Increased calcium levels and prolonged shelf life in tomatoes expressing Arabidopsis H+/Ca2+ transporters

Here we demonstrate that fruit from tomato (Lycopersicon esculentum) plants expressing Arabidopsis (Arabidopsis thaliana) H(+)/cation exchangers (CAX) have more calcium (Ca2+) and prolonged shelf life when compared to controls. Previously, using the prototypical CAX1, it has been demonstrated that, in yeast (Saccharomyces cerevisiae) cells, CAX transporters are activated when the N-terminal autoinhibitory region is deleted, to give an N-terminally truncated CAX (sCAX), or altered through specific manipulations. To continue to understand the diversity of CAX function, we used yeast assays to characterize the putative transport properties of CAX4 and N-terminal variants of CAX4. CAX4 variants can suppress the Ca2+ hypersensitive yeast phenotypes and also appear to be more specific Ca2+ transporters than sCAX1. We then compared the phenotypes of sCAX1- and CAX4-expressing tomato lines. The sCAX1-expressing tomato lines demonstrate increased vacuolar H(+)/Ca2+ transport, when measured in root tissue, elevated fruit Ca2+ level, and prolonged shelf life but have severe alterations in plant development and morphology, including increased incidence of blossom-end rot. The CAX4-expressing plants demonstrate more modest increases in Ca2+ levels and shelf life but no deleterious effects on plant growth. These findings suggest that CAX expression may fortify plants with Ca2+ and may serve as an alternative to the application of CaCl2 used to extend the shelf life of numerous agriculturally important commodities. However, judicious regulation of CAX transport is required to assure optimal plant growth.     

Arabidopsis monothiol glutaredoxin, AtGRXS17, is critical for temperature-dependent postembryonic growth and development via modulating auxin response

Global environmental temperature changes threaten innumerable plant species. Although various signaling networks regulate plant responses to temperature fluctuations, the mechanisms unifying these diverse processes are largely unknown. Here, we demonstrate that an Arabidopsis monothiol glutaredoxin, AtGRXS17 (At4g04950), plays a critical role in redox homeostasis and hormone perception to mediate temperature-dependent postembryonic growth. AtGRXS17 expression was induced by elevated temperatures. Lines altered in AtGRXS17 expression were hypersensitive to elevated temperatures and phenocopied mutants altered in the perception of the phytohormone auxin. We show that auxin sensitivity and polar auxin transport were perturbed in these mutants, whereas auxin biosynthesis was not altered. In addition, atgrxs17 plants displayed phenotypes consistent with defects in proliferation and/or cell cycle control while accumulating higher levels of reactive oxygen species and cellular membrane damage under high temperature. Together, our findings provide a nexus between reactive oxygen species homeostasis, auxin signaling, and temperature responses.     

Increased calcium in carrots by expression of an Arabidopsis H+/Ca2+ transporter

We demonstrate that carrots expressing the Arabidopsis H⁺/Ca²⁺ transporter CAX1 (Cation Exchanger 1) contained up to 50% more calcium (Ca) than plants transformed with control vectors. The CAX1-expressing carrots were fertile, and robust plant growth was seen in the majority of the transgenic plants. CAX1-expressing carrots were crossed to a commercial carrot variety to confirm that the increased Ca accumulation was mediated by CAX1-expression, and the increased Ca content was clearly correlated with the transgene. This study suggests that modulation of ion transporters could be an important means of increasing the Ca content of agriculturally important crops. To our knowledge, this study represents the first attempts to use biotechnology to increase the Ca content of an agriculturally important crop.     

Preliminary comparison of vetiver root essential oils from cleansed (bacteria- and fungus-free) versus non-cleansed (normal) vetiver plants

The ‘Sunshine’ cultivar of vetiver (Vetiveria zizanioides (L.) Nash, =Chrysopogon zizanioides (L.) Roberty) was subjected to meristem tissue culture in order to produce plants that were bacteria- and fungi-free. Tissue cultured (“cleansed” or phytosanitary) vetiver was grown for five months in pots of sterilized soil, and the oil compared to non-cleansed (normal) vetiver plants grown in the same conditions except using pots of unsterilized soil. The steam distilled (24 h) oil of the roots from tissue cultured (cleansed) vetiver yielded 0.02% clear oil compared to a 0.35% yield of light yellow oil for the normal vetiver plants, a 17-fold smaller yield. GC/MS analyses of the oils revealed that the non-cleansed (normal) vetiver had the typical vetiver oil profile, whereas the tissue cultured (cleansed) vetiver produced large amounts of C₁₉–C₂₉ alkanes plus several alkanols along with typical vetiver oil compounds, but lacked presumed fungal metabolites such as β-funebrene, prezizaene, -amorphene, and β-vetispirene. An unidentified biotic factor (apparently bacteria or fungi) appears to enhance the oil production in normal vetiver by both increasing yield and by the generation of signature oil compounds. These preliminary results of endogenous microbial transformations of plant chemistry may have broader physiological implications, especially among monocotyledons (including cereals).     

Ectopic expression of Arabidopsis H+-pyrophosphatase AVP1 enhances drought resistance in bottle gourd (Lagenaria siceraria Standl.)

Bottle gourd (Lagenaria sicerar ia Standl.) has been used as a source of rootstock for grafting watermelon to improve its fruit quality. We report here the development of a bottle gourd with resistance to drought by ectopic expression of the Arabidopsis AVP1 gene that encodes a vacuolar H⁺-pyrophosphatase. The drought resistance of AVP1-expressing and wild-type plants was assessed by growing plants under drought conditions. After 12 days of water deprivation, both AVP1-expressing and wild-type plants demonstrated reduced growth. After 10 days of re-watering, wild-type plants showed minimal growth while the AVP1-expressing plants resumed rapid growth. Further, AVP1-expressing plants displayed longer primary roots and more robust root systems than wild-type plants.     

Characterization of Arabidopsis Ca2+/H+ exchanger CAX3

Plant calcium (Ca(2+)) gradients, millimolar levels in the vacuole and micromolar levels in the cytoplasm, are regulated in part by high-capacity vacuolar cation/H(+) exchangers (CAXs). Several CAX transporters, including CAX1, appear to contain an approximately 40-amino acid N-terminal regulatory region (NRR) that modulates transport through N-terminal autoinhibition. Deletion of the NRR from several CAXs (sCAX) enhances function in plant and yeast expression assays; however, to date, there are no functional assays for CAX3 (or sCAX3), which is 77% identical and 91% similar in sequence to CAX1. In this report, we create a series of truncations in the CAX3 NRR and demonstrate activation of CAX3 in both yeast and plants by truncating a large portion (up to 90 amino acids) of the NRR. Experiments with endomembrane-enriched vesicles isolated from yeast expressing activated CAX3 demonstrate that the gene encodes Ca(2+)/H(+) exchange with properties distinct from those of CAX1. The phenotypes produced by activated CAX3-expressing in transgenic tobacco lines are also distinct from those produced by sCAX1-expressing plants. These studies demonstrate shared and unique aspects of CAX1 and CAX3 transport and regulation.