Plant and pathogen nutrient acquisition strategies

1Pathogenic bacteria infect host plants to acquire nutrients. Initially, bacterial pathogens colonize the plant surfaces namely, the phyllosphere and the rhizosphere and obtain nutrients. Later, majority of them gain access to the interior portions of plant tissues including the vascular elements and the intercellular spaces to obtain more nutrients and to avoid harsh and fluctuating environmental conditions. The nutrients and favorable environmental conditions inside the plants help pathogens to grow and multiply at high densities, eventually causing serious diseases. Bacterial pathogens enter plants through pre-existing openings such as stomata (for example, Pseudomonas syringae pv. tomato, causal agent of bacterial speck of tomato;; ), nectarthodes (for example, Erwinia amylovora, causal agent of fire blight of apple;; ), hydathodes (for example, Xanthomonas oryzae pv. oryzae, causal agent of bacterial blight of rice,; ), and lenticels (for example, E. carotovora pv. atroseptica, causal agent of blackleg of potato; ). They also enter through abrasions on leaf, stem or root and are transmitted through sucking insects (for example, Xylella fastidiosa, causal agent of Pierce’s disease of grapevine and variegated chlorosis of citrus) feeding on vascular elements. Once inside the plant, different pathogenic bacteria inhabit different parts of plant tissues termed in this review as a ‘niche’. Plant niches that harbor pathogens can be defined on the basis of distinct anatomical features of plant tissue, variation in nutrient contents and the difference in their access to pathogens. Based on this, we classified the nutrient niches in six different types namely, phyllosphere, rhizosphere, apoplast, xylem, phloem, and cell organelles (Figure 1). Bacterial pathogens are mainly limited to the apoplast, but some can inhabit xylem or phloem cells to obtain nutrients. Sugars and amino acids are predominantly present in the phloem and the leaf apoplast whereas, mineral nutrients and water are abundant in the xylem and the root apoplast.

2Biotrophic pathogens require living host tissues that ensure prolonged supply of the carbon and other nutrients to them. Necrotrophic pathogens cause host cell death and acquire nutrients released from dead cells. Hemibiotrophic pathogens obtain nutrients from the host cell by exhibiting the biotrophic phase in early stages of the infection process and then switching to the necrotrophic mode during later stages of their life cycle. These three types of pathogens utilize different strategies to acquire nutrients. They can either modulate their own metabolism or manipulate the plant cell machinery to acquire nutrients. For instance, bacterial pathogens use transporters for the uptake of desirable and readily available nutrients. Further, they can also take up less preferred nutrients as such or modify it to a desirable form using extracellular enzymes. Biotrophs and hemibiotrophs use type-III secretion system (TTSS) to inject the effector molecules that target different components of the host cell machinery and modulate the plant metabolism for releasing nutrients. For example, some effector molecules facilitate the sugar efflux by inducing the expression of genes encoding sugar transporters or genes involved in changing the membrane permeability characteristics of host cell. However, necrotrophs secrete cell wall-degrading enzymes that break down the components of host cell wall leading to the release of nutrients. In this review, we provide an overview of different plant niches and the critical nutrients that are utilized by the bacterial pathogens for their survival (Supplementary Table S1). We also enumerate different types of nutrient acquisition strategies used by pathogenic bacteria for their multiplication in the host plants.

