Trans-crop applications of atypical R genes for multipathogen resistance.

Genetic resistance to plant diseases is essential for global food security. Significant progress has been achieved for plant disease-resistance (R) genes comprising nucleotide-binding domain, leucine-rich repeat-containing receptors (NLRs), and membrane-localized receptor-like kinases or proteins (RLKs/RLPs), which we refer to as typical R genes. However, there is a knowledge gap in how non-receptor-type or atypical R genes contribute to plant immunity. Here, we summarize resources and technologies facilitating the study of atypical R genes, examine diverse atypical R proteins for broad-spectrum resistance, and outline potential approaches for trans-crop applications of atypical R genes. Studies of atypical R genes are important for a holistic understanding of plant immunity and the development of novel strategies in disease control and crop improvement.

Dissecting the causal polymorphism of the Lr67res multipathogen resistance gene.

Partial resistance to multiple biotrophic fungal pathogens in wheat (Triticum aestivum L.) is conferred by a variant of the Lr67 gene, which encodes a hexose-proton symporter. Two mutations (G144R and V387L) differentiate the resistant and susceptible protein variants (Lr67res and Lr67sus). Lr67res lacks sugar transport capability and was associated with anion transporter-like properties when expressed in Xenopus laevis oocytes. Here, we extended this functional characterization to include yeast and in planta studies. The Lr67res allele, but not Lr67sus, induced sensitivity to ions in yeast (including NaCl, LiCl, and KI), which is consistent with our previous observations that Lr67res expression in oocytes induces novel ion fluxes. We demonstrate that another naturally occurring single amino acid variant in wheat, containing only the Lr67G144R mutation, confers rust resistance. Transgenic barley plants expressing the orthologous HvSTP13 gene carrying the G144R and V387L mutations were also more resistant to Puccinia hordei infection. NaCl treatment of pot-grown adult wheat plants with the Lr67res allele induced leaf tip necrosis and partial leaf rust resistance. An Lr67res-like function can be introduced into orthologous plant hexose transporters via single amino acid mutation, highlighting the strong possibility of generating disease resistance in other crops, especially with gene editing.

Expression of the wheat multipathogen resistance hexose transporter Lr67res is associated with anion fluxes.

Many disease resistance genes in wheat (Triticum aestivum L.) confer strong resistance to specific pathogen races or strains, and only a small number of genes confer multipathogen resistance. The Leaf rust resistance 67 (Lr67) gene fits into the latter category as it confers partial resistance to multiple biotrophic fungal pathogens in wheat and encodes a Sugar Transport Protein 13 (STP13) family hexose-proton symporter variant. Two mutations (G144R, V387L) in the resistant variant, Lr67res, differentiate it from the susceptible Lr67sus variant. The molecular function of the Lr67res protein is not understood, and this study aimed to broaden our knowledge on this topic. Biophysical analysis of the wheat Lr67sus and Lr67res protein variants was performed using Xenopus laevis oocytes as a heterologous expression system. Oocytes injected with Lr67sus displayed properties typically associated with proton-coupled sugar transport proteins-glucose-dependent inward currents, a Km of 110 ± 10 µM glucose, and a substrate selectivity permitting the transport of pentoses and hexoses. By contrast, Lr67res induced much larger sugar-independent inward currents in oocytes, implicating an alternative function. Since Lr67res is a mutated hexose-proton symporter, the possibility of protons underlying these currents was investigated but rejected. Instead, currents in Lr67res oocytes appeared to be dominated by anions. This conclusion was supported by electrophysiology and 36Cl- uptake studies and the similarities with oocytes expressing the known chloride channel from Torpedo marmorata, TmClC-0. This study provides insights into the function of an important disease resistance gene in wheat, which can be used to determine how this gene variant underpins disease resistance in planta.

The Wheat Lr67 Gene from the Sugar Transport Protein 13 Family Confers Multipathogen Resistance in Barley.

