A study on cassava tolerance to and growth responses under salt stress

Soil salinity, commonly dominated by NaCl, is a major constraint to the production of food crops. Cassava (Manihot esculenta) is the most important tropical starch-producing crop. Reportedly, this crop is moderately sensitive to salt stress, and the A20/AN1-type zinc finger genes can confer abiotic stress tolerance to plants as the emerging target genes. However, neither cassava tolerance to salt nor the response of A20/AN1-type zinc finger genes from cassava to salt has yet to be characterized in detail. South China 124 (SC124), Fuxuan 01, South China 205, Kasetsart 50 and Argentina 7 (C4), somewhat differ in salt tolerance and starch content in storage roots under pre-established NaCl concentrations of 0, 10, 20, 30, 40, 50, 100, and 200 mM. In this study, we investigated responses of in vitro-grown plantlets of these five cultivars, focusing on plantlet growth, phytohormones, antioxidant enzymes, soluble protein and sugar, H2O2 content, and expression of A20/AN1-type zinc finger genes. The major results were as follows: the growth of cassava was obviously inhibited, starting at 20 mM NaCl depending on the cultivars. All five cultivars could not grow at 100 mM NaCl; 10 mM NaCl caused promoted growth by increasing number, volume, surface area, viability, length of fibrous rootlets, and elongated root cells; 20 mM NaCl facilitated both accumulation of total starch in fibrous rootlets and increase in total protein content in shoots; increased growth and starch accumulation did not correlate with phytohormone levels, antioxidant enzyme activities between shoots and rootlets, or, consequently, in the whole plantlets; and the A20/AN1 gene family was composed of at least 11 members and their expression was induced or suppressed in rootlets of plantlets of pot-grown SC124 under 200 mM NaCl, depending on gene members. In conclusion, low salt (NaCl) can not only promote the cassava growth but also lead to accumulation of total starch in the fibrous rootlets and increase in total protein content in shoots instead of rootlets. Low salt promoting growth effects are likely associated with lower levels of abscisic acid and higher levels of gibberellin and indole-3-acetic acid under salt stress. Such antagonistic relationship among these three phytohormones maybe involves expression of A20/AN1 genes under salt stress. The demand for H2O2 levels, osmoprotectants, and antioxidant enzyme activities in salt stress responses is cassava cultivar-dependent.