3Phyllosphere and rhizosphere are important plant surface nutrient niches for bacterial inhabitation, where the bacteria can survive and sometimes grow as epiphyte. However, the phyllosphere is a hostile environment for most of the bacteria due to many surface-associated stresses such as inconsistency in temperature and relative humidity, as well as direct exposure to ultraviolet rays. Bacteria also face nutrient limitation because waxy cuticle of leaves restricts diffusion of nutrients from the plant interior to the phyllosphere and no active transport of nutrients is reported from the cellular interior to the phyllosphere. Carbon sources including sugars, amino acids, and organic acids are the determinants of bacterial colonization and these are leached through stomata, hydathodes, and glandular trichomes or wounds. Besides, few minerals like phosphate, calcium, magnesium, potassium, sodium, and manganese also leach out passively from the leaf interior to the surfaces. Glucose, fructose, and sucrose are predominant carbon sources in the phyllosphere followed by amino acids like γ-amino butyric acid (GABA) and organic acids (Supplementary Table S2;;;;; ). However, the distribution of nutrients in phyllosphere is not homogeneous. At few places, they are abundant due to localized accumulation at the site of leakage. Such places can serve as a microhabitat for bacteria. Bacterial pathogens can actively consume the carbon sources present in the phyllosphere. There is a correlation between carbon source availability in the phyllosphere and its utilization profile of pathogen. For example, the in vitro carbon source utilization profile of P. syringae pv. tomato indicates the utilization of those carbon sources which are predominantly present in the phyllosphere such as glucose, sucrose, fructose, organic acids including succinic acid, malic acid, and citric acid and amino acids including alanine, serine, asparagine, glutamic acid, aspartic acid, and GABA. In fact, the pattern of their colonization is found to be similar to that of the nutrient distribution. In order to know the location and distribution of nutrients in the phyllosphere, green fluorescent protein (GFP)-based bacterial sugar-responsive biosensors were used. Bean plants were inoculated with Erwinia herbicola 299R (Eh299R), harboring plasmid carrying the promoter region of the fructose B (fruB) operon from Escherichia coli fused with GFP-encoding gene. Eh299R fructose-responsive biosensors demonstrated that the distribution of E. herbicola correlates with the availability of fructose in the phyllosphere. Similarly, in another study, E. herbicola harboring plasmid that carried sucrose-responsive promoter fused with GFP gene was used to demonstrate the spatially variable distribution of sucrose on the bean leaves. These studies depicted the location and heterogeneous distribution of nutrients in the phyllosphere. Such studies also suggest that the availability of nutritional resources determine the fate of bacterial colonization in the phyllosphere.

4The rhizosphere is another nutrient niche that harbors many non-pathogenic and pathogenic bacterial communities. Beneficial rhizosphere bacteria are well studied. However, only few bacterial pathogens are known that infect the plants through roots. They include Ralstonia solanacearum (causal agent of bacterial wilt of tomato), Agrobacterium tumefaciens (causal agent of crown gall tumor), Dickeya dadantii (causal agent of soft rot diseases of several crops) and Dickeya solani (causal agent of soft rot diseases of potato), Pectobacterium carotovorum (causal agent of soft rot diseases of several crops), and Pectobacterium atrosepticum (causal agent of black leg diseases of potato) and these were ranked among the top 10 plant pathogenic bacteria. Rhizosphere colonization is the primary step involved in the pathogenesis of bacterial pathogen. Nutrient sources available in rhizosphere can influence the bacterial population and play an important role in the niche colonization. The rhizosphere is enriched with large quantities of sugars, amino acids, organic acids, and minerals in the form of root exudates (Supplementary Table S2). The most abundant nutrients available in the root exudates determine the ability of bacteria to colonize the rhizosphere. For example, organic acids such as citric acid, malic acid, lactic acid, and succinic acid are the major nutrients present in tomato root at amount five times higher than that of the sugars. Psuedomonas fluorescens mutants impaired in their ability to utilize organic acids were not able to colonize tomato root and showed reduced growth in rhizosphere. On the other hand, a mutant impaired in their ability to utilize sugars normally colonized the rhizosphere. This suggests the direct correlation between nutrient abundance in the niche and nutrient utilization ability of the bacteria.