Fungal pathogens are a major constraint to global crop production; hence, plant genes encoding pathogen resistance are important tools for combating disease. A few resistance genes identified to date provide partial, durable resistance to multiple pathogens and the wheat (Triticum aestivum) Lr67 hexose transporter variant (Lr67res) fits into this category. Two amino acids differ between the wild-type and resistant alleles - G144R and V387L. Exome sequence data from 267 barley (Hordeum vulgare) landraces and wild accessions was screened and neither of the Lr67res mutations was detected. The barley ortholog of Lr67, HvSTP13, was functionally characterized in yeast as a high affinity hexose transporter. The G144R mutation was introduced into HvSTP13 and abolished Glc uptake, whereas the V387L mutation reduced Glc uptake by ∼ 50%. Glc transport by HvSTP13 heterologously expressed in yeast was reduced when coexpressed with Lr67res Stable transgenic Lr67res barley lines exhibited seedling resistance to the barley-specific pathogens Puccinia hordei and Blumeria graminis f. sp. hordei, which cause leaf rust and powdery mildew, respectively. Barley plants expressing Lr67res exhibited early senescence and higher pathogenesis-related (PR) gene expression. Unlike previous observations implicating flavonoids in the resistance of transgenic sorghum (Sorghum bicolor) expressing Lr34res, another wheat multipathogen resistance gene, barley flavonoids are unlikely to have a role in Lr67res-mediated resistance. Similar to observations made in yeast, Lr67res reduced Glc uptake in planta These results confirm that the pathway by which Lr67res confers resistance to fungal pathogens is conserved between wheat and barley.

Mechanisms of phloem unloading: shaped by cellular pathways, their conductances and sink function.

Phloem unloading represents a series of cell-to-cell transport steps transferring phloem-mobile constituents from phloem to sink tissues/organs to fuel their development or resource storage. Our analysis focuses on unloading of two major phloem-mobile constituents, sugars and water. Their unloading can occur across phloem plasma membranes (apoplasmic unloading), through plasmodesmata interconnecting phloem and sink cells (symplasmic unloading) or predominately symplasmically with an intervening post-phloem apoplasmic step. In planta studies of phloem unloading encounter substantial technical challenges in accessing phloem within a meshwork of vascular/ground tissues. Thus, current understanding of phloem-unloading mechanisms largely has been deduced from indirect experimental measures or modelling. Here we highlight recent advances in understanding phloem unloading mechanisms and identify where important knowledge gaps remain.

Yeast as a Heterologous System to Functionally Characterize a Multiple Rust Resistance Gene that Encodes a Hexose Transporter.

Recently, the Lr67 resistance gene was identified as a hexose transporter variant which confers adult plant rust and mildew resistance in wheat. Methodologies used to characterize the protein encoded by Lr67 may be of use to non-transporter experts conducting similar experiments with other hexose transporters. Hence, in this chapter, we detail a protocol for the functional characterization of hexose transporter proteins in the Saccharomyces cerevisiae expression system. We also provide guidance on the use of metabolic inhibitors and competing sugars to probe transporter structural features, energization, and specificity.

Contribution of sucrose transporters to phloem unloading within Sorghum bicolor stem internodes.

Sucrose produced in source leaves is loaded into collection phloem, transported to sinks and unloaded for utilization or storage. In the context of long distance transport, sucrose transporters (SUTs) can function to load sucrose into collection phloem, retrieve leaked sucrose during long distance transport, and load sucrose into sink cells. SUTs have also been proposed to efflux sucrose under conditions of low proton motive force and low extracellular sucrose. The involvement of sucrose transporters in phloem unloading in a representative monocot stem, Sorghum bicolor, was evaluated during different stages of internode development. Transcript levels and functional properties of selected key transporters were measured, with both cellular and subcellular localization determined.

Sucrose Transporter Localization and Function in Phloem Unloading in Developing Stems.

