GTR-Mediated Radial Import Directs Accumulation of Defensive Glucosinolates to Sulfur-Rich Cells in the Phloem Cap of Arabidopsis Inflorescence Stem

Abstract

In the phloem cap region of Arabidopsis plants, sulfur-rich cells (S-cells) accumulate >100 mM glucosinolates (GLS), but are biosynthetically inactive. The source and route of S-cell-bound GLS remain elusive. In this study, using single-cell sampling and scanning electron microscopy with energy-dispersive X-ray analysis we show that two GLS importers, NPF2.10/GTR1 and NPF2.11/GTR2, are critical for GLS accumulation in S-cells, although they are not localized in the S-cells. Comparison of GLS levels in S-cells in multiple combinations of homo-and heterografts of gtr1 gtr2, biosynthetic null mutant and wild-type plants indicate that S-cells accumulate GLS via symplasmic connections either directly from neighboring biosynthetic cells or indirectly to non-neighboring cells expressing GTR1/2. Distinct sources and transport routes exist for different types of GLS, and vary depending on the position of S-cells in the inflorescence stem. Based on these findings, we propose a model illustrating the GLS transport routes either directly from biosynthetic cells or via GTR-mediated import from apoplastic space radially into a symplasmic domain, wherein the S-cells are the ultimate sink. Similarly, we observed accumulation of the cyanogenic glucoside defensive compounds in high-turgor cells in the phloem cap of Lotus japonicus, suggesting that storage of defensive compounds in high-turgor cells may be a general mechanism for chemical protection of the phloem cap.

Tissue-specific transcriptome profiling of theArabidopsis thalianainflorescence stem reveals local cellular signatures

“…This comparison resulted in 575 phloem cap-associated genes (Supplemental Dataset 8, BH adjustment of p value in Wald test < 0.01, fold change >2). As expected(Xu et al, 2019), among this group of genes the GO term 'glucosinolate biosynthetic process'…”

In Arabidopsis thaliana Substrate Recognition and Tissue- as Well as Plastid Type-Specific Expression Define the Roles of Distinct Small Subunits of Isopropylmalate Isomerase

“…These cells are situated between the phloem parenchyma and the endodermis (Koroleva et al, 2000(Koroleva et al, , 2010. However, it seems unlikely that BCAT4 and IPMI SSU2/IPMI SSU3 are expressed in these cells, since these sulfur-rich cells are biosynthetically inactive (Xu et al, 2019). This is supported by the observation that BCAT4 promoter driven expression of GFP (pBCAT4:3XGFP-NLS) was seen in nuclei of relatively small and short cells with a morphology completely different from the extremely long S-cells (Supplementary Figures S5B,C).…”

GTR1 and GTR2 transporters differentially regulate tissue‐specific glucosinolate contents and defence responses in the oilseed crop Brassica juncea

“…Discovered first in Arabidopsis and functionally characterized using the Xenopus system, GTRs are high affinity H + /glucosinolate influx symporters, belonging to the ubiquitous peptide transporter (PTR/NRT1) superfamily (Léran et al, 2014; Nour‐Eldin et al, 2012). In Arabidopsis , two genes, GTR1 and GTR2 , were identified playing key roles in leaf distribution and long‐distance phloem translocation of glucosinolates, respectively (Madsen, Olsen, Nour‐Eldin, & Halkier, 2014; Nour‐Eldin et al, 2012; Xu et al, 2019).…”