5Apoplast provides conducive environment and more protection for host pathogens from many surface-associated responses. Therefore, it is the major site for colonization by bacterial pathogens. Nutrients present in the apoplast mainly flow from the cytosol of surrounding cells. Further, an active efflux is also reported. For example, during phloem loading in Arabidopsis thaliana, sucrose is actively exported from the phloem parenchyma cells into the apoplasm by AtSWEET11 (Sugars Will Eventually Be Exported Transporter) and AtSWEET12 sucrose efflux transporters. Major sources of carbon nutrients identified in the leaf apoplast include GABA, aspartate, glutamate, fructose, glucose, citrate, succinate, malate, and malonate (Supplementary Tables S1, S2, and S3; ). However, most of the apoplastic nutrients are bound in the region of cell wall or stored in the vacuole present inside the cell. Thus, the apoplastic nutrients are not easily available to pathogenic bacteria as reported for P. syringae pv. tomato. In addition, apoplast has less water availability compared to the phyllosphere and this is one of the major factors that determine the colonization of a bacterial pathogen. Owing to these conditions, host pathogen has to employ strategies to release nutrients and water into the apoplast. Apart from this, pH is another determining factor for bacterial multiplication. Low pH restricts the multiplication of phyto-pathogenic bacteria. For example, P. syringae pv. syringae experience slight acidic environment in the apoplast, therefore, during compatible interactions they promote apoplast alkalinization for their favorable multiplication. Also P. syringae pv. syringae produce syringomycin and disrupts H+ channel to alkalinize the apoplast. Alkalinization increases the availability of nutrients to the bacteria. Studies suggest that pathogens have evolved nutrient acquisition strategies based on nutrient abundance in the host plant. Bacterial pathogens express specific nutrient utilization pathways to utilize the most abundant nutrient present in apoplast. For example, P. syringae pv. tomato inhabiting tomato (Solanum lycopersicum) apoplast uses GABA, the most abundant amino acid present in tomato apoplast. Accordingly, the genome of P. syringae pv. tomato has evolved in such a way that it contains three copies of two genes needed for GABA metabolism, whereas other Pseudomonas strains contain a single copy. This study indicates the relation between the nutrient utilization abilities of bacterial pathogens and the corresponding nutrient availability in host plants. This line of research is expected to reveal in-depth information on evolution of the bacterial nutrient acquisition strategies in accordance with nutrient abundance. This necessitates more systematic studies for deeper mechanistic understanding in this direction.

6Vascular tissue comprising xylem and phloem serve as another nutrient niche for pathogens. Phloem consists of sieve tube elements associated with companion cells and functions in the transport of sugar from source to sink tissues. Sucrose is loaded from the apoplast into the companion cells and then it flows into the phloem sap of sieve tube elements. The nutrient-rich phloem sap includes sugars, organic acids, amino acids, sugar alcohols, and some minerals (Supplementary Tables S2 and S3). For example, metabolomic analysis of squash (Cucurbita maxima) phloem sap revealed sucrose, glucose, and fructose as the predominant source of carbon, followed by organic acids including citrate and malate. Similarly, phloem of the plants belonging to the family Rosaceae, contains sugar alcohol sorbitol as a major carbon source instead of sucrose which is otherwise predominant in other plants. Pathogenic bacteria such as E. amylovora that infect Rosaceae family plants uses sorbitol as preferred carbon source. This supports the notion that the type of nutrient present in a plant determines the type of pathogen it harbors. Nevertheless, host and pathogen are involved in a process of co-evolution that happens at all levels, not only at the level of nutrient acquisition. Spiroplasmas and phytoplasmas are strictly limited to the phloem, whereas some strains of Pseudomonas such as P. syringae pv. actinidiae (causal agent of bacterial canker of kiwifruit) and P. syringae pv. aesculi (causal agent of bleeding canker of horse chestnut) inhabit phloem as well as other nutrient niches (Supplementary Table S2;;; ).

7Xylem is mainly composed of dead and lignified cells and, therefore, xylem sap has cell wall breakdown products. Compared to other nutrient niches xylem contains the lowest concentration of organic carbon (Supplementary Table S3). Only traces of diverse organic acids, amino acids and few sugars present in xylem serve as nutrient source for pathogenic bacteria (Supplementary Table S2; ). For example, major organic acids found in the sugar beet xylem sap include malate, citrate, and succinate. Similarly, tomato xylem sap contains small amount of glucose and fructose as well as low concentrations of glutamine followed by asparagine, GABA and other amino acids (Supplementary Table S3; ). Xylem also contains monomeric inorganic ions that are essential for the bacteria. Estimation of xylem sap content from different plant species showed potassium as the highly abundant mineral nutrient followed by nitrate and chloride (http://plantsinaction.science.uq.edu.au/edition1, Supplementary Table S3). Xanthomonas albilineans (causal agent of sugarcane leaf scald) and Xylella fastidiosa are xylem limited bacteria. Some bacteria belonging to genus Erwinia, Pseudomonas, Ralstonia, and Xanthomonas also inhabit the xylem apart from other niches (Supplementary Table S2; ).

