drought stress, these characteristics can increase water intake efficiency and decrease water loss . Wheat that can tolerate drought benefits from root hair length because it can improve water uptake efficiency and lower water loss. Under changing climatic conditions, maintaining wheat yield and food security depends on drought resistance. Numerous other root characteristics, including deep roots, ideal root length density and xylem diameter, and enhanced root surface area, are also linked to drought resistance. Additionally, a large number of genes and quantitative trait loci control how wheat roots develop and how their root systems are organised in response to drought. The development of molecular markers, markerassisted selection, and genetic improvement in breeding for drought tolerance may benefit from RSL like genes, one of the major genes controlling root development in plants .
Root surface area is made up in part by root hair, which is developed by the RSL-like gene. Root hairs are filamentous protrusions from the root epidermis that widen the soil contact area and improve nutrient and water uptake. The root hairforming cells express the RSL like gene, which positively controls their differentiation and lengthening. According to , the RSL like gene is a member of a family of fundamental helix-loop-helix transcription factors that bind to certain DNA sequences and stimulate the expression of genes involved in the production of root hair.
By interacting with other genes and stress responses, the RSL-like gene may also contribute to the ability to withstand drought. For instance, some RSL genes are activated by the stress of a drought and may modify the expression of other genes involved in the tolerance to a drought. Auxin, a plant hormone that controls a variety of developmental processes, including the growth of root hairs, is also responsive to several RSL genes . The balance between root and shoot growth under drought stress, which is crucial for preserving plant productivity and survival, may also be impacted by auxin.
Using various techniques, including linkage mapping, association mapping, transcriptome analysis, and gene editing, several studies have been carried out to uncover QTLs and genes affecting RSA and drought tolerance in wheat. Numerous QTLs and genes have been identified in these research that are linked to a variety of RSA features, including root length, root number, root angle, root depth, root diameter, root surface area, and root biomass. Other variables related to drought resistance, including as water absorption efficiency, transpiration efficiency, stomatal conductance, photosynthetic rate, biomass build-up, grain yield, and harvest index are also influenced by some of these QTLs and genes. However, due to the complexity of these traits, the size of wheat's genome, the absence of a high-quality reference genome sequence for wheat, the limited phenotyping techniques for RSA traits, and the genotype-by-environment interactions, the genetic analysis of RSA and drought tolerance in wheat is still difficult. Numerous gene families or pathways involved in numerous facets of plant growth and stress responses contain some of the important genes that have been discovered to control RSA and drought tolerance in wheat. According to , root hairs are filamentous protrusions of the root epidermis that expand the surface area in contact with the soil and improve nutrient and water absorption. Root hair-forming cells contain RSL-like genes, which positively control their differentiation and lengthening. Land plants like Arabidopsis, rice, moss, and hornwort have RSL-like genes that are conserved . A gene named TaRSL4 exists in wheat and is an orthologue of AtRSL4 in Arabidopsis. In both diploid and allotetraploid wheats, TaRSL4 is also involved in the production of root hair, and its expression is positively linked with root hair length. Hybrids of two diploid wheats with four sets of chromosomes are known as allotetraploid wheats. Genome interaction, which describes how various genomes interact to alter gene expression, has an impact on TaRSL4 expression. Longer root hairs in allotetraploid wheats may result from the A genome of wheat's increased expression of TaRSL4 compared to the other genomes . As it can improve water uptake efficiency and decrease water loss, root hair length is a crucial characteristic for wheat drought tolerance .
The Lr genes, which provide resistance to leaf rust, a fungal disease that affects wheat leaves, are another illustration of important genes that control RSA and drought tolerance in wheat. Basic helix-loop-helix proteins encoded by Lr genes interact with other proteins to trigger pathogen defence mechanisms. In wheat, there are 14 additional genes for resistance to leaf rust in addition to more than 80 Lr genes. Some RSA features, including root length, root number, root angle, root depth, and root biomass, are likewise correlated with Lr genes. For instance, the pleiotropic gene Lr34 / Yr18 / Pm38 / Bdv1 / Sb1 / Ltn1 gives resistance to a variety of diseases, including stem rust, leaf rust, stripe rust, powdery mildew, barley yellow dwarf virus, and Septoria tritici blotch. Additionally, during drought stress, this gene increases root length, root number, root angle, root depth, and root biomass . An ATP-binding cassette transporter protein that may transport metabolites or hormones involved in plant growth and stress responses is encoded by the genes Lr34/Yr18/ Pm38/Bdv1/Sb1/Ltn1.Nutrients as a Key Factor in Shaping Root Growth and Development
Due to the fact that they supply the energy and building blocks for numerous metabolic processes, nutrients are crucial for the growth and development of plants. Additionally, nutrients can influence gene expression, which is the process of transforming genetic material into useful substances like proteins and RNAs. Numerous elements, including transcription factors, chromatin alterations, epigenetic markers, and short RNAs, control the expression of genes. Some foods may influence gene expression by participating directly or indirectly in these regulatory systems.
Both the internal nutritional status of the plant and the availability of nutrients in the soil may have an impact on how RSL-like genes are expressed. For instance, several studies have demonstrated that a lack of nitrogen can cause the expression of RSL-like genes in rice and Arabidopsis, increasing the length and production of root hair. Other genes including GLABRA2, CAPRICE, and WEREWOLF that are involved in the growth of root hair may also be impacted by nitrogen deprivation. By changing the amounts of auxin, a plant hormone that controls several developmental processes, including the creation of root hairs, nitrogen shortage may activate RSLlike genes. The balance between root and shoot growth under nutritional stress may also be impacted by auxin, which is crucial for preserving plant productivity and survival .
