The Medicago truncatula Sugar Transport Protein 13 and Its Lr67res-Like Variant Confer Powdery Mildew Resistance in Legumes via Defense Modulation Megha Gupta 1,2 , Shubham Dubey 1,4 , Deepti Jain 3 and Divya Chandran 1, * 1 Laboratory of Plant-Microbe Interactions, Regional Centre for Biotechnology, NCR Biotech Science Cluster, Faridabad 121001, Haryana, India 2 Kalinga Institute of Industrial Technology, Bhubaneswar, Orissa, India 3 Transcription Regulation Lab, Regional Centre for Biotechnology, NCR Biotech Science Cluster, Faridabad 121001, Haryana, India 4 Present address: Department of Biological Sciences, Hockmeyer Hall of Structural Biology, Purdue University, West Lafayette, IN 47906, USA. *Corresponding author: E-mail, divya.chandran@rcb.res.in; Fax, þ91 0129 2848803. (Received 16 November 2020; Accepted 2 February 2021) Obligate biotrophic pathogens like the pea powdery mildew (PM) Erysiphe pisi establish long-term feeding relationships with their host, during which they siphon sugars from host cells through haustoria. Plants in turn deploy sugar transporters to restrict carbon allocation toward pathogens, as a defense mechanism. Studies in Arabidopsis have shown that sugar transport protein 13 (STP13), a proton-hexose symporter involved in apoplasmic hexose retrieval, con- tributes to bacterial and necrotrophic fungal resistance by limiting sugar flux toward these pathogens. By contrast, expression of Lr67res, a transport-deficient wheat STP13 variant harboring two amino acid substitutions (G144R and V387L), conferred resistance against biotrophic fungi in wheat and barley, indicating its broad applicability in disease management. Here, we investigated the role of STP13 and STP13 G144R in legume–PM interactions. We show that Medicago truncatula STP13.1 is a proton-hexose symporter involved in basal resistance against PM and in- directly show that Lr67res-mediated PM resistance, so far reported only in monocots, is transferable to legumes. Among the 30 MtSTPs, STP13.1 exhibited the highest fold induction in PM-challenged leaves and was also responsive to chitosan, ABA and sugar treatment. Functional assays in yeast showed that introduction of the G144R mutation but not V388L abolished MtSTP13.1’s hexose uptake ability. Virus-induced gene silencing of MtSTP13 repressed pathogenesis-related (PR) gene expression and enhanced PM susceptibility in M. truncatula whereas transient over- expression of MtSTP13.1 or MtSTP13.1 G144R in pea induced PR and isoflavonoid pathway genes and enhanced PM re- sistance. We propose a model in which STP13.1-mediated sugar signaling triggers defense responses against PM in legumes. Keywords: 2-NBDG • Chitosan • Pea • PR genes • Salicylic acid • Sugar signaling. Introduction The powdery mildew (PM), Erysiphe pisi (Ep), is an obligate biotrophic ascomycete fungus that infects a number of grain and forage legumes, including pea, Viciae species, alfalfa and Medicago truncatula (Rispail et al. 2020). This pathogen exclu- sively infects epidermal cells of aerial organs of plants, with distinct infection stages. Early infection stages include conidial germination [1–2 h postinoculation (hpi)], appressorium for- mation (3–6 hpi), host epidermal cell penetration (6–18 hpi) and primary haustorium development (by 24 hpi). Later infec- tion stages include the formation of epiphytic mycelial net- works and secondary haustoria (24–72 hpi), followed by asexual reproduction (120 hpi). The ability of PMs to evade/suppress immune responses and siphon nutrients, espe- cially sugars, from the host largely determines their infectivity (H€ uckelhoven 2005). Hence, source leaf tissues turn into local sinks when colonized by PMs, hampering normal carbon par- titioning and reducing the yield of diseased crops (Edwards 1971, Gao et al. 2014). There is mounting evidence that plant sugar transporters, which coordinate photoassimilate transport from source to sink organs, regulate the allocation of carbon resources toward pathogens and consequently in- fluence the outcome of plant–pathogen interactions (Lemoine et al. 2013). In particular, members of the sugar transport protein (STP) family of proton-hexose symporters, which are involved in the transport of apoplasmic hexoses across the plasma membrane of plant cells, have emerged as a significant component of the host defense response against diverse pathogens (reviewed in Pommerrenig et al. 2020). However, their role in legume–PM interactions is unexplored. Since PMs primarily utilize haustoria for sugar uptake (Mendgen and Nass 1988), spatio-temporal regulation of sugar availability in the haustorium-containing and/or neigh- boring host cells by STPs may be a critical factor impacting PM disease progression. Plant Cell Physiol. – # The Author(s) 2021. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com Regular Paper 62(4): 650 667 doi:https://doi.org/10.1093/pcp/pcab021, Advance Access publication on 8 February 2021, available online at https://academic.oup.com/pcp Downloaded from https://academic.oup.com/pcp/article/62/4/650/6130797 by guest on 14 June 2022