8Plant cell organelles are another important niche for many necrotrophic pathogens. It serves as a prime nutrient source for other nutrient niches like apoplast. Bacterial pathogens use various nutrient acquisition strategies to drive nutrients from the cytosol into the apoplast. Cytosol contains glucose, fructose and sucrose as major sugars, as well as organic acids, amino acids, and some minerals. The vacuole is a long-term and large store house for soluble sugars. Fructose is a predominant form of stored sugar found in the vacuole of barely (Horduem vulgare) and Arabidopsis.

9Quorum sensing is efficient way of regulating the expression of specific bacterial genes in response to bacterial cell density. This is based on the constant production and detection of signaling molecules known as auto inducers. N-acylhomoserine lactones (AHLs) produced in Erwinia sp and Pseudomonas sp are common group of autoinducer signals. Other examples of autoinducer signals are 3-hydroxy palmitic acid methyl ester (3-OH PAME) in R. solanacearum and α, β unsaturated fatty acid (cis-11-methyl-2-dodecenoic acid) in X. campestris. Quorum sensing allows the co-ordination of bacterial population to enhance access to the nutrient-rich niches. Quorum sensing plays a role in the formation of biofilms. Biofilms are highly organized structures, composed of bacterial communities enmeshed in the polysaccharide matrix that protects the bacterial communities from water loss. It is known that bacteria initiate biofilm formation in response to nutrient availability. Based on these facts, we predict that bacteria sense the nutritional content of the niches through quorum sensing and trigger the formation of biofilms as a mechanism for nutrient acquisition and desiccation prevention. Apart from this, chemotaxis and motility also play role in recognition of nutrient niches and its colonization.

10Further, we suggest that after sensing the scarcity of nutrients through quorum sensing, bacterial pathogens regulate the expression of Hrp (for hypersensitive reaction and pathogenicity) and other genes that facilitate them to overcome the nutrient limitation by acquisition of more nutrients. Hrp gene expression is induced in minimal medium or in planta but not in nutrient-rich medium. This suggests that nutrient and environmental signals regulate the Hrp gene expression in phytopathogenic bacteria. Quorum sensing indirectly plays role in regulating the expression of Hrp genes by sensing the nutrient concentration. For example, in R. solanacearum HrpG is the response regulator of TTSS activation cascade and HrpB is the transcriptional activator of Hrp gene expression. HrpG induces the expression of HrpB gene that in turn activates the expression of Hrp genes encoding TTSS and effector molecules. Quorum sensing also regulate the expression of genes encoding plant cell wall-degrading enzymes in Pectobacterium atrosepticum and Pectobacterium carotovorum and these enzymes facilitate the release of nutrients by degrading cell wall.

11Similarly, quorum sensing is proposed to act as master regulator for virulence genes that facilitates the transition from biotrophy to necrotrophy in hemibiotrophic bacterial pathogens. Based on this, we propose a hypothesis that in hemibiotrophic bacterial pathogen, nutrient limitation sensed through quorum sensing plays a role in mediating transition from biotrophy to necrotrophy. During asymptomatic biotrophic phase, when hemibiotrophic bacterial pathogens sense their population density and nutrient scarcity, they switch themselves to the necrotrophic mode of infection. Therefore, during necrotrophic phase, hemibiotrophic bacterial pathogens can grow into large population sizes by obtaining more nutrients through plant tissue degradation. Further, effector molecules are known to regulate the switch from biotrophy to necrotrophy in fungus. For example, Phytophthora infestans, a hemibiotrophic fungal pathogen secrete SNE1 (secreted effector protein) specifically in biotrophic phase that suppresses necrosis inducing effectors. Such effectors are not yet identified in bacterial pathogens. More systematic studies are needed to relate the role of these effectors in mediating transition from biotrophy to necrotrophy.

