Modulating Plant Calcium for Better Nutrition and Stress Tolerance

Abstract

External Ca2+ supplementation helps plants to recover from stress. This paper considers genetic methods for increasing Ca2+ to augment stress tolerance in plants and to increase their nutritional value. The transport of Ca2+ must be carefully controlled to minimize fluctuations in the cytosol while providing both structural support to new cell walls and membranes, and intracellular stores of Ca2+ for signaling. It is not clear how this is accomplished in meristems, which are remote from active transpiration—the driving force for Ca2+ movement into shoots. Meristems have high levels of calreticulin (CRT), which bind a 50-fold excess of Ca2+ and may facilitate Ca2+ transport between cells across plasmodesmatal ER. Transgenes based on the high-capacity Ca2+-binding C-domain of CRT1 have increased the total plant Ca2+ by 15%–25% and also increased the abiotic stress tolerance. These results are compared to the overexpression of sCAX1, which not only increased total Ca2+ up to 3-fold but also caused Ca2+ deficiency symptoms. Coexpression of sCAX1 and CRT1 resolved the symptoms and led to high levels of Ca2+ without Ca2+ supplementation. These results imply an important role for ER Ca2+ in stress tolerance and signaling and demonstrate the feasibility of using Ca2+-modulating proteins to enhance both agronomic and nutritional properties.

CAX1 Vacuolar Antiporter Overexpression in Potato Results in Calcium Deficiency in Leaves and Tubers by Sequestering Calcium as Calcium Oxalate

“…In addition, cax1/ cax3 double mutants displayed a severe reduction in growth, including leaf tip and flower necrosis and pronounced sensitivity to exogenous Ca 2+ and other ions (Cheng et al, 2005). In leaves, the vacuole serves as the reservoir for excess accumulation of Ca 2+ rather than the cell wall (Robertson, 2013). CAX1 keeps apoplastic Ca 2+ low through compartmentation into mesophyll vacuoles that is essential for normal plant function and productivity .…”

“…The unbound extracellular Ca 2+ must be maintained in equilibrium across the apoplast-symplast boundary. In leaves, the vacuole serves as the reservoir for excess accumulation of Ca 2+ rather than the cell wall (Robertson, 2013). Park et al (2005a) transformed the potato cultivar 'Russet Norkotah' with a small version of CAX1 and reported that increased levels of CAX1 do not appear to alter potato growth and development, nor tuber morphology or yield.…”

The Contribution of Calcium to Changes in Leaf Anatomical Character of Oil Palm Seedlings (Elaeis guineensis Jacq.) under Drought Stress

“…Following anaphase, a new cell wall is formed. During this process, Ca is very necessary to stabilize the developing cell wall by binding to pectate (Robertson, 2013).…”

Fractionating of Calcium in Tuber and Leaf Tissues Explains the Calcium Deficiency Symptoms in Potato Plant Overexpressing CAX1

“…CAX1 has been recognized as a key regulator of apoplastic Ca 2+ concentrations, and its product maintains low apoplastic Ca 2+ levels via compartmentation into mesophyll vacuoles, which is essential for sufficient plant function and productivity (Conn et al, 2011). The vacuole is known to serve as the reservoir for excess accumulation of Ca 2+ (Robertson, 2013).The over-expression of sCAX1 in carrot (Park et al, 2004), tomato (Park et al, 2005a) and potato (Park et al, 2005b) has been shown to increase calcium contents in the edible parts of those species. Some studies showed that this over-expression can result in calcium deficiency symptoms such as blossom-end rot in tomato fruit (de Freitas et al, 2011) and leaf necrosis in tobacco (Hirschi, 1999).…”