How sucrose transporters (SUTs) regulate phloem unloading in monocot stems is poorly understood and particularly so for species storing high Suc concentrations. To this end, Sorghum bicolor SUTs SbSUT1 and SbSUT5 were characterized by determining their transport properties heterologously expressed in yeast or Xenopus laevis oocytes, and their in planta cellular and subcellular localization. The plasma membrane-localized SbSUT1 and SbSUT5 exhibited a strong selectivity for Suc and high Suc affinities in X. laevis oocytes at pH 5-SbSUT1, 6.3 ± 0.7 mm, and SbSUT5, 2.4 ± 0.5 mm Suc. The Suc affinity of SbSUT1 was dependent on membrane potential and pH. In contrast, SbSUT5 Suc affinity was independent of membrane potential and pH but supported high transport rates at neutral pH. Suc transport by the tonoplast localized SbSUT4 could not be detected using yeast or X. laevis oocytes. Across internode development, SUTs, other than SbSUT4, were immunolocalized to sieve elements, while for elongating and recently elongated internodes, SUTs also were detected in storage parenchyma cells. We conclude that apoplasmic Suc unloading from de-energized protophloem sieve elements in meristematic zones may be mediated by reversal of SbSUT1 and/or by uniporting SWEETs. Storage parenchyma localized SbSUT1 and SbSUT5 may accumulate Suc from the stem apoplasms of elongating and recently elongated internodes, whereas SbSUT4 may function to release Suc from vacuoles. Transiting from an apoplasmic to symplasmic unloading pathway as the stem matures, SbSUT1 and SbSUT5 increasingly function in Suc retrieval into metaphloem sieve elements to maintain a high turgor to drive symplasmic unloading by bulk flow.

Cellular pathways of source leaf phloem loading and phloem unloading in developing stems of Sorghum bicolor in relation to stem sucrose storage.

Cellular pathways of phloem loading in source leaves and phloem unloading in stems of sweet Sorghum bicolor (L.) Moench were deduced from histochemical determinations of cell wall composition and from the relative radial mobilities of fluorescent tracer dyes exiting vascular pipelines. The cell walls of small vascular bundles in source leaves, the predicted site of phloem loading, contained minimal quantities of lignin and suberin. A phloem-loaded symplasmic tracer, carboxyfluorescein, was retained within the collection phloem, indicating symplasmic isolation. Together, these findings suggested that phloem loading in source leaves occurs apoplasmically. Lignin was restricted to the walls of protoxylem elements located in meristematic, elongating and recently elongated regions of the stem. The apoplasmic tracer, 8-hydroxypyrene-1,3,6-trisulfonic acid, moved radially from the transpiration stream, consistent with phloem and storage parenchyma cells being interconnected by an apoplasmic pathway. The major phase of sucrose accumulation in mature stems coincided with heavy lignification and suberisation of sclerenchyma sheath cell walls restricting apoplasmic tracer movement from the phloem to storage parenchyma apoplasms. Phloem unloading at this stage of stem development followed a symplasmic route linking sieve elements and storage parenchyma cells, as confirmed by the phloem-delivered symplasmic tracer, 8-hydroxypyrene-1,3,6-trisulfonic acid, moving radially from the stem phloem.

Are sucrose transporter expression profiles linked with patterns of biomass partitioning in Sorghum phenotypes?

Sorghum bicolor is a genetically diverse C4 monocotyledonous species, encompassing varieties capable of producing high grain yields as well as sweet types which accumulate soluble sugars (predominantly sucrose) within their stems to high concentrations. Sucrose produced in leaves (sources) enters the phloem and is transported to regions of growth and storage (sinks). It is likely that sucrose transporter (SUT) proteins play pivotal roles in phloem loading and the delivery of sucrose to growth and storage sinks in all Sorghum ecotypes. Six SUTs are present in the published Sorghum genome, based on the BTx623 grain cultivar. Homologues of these SUTs were cloned and sequenced from the sweet cultivar Rio, and compared with the publically available genome information. SbSUT5 possessed nine amino acid sequence differences between the two varieties. Two of the remaining five SUTs exhibited single variations in their amino acid sequences (SbSUT1 and SbSUT2) whilst the rest shared identical sequences. Complementation of a mutant Saccharomyces yeast strain (SEY6210), unable to grow upon sucrose as the sole carbon source, demonstrated that the Sorghum SUTs were capable of transporting sucrose. SbSUT1, SbSUT4, and SbSUT6 were highly expressed in mature leaf tissues and hence may contribute to phloem loading. In contrast, SbSUT2 and SbSUT5 were expressed most strongly in sinks consistent with a possible role of facilitating sucrose import into stem storage pools and developing inflorescences.