12Necrotrophic bacteria employ cell wall degrading exoenzymes, necrosis inducing proteins and toxins to provoke cell death. Some plant cell wall-degrading exoenzymes used by the bacteria are pectate lyase, pectin lyase, pectin methylesterase, cellulase, polygalacturanase, and protease. All of these exoenzymes except protease are secreted by type II secretion system. These exoenzymes facilitate the bacterial entry in the plant tissues by breaking down the component of host cell wall and expose the nutrients from the cytosol and the various organelles. Biotrophs and hemibiotrophs obtain nutrients by manipulating the expression of plant genes regulating cell membrane permeability or the genes encoding transporters. Apart from this, some bacteria modulate their own machinery to suitably utilize the available host nutrients. We discussed the strategies used by biotrophs and hemibiotrophs for nutrient acquisition in subsequent section.

13The first strategy for obtaining nutrients from niches involves the use of transporters present in the pathogen. Porins present in bacterial membrane allow the passive diffusion of nutrients available in plant niches. These pore-forming proteins are abundant in several species of Pseudomonas. Bacterial pathogens also use different types of transporters for the active uptake of nutrients (Figure 2). ATP-binding cassette transporters (ABC-transporters) and TonB-dependent transporters (TBDTs) are the important transporters involved in nutrient uptake. For example, ABC-transporters present in Pseudomonas species facilitate the uptake of maltose, glucose and sucrose as well as amino acids and minerals. TBDTs are known to be involved in iron or vitamin B12 uptake. These transporters are also shown to facilitate the uptake of carbohydrates present in low amounts on the leaf surfaces. Genomes of several species of Xanthomonas, Sphingomonas, and Pseudomonas are known to have high representation of TBDT genes. For example, the genomes of X. campestris pv. campestris (causal agent of black rot of crucifers), P. syringae pv. tomato DC3000 and P. syringae pv. syringae (causal agent of blight of barely) have 72, 25, and 19 TBDT genes, respectively. During phyllosphere colonization, X. campestris pv. campestris encounters nutrient limited environment on the leaf surfaces. Under these conditions, TBDT has been shown to transport sucrose available in the phyllosphere. Similarly, the genome of X. albilineans (causal agent of sugarcane leaf scald) has 35 putative TBDT genes. During their xylem colonization, TBDT is involved in the transport of plant cell wall derived nutrients like maltose, xylan, xylose, pectin, polygalacturonate, and arabinose. These studies show that bacterial pathogens are adapted to live in nutrient-poor environments by scavenging the plant carbohydrates through TBDTs. Other studies indicate that bacterial pathogens preferentially utilize some carbon sources over others. Therefore, they highly express genes encoding transporters that specifically facilitate the uptake of preferred carbon sources present in the niche. For example, some species of Pseudomonas and Xanthomonas preferentially utilize dicarboxylates such as malate, citrate and succinate over other carbon sources present in the apoplast. Dicarboxylate transporter (DctA1) of P. syringae pv. tomato strain facilitates the uptake of TCA cycle intermediates. Similarly, citrate transporter (CitH) of X. campestris pv. vesicatoria facilitates the citrate uptake and it is important for this bacterial virulence in tomato plants.

14The second strategy employed by the bacterial pathogens involves changes in their metabolism for the better utilization of less preferred nutrients. During non-availability of desirable nutrients, bacterial pathogens utilize less preferred nutrients. R. solanacearum colonizes the tomato xylem which contains sucrose but not glucose. However, glucose is a preferred carbon source under in vitro conditions for this pathogen. R. solanacearum metabolizes sucrose present in the xylem by using sucrose-specific phosphoenol pyruvate-carbohydrate phosphotransferase system (SSPTS) encoded by scrRABY gene cluster. SSPTS facilitates the sucrose uptake and convert it into glucose-6-phosphate and fructose-6-phosphate in a two-step process. This strategy not only provides the significant fitness advantage during xylem colonization, but also allows the bacteria to exploit other nutrient sources that are otherwise less preferred by them.

