Potassium (K) is one of the essential macronutrients required for plant growth and development (Sale et al., 2003). It plays crucial roles in many physiological and developmental processes of plants, such as osmotic adjustment, phloem loading and sugar transport to heterotrophic organs, water relations, stomatal regulation, enzyme activation and resistance to biotic and abiotic stresses (Urrego et al., 2014; Wang et al., 2013). However, with the increases in nitrogen (N) and phosphorus (P) fertilizer applications and the release of higher-yielding crop varieties (Dong et al., 2010; Hu et al., 2016), a negative K+ balance in soil (around − 60 kg ha− 1 every year) occurred and is becoming worse (Balik et al., 2020; Steiner et al., 2012).
Cotton needs K as much as N or even more (Rochester, 2007), and it is more sensitive to K deficiencies than most other field crops due to its sparse root system (Cassman et al., 1989; Mullins & Burmester, 2010). Since the 1990s, the premature senescence of cotton caused by K deficiencies has occurred frequently and with greater intensity worldwide (Wright et al., 1999), which coincided with the commercialization and popularization of transgenic Bacillus thuringiensis (Bt) cotton cultivars that was developed to produce proteins toxic to lepidopterous insects and thus reduce their damage to cotton yield (Perlak et al., 1990).
Genetic engineering and plant transformation have played a pivotal role in crop improvement by introducing beneficial foreign gene(s) into crop plants (Kumar et al., 2020). However, the improvement of a plant variety by inserting one or two qualitative genes may lead to unintended effects (i.e. going beyond that of the original genetic modification) (Ladics et al., 2015; Verhalen et al., 2003) because of random gene insertion (that could disrupt the function of native gene of the host genome) (Marrelli et al., 2006), random mutation, somaclonal variation, pleiotropy, position effect, the tissue culture process during the development of genetically modified plants (Ladics et al., 2015; Miki et al., 2009; Schnell et al., 2015), and the added burden by the constitutive over-expression of the alien transgene (Gurr & Rushton, 2005).
In order to reveal whether Bt cotton was responsible for the K deficiency of cotton production due to its lower K efficiency (one of unintended effects), we previously compared K efficiency between 33 Bt- and 15 conventional cotton cultivars/lines, and found that Bt cotton showed more severe K deficiency symptoms than conventional cotton at the seedling stage, and yielded less than the latter in the field (Tian et al., 2009). However, we are not sure yet if the Bt gene transformation directly decreased cotton K use efficiency since the genetic background of tested Bt- and conventional cotton cultivars/lines was different.
Therefore, we generated two independent Bt cotton lines by introducing Bt gene into a wild type (WT) of cotton variety. In this study, we used virus-induced gene silencing (VIGS) method to knock down the Bt gene of transgenic lines and compared their K efficiency with VIGS-Ctrl (control) plants. Also, the K efficiency of Bt lines was compared with WT via field experiments. The results will provide direct evidences as to whether the introduction of Bt gene influences the K use efficiency of cotton, and could be helpful in K management of Bt cotton production.