Investigating the in vivo calcium transport path to developing potato tuber using 45Ca: a new concept in potato tuber calcium nutrition

Calcium is believed to be transported with water in the xylem. Consistent with this proposal, low‐transpiring organs such as potato Solanum tuberosum tubers are known to suffer from calcium deficiency. Although roots on tubers and stolons have been shown to supply water to tubers, there is no direct evidence for the calcium transport pathway to tubers. Both a xylem and a phloem transport pathway have been suggested. We investigated in vivo calcium transport to developing potato, cv. Dark Red Norland and cv. Russet Burbank, tubers using ⁴⁵Ca in a controlled environment facility. Whole plant split pot experiments allowed the placement of ⁴⁵Ca either in the main (basal) root or the tuber and stolon areas of the pot. The results showed that ⁴⁵Ca was transported to the shoot with the transpiration stream from both areas but was not re‐translocated to tubers or the main (basal) root system even 57 days after ⁴⁵Ca application. Radioactivity could only be detected in the tuber when ⁴⁵Ca was fed to the stolon and tuber area. When ⁴⁵Ca was fed to specific tubers, radioactivity was detected in the aerial shoot; however, no activity was detected in other tubers or the main (basal) roots. In another set of experiments, roots on a stolon near a tuber were precisely fed ⁴⁵Ca and Safranin O. The radioactive signal exactly overlapped the water transport pathway in the tuber marked with Safranin O dye, suggesting that water and calcium can be simultaneously transported from stolon roots to the tuber. No transport of ⁴⁵Ca across the tuber periderm was detected 8 days after ⁴⁵Ca was applied to the tuber periderm. This indicated that no significant transport of calcium occurs from the soil across the periderm. Our results provide evidence that: (1) calcium is not re‐translocated via the phloem from the aerial shoot tubers and main (basal) roots; (2) the main root system does not supply calcium to the tuber; (3) calcium is not transported across the periderm to the interior tuber tissue; (4) calcium is transported to the tuber via the xylem along with water, and the roots on the stolon associated with the tuber supply water and calcium to the developing tuber; and (5) transpirational demand is a significant determinant of calcium distribution within the plant.     

Carbohydrate Metabolism in Tuberizing Stolon Tips of the Strictly Short Day-Dependent Potato Species, Solanum Demissum Lindl.

Abstract: Tuber formation in the strictly short day-dependent potato species Solanum demissum Lindi. was studied. In order to accurately determine the developmental stage of individual stolon tips, a reliable parameter describing tuber formation is defined. Dry matter percentage of stolon tips was highly correlated with starch concentration and with swelling of the stoIon tips, independent of plant age and stolon diameter. Based on this parameter, changes in carbohydrate metabolism during the initial stages of tuber formation were analyzed. Glucose and fructose levels decreased significantly upon tuber formation. This decrease could be explained by a change in sucrose hydrolysis, dominated by acid invertases before tuber formation, and by sucrose synthase afterwards. A model of the temporal and spatial changes in sucrose unloading and hydrolysis in swelling stolon tips is discussed. The observed changes in carbohydrate metabolism are similar to changes observed in a simplified in vitro system, indicating that such model systems adequately reflect tuber development in intact plants.     

Xylem and Phloem Transport of Mineral Nutrients from Solanum tuberosum Roots

The xylem and phloem transport of mineral elements from stemnodal roots to the stem and stolon of growing potato (Solanumtuberosum L. cv. ‘Russet Burbank’) plants was investigated.Adventitious roots, originating from below-ground nodes of thestem of potato seedlings, were exposed to solutions of SrCI₂or MnSO₄. Relative elemental concentrations were measured inthe conductive tissues using energy dispersive X-ray analysis.After a 5 h daylight uptake period, Sr (a Ca-transport analogue)levels were elevated in the stem xylem tissue, but Sr did notincrease in the stem phloem, nor was it present in either ofthe conductive tissues of stolons located 1–2 nodes abovethe treated roots. In contrast, elevated levels of Cl, S, andMn were found in stolon xylem and phloem tissue during the sameperiod. The absence of Sr in the stolon after 5 h suggests thatno xylem flow into the stolon occurred during the uptake periodand, furthermore, phloem flow is responsible for the transportof the Cl, S, and Mn into the stolon. Elevated levels of thesemobile nutrients in the xylem of the stolon were attributedto xylem-to-phloem transfer in the stem or leaves, transportto the stolon in the phloem, and phloem-to-xylem transfer inthe stolon. During a 19 h uptake period, some Sr was observedin the phloem tissue of the stem, demonstrating slow exchangeof Sr with sieve elements or proximal phloem parenchyma andcompanion cells. Key words: Calcium, manganese, X-ray analysis     

Molecular changes associated with tuberisation in Solanum tuberosum. Differential expression of α-tubulin isotypes

Changes in gene expression that occur in the stolon tips of potato ( Solanum tuberosum L.) cv. Record during tuberisation were investigated. Protein extracts from stolon tips at various stages in the tuberisation process were analysed by two-dimensional gel electrophoresis. A number of quantitative and qualitative changes in polypeptide composition accompanied the very early stages of tuberisation. In vitro translation of RNA extracted from stolon tips also revealed quantitative and qualitative changes associated with tuberisation. Immunoblotting of protein extracts with monoclonal antibodies raised against α- and β-tubulin showed quantitative changes in the relative level of β-tubulin, but not α-tubulin, as the stolon tips tuberised. Changes in the pattern of α-tubulin isotype expression were shown to occur at early stages in the tuberisation process.     

Key players associated with tuberization in potato: potential candidates for genetic engineering

Tuberization in potato (Solanum tuberosum L.) is a complex biological phenomenon which is affected by several environmental cues, genetic factors and plant nutrition. Understanding the regulation of tuber induction is essential to devise strategies to improve tuber yield and quality. It is well established that short-day photoperiods promote tuberization, whereas long days and high-temperatures inhibit or delay tuberization. Worldwide research on this complex biological process has yielded information on the important bio-molecules (proteins, RNAs, plant growth regulators) associated with the tuberization process in potato. Key proteins involved in the regulation of tuberization include StSP6A, POTH1, StBEL5, StPHYB, StCONSTANS, Sucrose transporter StSUT4, StSP5G, etc. Biomolecules that become transported from “source to sink” have also been suggested to be important signaling candidates regulating the tuberization process in potatos. Four molecules, namely StSP6A protein, StBEL5 RNA, miR172 and GAs, have been found to be the main candidates acting as mobile signals for tuberization. These biomolecules can be manipulated (overexpressed/inhibited) for improving the tuberization in commercial varieties/cultivars of potato. In this review, information about the genes/proteins and their mechanism of action associated with the tuberization process is discussed.