15The third strategy involves the release of bound nutrients from host cell wall during nutrient starvation. Xanthomonas albilineans, Xylella fastidiosa, and R. solanacearum use cellulose-degrading enzymes such as endoglucanase (EGL) and cellobiohydrolase (CBH) that alter plant cell wall components to release sucrose. These enzymes are also required for degradation of the xylem pit membrane and nutrient release. This helps successful colonization of the xylem vessels. Further, plants and bacterial pathogens are coevolving with each other and during this plants can evolve strategies to modulate their nutrient availability to the pathogen. Therefore, we predict that successful colonization of niches depends upon how rapidly and effectively the bacterial pathogens can evolve and adapt various nutrient acquisition strategies, involving modulation of their own machinery. This helps the bacterial pathogens to exhaustively utilize different nutrients that are otherwise less prefered by them or not easily accessible from plants. However, we have little knowledge about role of nutrients in plant–pathogen co-evolution and more specific studies are needed to investigate this interesting point.

16During the apoplast colonization, bacterial pathogens manipulate host cell machinery for nutrient release from the cell into the apoplast. For achieving this, the pathogens belonging to genus Xanthomonas and Pseudomonas use two different strategies. The first strategy involves the use of TTSS for reprogramming of plant carbon metabolism (Supplementary Table S4). Bacterial pathogens deliver effector molecules to enhance the sugar efflux and target SWEET transporters. For example, X. oryzae pv. oryzae PXO99 (causal agent of bacterial leaf blight of rice) injects effector PthXo1, a transcriptional activator-like (TAL) protein that binds directly to the promoter of gene-encoding SWEET11 transporter and induces its expression. It increases the efflux of glucose from the cytosol that can be acquired by pathogen for their enhanced growth in the apoplast. Similarly, X. oryzae pv. oryzae PXO99 also secretes effector AvrXa7 that up-regulates the expression of plant gene-encoding SWEET14 transporter. Recently, OsSWEET13 a sucrose transporter in rice is also known to be the target of X. oryzae pv. oryzae. This pathogen induces the expression of OsSWEET13 gene by using PthXo2 effector. This eventually enhances the release of sucrose from host cell into the apoplast that can be exploited by pathogen as a source of nutrition. During compatible interactions, bacterial pathogens induce the expression of gene-encoding cell wall-bound invertase (Cw-Inv). Cw-Inv is an extracellular enzyme that converts the sucrose into glucose and fructose in the apoplast and these serve as nutrients for pathogens. However, early induction of cw-Inv activity and the rapid accumulation of soluble sugars during incompatible interactions can induce defense responses like callose deposition and phenolic compounds production and establish resistance against the pathogens. Induction of PR 1 (pathogenesis related) and PR 3 gene expression are also known to be triggered by hexose signals. Further bacterial pathogens are known to secrete the transport protein via TTSS and facilitate the nutrient export from host cell to the pathogen. For example, in rice X. oryzae pv. oryzae PXO99 injects ketoglutarate transport protein (KgtP). This protein is localized onto the host cell membrane and facilitates the export of α-ketoglutaric acid from host cell to the pathogen. In rice, α-ketoglutaric acid is synthesized by enzyme, iso-citrate dehydrogenase (IDH). Upon infection in rice, this pathogen indirectly enhances the expression of OsIDH gene by exporting α-ketoglutaric acid. This suggests that KgtP present in pathogen indirectly functions in manipulating the plant cell metabolism.

