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

Abstract

Consistent with reports on other plants we recently reported that a potato transgenic line (AT010901) overexpressing sCAX1 show classic symptoms of calcium deficiency shoot tip injury, leaf curling, leaf margin necrosis and tuber internal defects such as hollow heart and brown spots. The present study was undertaken to quantify calcium in various fraction of leaf and tuber tissues of this transgenic and wild type potato clones to understand the development of these deficiency symptoms at normal calcium nutrition (1mM) and its mitigation at higher calcium nutrition (10mM). Plants were grown in controlled environment growth chamber and watered with balanced nutrient solution containing either 1 or 10 mM calcium. The plants overexpressing sCAX1 showed calcium deficiency symptoms while sequestering calcium in the vacuole as calcium oxalate crystals. Various fractions of calcium were qualified in the young and mature leaves as well as tuber tissue. A reduced concentration of water soluble fraction of calcium was most important factor related to the development of calcium deficiency symptoms in the line overexpressing sCAX1. Furthermore, an increase in this fraction appear to explain the alleviation of the deficiency symptoms in these transgenic plants.Ours is the first study to document the significance of water-soluble calcium in the development of calciumdeficiency symptoms in the potato transgenic lines overexpressing sCAX1. Furthermore, our result demonstrates that an increase in this fraction plays a significant role in the alleviation of calcium deficiency symptoms when calcium concentration in the nutrient media is increased. These results provide important insight on the role of sCAX1 in the calcium homeostasis.

Identification of the Causative Genes of Calcium Deficiency Disorders in Horticulture Crops: A Systematic Review

“…In these two reports, no deleterious alternations in plant growth and morphological traits between transgenic plants and wild types were observed. In contrast, Zorrilla et al [48] and Gao et al [49] reported that sCAX-expressing potatoes showed shoot rip necrosis and hollow heart, while the total and tuber Ca 2+ concentrations were higher than those of the wild-type. The authors demonstrated that this result was due to sCAX-expressing potatoes sequestering Ca 2+ in vacuoles as calcium oxalate crystals, reducing the water-soluble Ca 2+ concentration.…”

“…To determine the role of gene expression in the occurrence of Ca deficiency disorders and/or Ca accumulation, 37 articles were reviewed in detail (Tables 1 and 2 and Figures 1 and 2). Twenty-six of the thirty-seven reports showed an association between Ca deficiency and/or Ca accumulation and the expression of Ca 2+ -movement-related genes (Ca 2+ antiporters [27,29,32,35,36,39,[42][43][44][45][46][47][48][49][50]53], calreticulin [30,[51][52][53], Ca 2+ -ATPase [35,36,39,54,55], Ca 2+ channels [55][56][57][58], and PME [35,59]). Of the remaining studies, five studies identified QTLs that described variation in tipburn incidence [25,26,33,34,38].…”

“…Only CAXs (Ca 2+ /H + antiporter) and CRT (calreticulin) were identified as genes associated with the occurrence of Ca deficiency disorders in studies obtained from both document retrievals (Searches 1 and 2). In particular, the relevance of CAXs expression to Ca deficiency disorders and/or Ca accumulation has been well-documented [29,32,35,36,39,[42][43][44][45]48,49,53]. Thus, it is plausible that CAXs act as a causative gene for Ca deficiency disorders in horticultural crops.…”

“…However, it should be noted that two diametrically opposed hypotheses as to the mechanism by which expression of CAXs is associated with Ca deficiency disorders are advanced by these studies. In Search 2 (Table 2), many studies [42,43,45,48,49] indicated that CAX1-expressing transgenic plants exhibited Ca deficiency disorders because excessive Ca accumulation in vacuoles inhibited the distribution of Ca to new leaves and fruits. Based on this phenomenon, two studies suggested CAXs as a causative gene for Ca deficiency disorders using transcriptome analysis [35,36].…”

Understanding calcium functionality by examining growth characteristics and structural aspects in calcium-deficient grapevine

“…That is because linking nutrient deficiency symptoms with physiological functions offers the simplest yet wide-reaching means capable of explaining and predicting the Ca 2+ status in crop plants with high genetic diversity 1 . Because of this reason, the Ca 2+ deficiency symptoms, relatively appearing first and very early 9 , 21 , 22 , have been investigated 4 , 9 , 19 , 20 , 23 and used as diagnostic criteria for plants and soils 1 , 24 . This knowledge is acutely needed, especially now to sustain plant productivity against climate-change-induced challenging conditions such as mineral stress, water scarceness, increased groundwater salinity, soil pests build-up, etc.…”