Characterization of AgMaT2, a Plasma Membrane Mannitol Transporter from Celery, Expressed in Phloem Cells, Including Phloem Parenchyma Cells

Abstract

A second mannitol transporter, AgMaT2, was identified in celery (Apium graveolens L. var. dulce), a species that synthesizes and transports mannitol. This transporter was successfully expressed in two different heterologous expression systems: baker's yeast (Saccharomyces cerevisiae) cells and tobacco (Nicotiana tabacum) plants (a non-mannitol-producing species). Data indicated that AgMaT2 works as an H 1 /mannitol cotransporter with a weak selectivity toward other polyol molecules. When expressed in tobacco, AgMaT2 decreased the sensitivity to the mannitol-secreting pathogenic fungi Alternaria longipes, suggesting a role for polyol transporters in defense mechanisms. In celery, in situ hybridization showed that AgMaT2 was expressed in the phloem of leaflets, petioles from young and mature leaves, floral stems, and roots. In the phloem of petioles and leaflets, AgMaT2, as localized with specific antibodies, was present in the plasma membrane of three ontologically related cell types: sieve elements, companion cells, and phloem parenchyma cells. These new data are discussed in relation to the physiological role of AgMaT2 in regulating mannitol fluxes in celery petioles.

Lost in Transit: Long-Distance Trafficking and Phloem Unloading of Protein Signals in Arabidopsis Homografts

“…Our study shows that the transit/ signal sequences responsible for directing proteins to organelles in CCs are insufficiently strong to prevent protein loss to the translocation stream. The 35S promoter is expressed strongly in CCs (Juchaux-Cachau et al, 2007;Corbesier et al, 2007;Mathieu et al, 2007), as is the SUC2 promoter. For example, FT driven from the 35S promoter induces flowering in an identical fashion to that seen with the CC promoter, SUC2 (Mathieu et al, 2007), suggesting that the level of 35S expression in CCs is sufficiently high to promote FT movement into SEs.…”

“…Figures 2A and 2G). As the 35S promoter is expressed in CCs (Juchaux-Cachau et al, 2007;Corbesier et al, 2007;Mathieu et al, 2007), the most likely origin of the mobile fusion protein observed in roots was from CCs adjacent to the mature SEs in the scion.…”

Phloem Loading Strategies in Three Plant Species That Transport Sugar Alcohols

“…PCMBS severely inhibits the function of Suc transporters. Unfortunately, sugar alcohol transporters tested to date are not affected by PCMBS (Flora and Madore, 1993;Noiraud et al, 2001a;Gao et al, 2003;RamspergerGleixner et al, 2004;Watari et al, 2004;Juchaux-Cachau et al, 2007), with the exception of two sorbitol transporters (Ramsperger-Gleixner et al, 2004;McQueen et al, 2005).…”

Physiological Roles of Polyols in Horticultural Crops

“…The overexpression of celery MTD, which plays a role in mannitol catabolism, in tobacco plants enhances disease resistance (Jennings et al, 2002). When the celery mannitol transporter is expressed in tobacco plants, sensitivity to mannitol-secreting pathogenic fungi is decreased, suggesting a role for polyol transporters in defense mechanisms (Juchaux-Cachau et al, 2007).…”

“…Celery MTD has been purified and its cDNA cloned Williamson et al, 1995). Mannitol transporters have also been isolated and characterized from celery, and play a role in mannitol transport in phloem tissues through the proton symport mechanism (Juchaux-Cachau et al, 2007;Noiraud et al, 2001).…”