Abstract
The plant growth retardant mepiquat chloride (MC) has been extensively used to produce compact plant canopies and increase yield in cotton (Gossypium hirsutum L.). Previous studies showed that MC reduced plant height and internode length by inhibiting GA biosynthesis and cell elongation. However, whether there are other molecular mechanisms underlying MC-induced growth retardation has remained largely unknown. In the present study, we conducted histological, transcriptomic, and phytohormone analyses of the second elongating internodes of cotton seedlings treated with MC. Histological analysis revealed that the MC shortened the internodes through suppressing both cell division and cell elongation. Consistent with the observed phenotype, many genes related to cell growth were significantly downregulated by MC. Transcriptome profiling showed that the expression of genes related not only to GA, but also to auxin, brassinosteroid (BR), and ethylene metabolism and signaling was remarkably suppressed, whereas that of genes related to cytokinin (CK) and abscisic acid (ABA) metabolism was induced by MC. Consistent with the expression pattern, significant decrease of endogenous GA, auxin, and BR content, but an increase in CK content was observed after MC treatment. Most of these hormone related genes displayed opposite regulation pattern by exogenous GA3 treatment compared to MC; simultaneous application of MC and GA3 could alleviate the genes expression changes induced by MC treatment, indicating MC does not directly affect other plant hormones, but through the inhibition of the GA biosynthesis. In addition, the expression of genes related to secondary metabolism and many transcription factors (TFs) were differentially regulated by MC. In summary, we confirmed the important role of GA in MC-induced growth inhibition of cotton, and further found that other hormones were also involved in this process in a GA-dependent manner. This study provides novel insights into the molecular mechanism underlying the MC-mediated inhibition of internode elongation in cotton seedlings.
Highlights
Cotton (Gossypium hirsutum L.) is an important economic crop; it is widely cultivated worldwide for its natural fiber and oilseed
mepiquat chloride (MC) has been the most successful and widely used plant growth regulator in cotton; it allows to compact plants by reducing internode length and leaf size (Reddy et al, 1990; Siebert and Stewart, 2006; Ren et al, 2013)
We showed that MC reduced the synthesis of GA, resulting in the suppression of cell elongation in the internodes of cotton seedlings (Wang et al, 2014)
Summary
Cotton (Gossypium hirsutum L.) is an important economic crop; it is widely cultivated worldwide for its natural fiber and oilseed. Excessive vegetative growth often occurs, leading to serious production problems such as auto-shading, fruit abortion, delayed maturity, and yield reduction (Zhao and Oosterhuis, 2000). Gibberellin (GA) is a key kind of hormones that regulate stem and internodal elongation by affecting cell division and expansion (Wang et al, 2018). GAs promote plant growth by stimulating the degradation of DELLA proteins—the major repressors of GA signaling—by binding to the receptor— GIBBERELLIN INSENSITIVE DWARF1 (GID1)—through the ubiquitin–proteasome pathway (Velde et al, 2017). When the content of active GAs decreases, DELLA proteins accumulate and interact with transcription factors (TFs) to suppress the expression of their target genes and further suppress cell division or cell elongation (Fukazawa et al, 2017; Velde et al, 2017)
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