Sucrose transport in cassava Manihot esculenta Crantz

Abstract

Sucrose is a major translocated form of carbon assimilates in many plant species and plays a key role in crop production. Sucrose has been shown in many plants to be actively transported across membrane by a transport protein to heterotrophic organs for its growth, development and storage. In cassava, sucrose is stored in root as starch. Sucrose transport in cassava was studied by uptake experiment. Cassava leaf and root discs, 12 mm in diameter were incubated in the medium containing 14C-sucrose. It was found that the Km 1.31 mM, Vmax 3.7 nmole/hr/cm2 in leaf disc and Km 20 mM, Vmax 0.018 nmole/hr/cm2 in root disc, indicating that sucrose transport in cassava was carrier- mediated. Sucrose uptake was inhibited by N-ethylmaleimide (NEM), p-chloromercuribenzensulfonic acid (PCMBS) and iodoacetic acid (IAA) suggesting that the sucrose transport protein consist thiol groups. The sucrose uptake in leaf disc was strongly inhibited by protonophore such as dinitrophenol (DNP) and carbonylcyanide m-chlorophenylhydrazone (CCCP) and inhibited by metabolic inhibitors such as vanadate, erythrosin B, and KCN, suggesting that the uptake involved proton transport and was energy-dependent. Cassava is a known cyanophoric plant. All cassava tissues, with the exception of seeds, contain the cyanogenic glycosides, linamarin (>93% total cyanogen) and lotaustralin (<10% total cyanogen), which were synthesized in the leaves. The effect of cyanogenic glucosides on sucrose transport was studied. It was found that in presence of linamarin, sucrose transport was inhibited with I50 value at 0.1 mM linamarin. On the contrary, sucrose transport was significantly the activated in the root disc.Plasma membrane of cassava leaves was purified by aqueous two-phase system consisted of Dextran T500 and PEG 3350. The membrane preparation appeared as vesicles and exhibited linear uptake of 14C-sucrose with time. Sucrose binding protein was purified by using sucrose-affinity column and it showed MW of 62 kDa on SDS-PAGE.