Moderately enhancing cytokinin level by down-regulation of GhCKX expression in cotton concurrently increases fiber and seed yield

Abstract

Cotton is the leading natural fiber crop in the world. Cotton seeds are also an important oil and protein source. However, enhancement of fiber abundance usually leads to a smaller seed. Thus, it has become a challenge for cotton breeding to concurrently increase fiber yield and seed yield. To improve cotton yield, we elevated the endogenous cytokinin level in transgenic cotton by constitutive suppression of cytokinin dehydrogenase (CKX), a key negative regulator controlling endogenous cytokinin in plants. The slightly and moderately suppressed transgenic cotton plants showed normal growth and development, while the severely suppressed plants exhibited a typical cytokinin-overproduction alteration. The suppression of CKX led to an enhancement of endogenous cytokinins in transgenic cotton plants. Total cytokinins in moderately suppressed lines, CR-3 and CR-6, increased by 20.4 and 55.5 % respectively, and that in the severely suppressed line (CR-13) increased by 134.2 % compared to the wild type. The moderately suppressed lines showed a delay in leaf senescence, higher photosynthesis, more fruiting branches and bolls, and bigger seed size. Field trials showed that seed yield and lint yield of the moderately suppressed CR-6 line increased by 15.4 and 20.0 %, respectively. Meanwhile, the enhanced cytokinin level in transgenic cottons did not show significant influence on fiber qualities. Our data demonstrated that CKX is a promising gene for crop yield improvement.

Keywords: CKX; Cotton fiber; Cytokinin; Seed; Yield.

Fig. 1

Relative expression levels of GhCKX in T₁ transgenic cotton plants. RNAs were isolated from young stems of transgenic cotton plants and analyzed by real-time RT-PCR. WT wild type, NT nontransgenic plants segregated from selfed progenies of transgenic cottons. Error bars indicate SD of three replicates

Fig. 2

Chlorophyll contents of upper, middle, and lower leaves in 90-, 105- and 120-DAS plants. Results are presented as mean ± SD (n = 3). Asterisks (*) indicate a significant difference (P < 0.05). FW fresh weight

Fig. 3

Comparison of photosynthetic rate in leaves, and soluble sugar and protein concentrations in bolls between transgenic lines and wild type. a Photosynthetic rate of the fourth leaf from the apex at 120 DAS. Error bars indicate SD of 10 randomly selected plants of each line. b Soluble sugar content of 35-DPA ovules and boll shells. c Soluble protein content of 35-DPA ovules and boll shells. Error bars indicate SD of data of three replicates for each line. For each column, means that are not followed by the same letter are significantly different according to Tukey’s range test at the 0.05 level