Phosphorus, iron, zinc, and copper are additional nutrients that may have an impact on the expression of genes similar to RSL. In Arabidopsis and rice, phosphorus shortage can also cause the production of RSL-like genes, which enhances the formation of root hair. Other genes involved in the growth of root hair, like PHOSPHATE STARVATION RESPONSE1, PHOSPHATE TRANSPORTER1, and PHOSPHATE1, may also be affected by phosphorus shortage. By changing the amounts of cytokinin, a different plant hormone that controls a variety of developmental processes, including root growth, phosphorus shortage may activate RSL-like genes. Auxin and cytokinin may also interact to modify the formation of root hair under nutritional stress .
In Arabidopsis and rice, iron deprivation can also cause the expression of RSL-like genes, increasing the length and production of root hair. Other root hair development genes including IRON-REGULATED TRANSPORTER1, FERRIC REDUCTION OXIDASE2, and FERRIC CHELATE REDUCTASE1 may also be affected by iron shortage. By changing the amounts of ethylene, a different plant hormone that controls a number of developmental processes, including the creation of root hairs, iron shortage may activate RSL-like genes. Under nutritional stress, ethylene and auxin may also interact to modify the formation of root hair.
In addition, zinc deprivation can cause the production of RSL-like genes in Arabidopsis, which promotes the growth of root hair. Other root hair-related genes including ZINC DEFICIENCY INDUCED TRANSCRIPTION FACTOR1, ZINC TRANSPORTER1, and ZINC-INDUCED FACILITATOR-LIKE1 may also be affected by zinc deficiency. By changing the amounts of abscisic acid, another plant hormone that controls numerous developmental processes, including root growth, zinc deprivation may activate RSL-like genes. According to , auxin and abscisic acid may work together to modify the formation of root hair under nutrient stress.
In Arabidopsis, a copper deficit can also cause the expression of RSL-like genes, increasing the length and production of root hair. Other root hair growth genes including COPPER TRANSPORTER1, COPPER AMINE OXIDASE1, and COPPER/ZINC SUPEROXIDE DISMUTASE1 may also be affected by copper shortage. By changing the amounts of jasmonic acid, another plant hormone that controls numerous developmental processes, including root growth, copper shortage may activate RSL-like genes. According to de Mateo , auxin and jasmonic acid may work together to modify the formation of root hair in nutrient-stressed plants.Other Key Players in Root Development
FAMA/RHD6-LIKE, also known as FRH, is a group of fundamental helix-loop-helix transcription factors that regulates the elongation of root hair cells by being expressed in these cells. RSL proteins, a different class of fundamental helix-loop-helix transcription factors that regulate root hair initiation and elongation, interact with FRH proteins. By transcriptionally controlling the expression of genes necessary for cell growth, such as EXPANSIN A7 and ERF109, FRH proteins aid in the elongation of root hairs .
In Neurospora crassa, the FRH homologous gene is a homolog of the protein frequency-interacting RNA helicase (FRH), which regulates the circadian clock and is involved in RNA metabolism [27, 28]. The FRH like gene and RSL like gene interact to control the development of rhizoids, slime papillae, mucilage papillae, and gemmae, which are structures derived from single epidermal cells that expand out of the epidermal plane, in the liverwort Merchantia polymorpha, a model organism for studying plant evolution . By stabilising its mRNA and limiting its degradation by the exosome complex, FRH like gene positively regulates the expression of RSL like gene . For the differentiation of epidermal structures, RSL like gene promotes the transcription of genes involved in cell wall production and cell growth. By influencing the stability and activity of the White Collar Complex (WCC), a transcription factor that regulates the production of circadian clock genes, the FRH-like gene and the RSL-like gene also modify the circadian rhythm of Merchantia polymorpha [28,31]. Insights into the molecular mechanisms underlying the adaptation and diversification of plant morphology and physiology are provided by the functional conservation and divergence of the FRH-like gene and the RSL-like gene between Merchantia polymorpha and other land plants [27, 29, 30]. For the differentiation of epidermal structures, RSL like gene promotes the transcription of genes involved in cell wall production and cell growth. By influencing the stability and activity of the White Collar Complex (WCC), a transcription factor that regulates the production of circadian clock genes, the FRH-like gene and the RSL-like gene also modify the circadian rhythm of Merchantia polymorpha [28, 31]. Insights into the molecular mechanisms underlying the adaptation and diversification of plant morphology and physiology are provided by the functional conservation and divergence of the FRH-like gene and the RSL-like gene between Merchantia polymorpha and other land plants [27, 29, 30].
Thus, changes in hormone signalling, chromatin alterations, epigenetic markers, and short RNAs can all affect gene expression either directly or indirectly. Different nutrients, such as nitrogen, phosphorus, iron, zinc, and copper, can affect the expression of RSL-like genes. Nutrient shortage can activate RSL-like genes, which can promote root hair formation and increase nutrient and water uptake efficiency in stressed plants. In conclusion, RSA is a complex feature that influences wheat's ability to withstand drought. To modulate RSA and drought tolerance in wheat, numerous QTLs and genes have been discovered using various methods. The genetic analysis of these features is still difficult, nevertheless, for a variety of reasons. Some important genes that control RSA and drought resistance in wheat are a part of multiple gene families or pathways that are involved in diverse facets of plant growth and stress responses. The development of molecular markers, marker-assisted selection, and genetic improvement in breeding for drought tolerance may benefit from the knowledge provided by these genes.
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