The role of haustoria in sugar supply during infection of broad bean by the rust fungusUromycesfabae

Abstract

Biotrophic plant pathogenic fungi differentiate specialized infection structures within the living cells of their host plants. These haustoria have been linked to nutrient uptake ever since their discovery. We have for the first time to our knowledge shown that the flow of sugars from the host Vicia faba to the rust fungus Uromyces fabae seems to occur largely through the haustorial complex. One of the most abundantly expressed genes in rust haustoria, the expression of which is negligible in other fungal structures, codes for a hexose transporter. Functional expression of the gene termed HXT1 in Saccharomyces cerevisiae and Xenopus laevis oocytes assigned a substrate specificity for D-glucose and D-fructose and indicated a proton symport mechanism. Abs against HXT1p exclusively labeled haustoria in immunofluorescence microscopy and the haustorial plasma membrane in electron microscopy. These results suggest that the fungus concentrates this transporter in haustoria to take advantage of a specialized compartment of the haustorial complex. The extrahaustorial matrix, delimited by the plasma membranes of both host and parasite, constitutes a newly formed apoplastic compartment with qualities distinct from those of the bulk apoplast. This organization might facilitate the competition of the parasite with natural sink organs of the host.

Signal and nutrient exchange at biotrophic plant–fungus interfaces

“…Although AAT1 is expressed in haustoria and intercellular hyphae, the transcripts and the proteins encoded by AAT2 and HXT1 have been detected only in haustoria [46,47]. Heterologous expression in transgenic yeast and Xenopus oocytes demonstrated that HXTP1p is a proton co-transporter specific for glucose and fructose [48]. Taken together, these findings support the idea that haustorial interfaces represent the preferred route for the translocation of host metabolites into rust fungi.…”

Glucose sensing and signaling in Saccharomyces cerevisiae through the Rgt2 glucose sensor and casein kinase I

“…However, our finding that the glucose sensor tail is not required for its function raises the possibility that similar glucose sensors exist in other organisms. Proteins similar to the yeast glucose sensors, but without the long C-terminal tails, have been identified in some fungal species [e.g., Neurospora crassa Rco3 (34), Uromyces fabae Hxt1 (35), and Amanita muscaria MstI (GenBank accession no. CAB06078)].…”

Plant infection and the establishment of fungal biotrophy

“…Another gene (HXT1) that is abundantly expressed in rust haustoria but barely transcribed in other fungal structures encodes a hexose transporter [32]. Functional expression in Saccharomyces cerevisiae and Xenopus laevis oocytes revealed a substrate specificity of the HXT1-encoded protein towards the hexoses glucose and fructose, and demonstrated a proton-driven symport mechanism.…”