17The second strategy involves the manipulation of host cell membrane permeability characteristics by bacterial pathogens for obtaining the nutrients from host cell. For example, stigmasterol content plays important role in determining the plant membrane permeability characteristics. A change in the ratio of stigmasterol and sitosterol content alters the membrane permeability. Arabidopsis AtCYP710A1 gene-encoding C22-sterol desaturase enzyme catalyzes the conversion of β-sitosterol into stigmasterol and plays an important role in maintaining the membrane permeability characteristics. During both PAMP triggered immunity (PTI) and effector triggered immunity (ETI) the expression of AtCYP710A1 gene is induced. Atcyp710a1 mutant in Arabidopsis showed release of nutrients from cytosol into the apoplast and supported higher multiplication for the bacteria belonging to Pseudomonas genus. Consistent with the fact that bacterial effectors modulate host machinery, we propose that pathogens inject effectors into the plant cell that interfere with the expression of stigmasterol biosynthesis genes to enhance the release of nutrients from the cytosol into the apoplast.

18Non-host resistance of plants prevents a wide range of pathogen infection. We propound that the non-availability of nutrients in plant and the lack of nutrient acquisition ability are the two considerable factors among several others that dictate the failure of non-host pathogen to infect the non-host plant. Sometimes, nutrients available at particular niche in non-host plant do not match with the nutrients required by the pathogen. Further, less- or non-availability of desirable nutrients restricts the non-host pathogen infection. Many nutrients are sequestered and not easily accessible by non-host pathogens. Another possible reason could be the lack of nutrient acquisition strategies such that non-host pathogens are not able to modulate their own metabolism and incapable of reprogramming the host cell machinery for acquiring nutrients. On the contrary, the host pathogens adapt various strategies to gain access to these nutrients present in the niches. Taken together, we suggest that nutrient availability or its limitation to pathogens due to plant defense is one of the reasons for non-host resistance. This is supported by a recent study involving SQS-encoding squalene synthase and SMT2-encoding sterol methyltransferase 2 that participate in sterol biosynthesis that determines the membrane permeability (Supplementary Table S5). Due to increases in membrane permeability of NbSQS-silenced Nicotiana benthamiana and atsmt-2 Arabidopsis mutant plants, the nutrient release in the apoplast increased. Therefore, upon inoculation with non-host pathogens these plants were compromised in non-host resistance. Notably, the wild type plants were resistant to these pathogens. This finding implicates that restriction of nutrient availability to invading pathogens can impart non-host resistance. Currently, only this study demonstrates the significance of nutrient availability in non-host resistance. Further, more systematic studies are needed to support this view.

19Plant nutrient niches are widely and differentially targeted by the bacterial pathogens and availability of nutrients plays a major role in determining the ability of pathogens to grow and multiply in these niches. Wide variability exists in the nutrient content of different nutrient niches. This indicates that bacterial pathogens have to cope with the inconsistent environment for which they modulate their metabolism in accordance with the nutrient content of different nutrient niches. Bacterial pathogens adapt their metabolic machinery in such a way that they preferentially use the most abundant nutrient sources of particular niche. Notably, different species of bacterial pathogens preferentially utilize some nutrient sources over others. During apoplast colonization, bacterial pathogens secrete effector molecules that target different components of plant cell machinery which enhance the availability of nutrients. Xylem-limited bacterial pathogens possess novel strategies to live in nutrient-poor and cloistered environment.

20To date, the role of nutrient availability in biotrophy to necrotrophy transition in hemibiotrophic bacterial pathogens is not well understood. This is one of the interesting future research area. Further, nutrient acquisition strategies used by bacterial pathogens are not well understood. Thus, future studies need to be focussed on the identification of more repertoires of bacterial effectors and their target genes in plant cell that are manipulated to facilitate the nutrient efflux for pathogen nutrition. It will be advantageous to mutate effector-binding site of the target gene that prevent effector binding and thereby prevents the pathogen from manipulating nutrient efflux system. Recently, transcription activator-like effector nucleases (TALEN)-based promoter editing of OsSWEET14 gene was done to disrupt TAL-effector binding site and this provided disease resistance in rice against X. oryzae pv. oryzae. Further, using host-induced gene silencing (HIGS), plants resistant to a fungal pathogens were developed. In the future, HIGS can be used as a powerful approach for silencing the genes involved in metabolism, growth and virulence of invading bacterial pathogens. Another interesting area of research in the near future is to understand non-host resistance strategies used by plants for limiting the nutrient availability to non-host pathogens.

21The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.