Fiber up! Gibberellin–strigolactone crosstalk during cotton development

Strigolactones (SLs) are a group of phytohormones that regulate many aspects of plant development, such as shoot branching and the formation of adventitious roots. Many of these functions and the intersections of SL signaling with other plant hormone pathways have been extensively studied. However, less is known about SL action at the single-cell level. Cotton (Gossypium hirsutum) is an economically important crop that provides the largest source of natural fiber. Amazingly, each fiber is a single cell, making them excellent single-cell models for research.

Two important contributors to fiber development are cell elongation and secondary wall thickness. In a new publication, Yuzhou Zhang, Zailong Tian, and colleagues (Zhang et al., 2022) show that gibberellins (GAs) activate SL biosynthesis to enhance cell wall thickness and the elongation of fiber cells in cotton. The authors showed that the amount of epi-5-deoxystrigol, an endogenous SL, and the expression levels of the SL biosynthesis gene DWARF27 (D27) were higher during fiber elongation than at the initiation stage. Application of the synthetic SL analog GR24 promoted cotton fiber elongation, whereas the SL biosynthesis inhibitor Tis108 impeded elongation. In addition, a d27 knockout line had smaller cotton balls, while a D27 overexpression line produced larger cotton balls. Zhang et al. then used RNA-sequencing (RNA-seq) to identify differentially regulated genes after GR24 and Tis108 treatments. Among these, many genes belonged to fatty acid-related pathways. Very-long-chain fatty acids (VLCFAs), whose biosynthesis is catalyzed by ketoacyl-CoA synthase (KCS), are known to promote cotton fiber elongation (Qin et al., 2007; Wang et al., 2017). The authors identified four KCS genes that were upregulated after GR24 treatment and downregulated after Tis108 application, suggesting that SLs activate KCS-mediated VLCFA biosynthesis, thereby promoting fiber elongation. The authors then investigated whether SLs affect cell wall thickening. Using electron microscopy, they determined that cell walls were thicker after GR24 treatment but thinner after applying Tis108, relative to untreated controls. In addition, they saw an upregulation of four cellulose synthase (CesA) genes after GR24 treatment and in the D27 overexpression line but observed their downregulation in the d27 knockout line.

Zhang et al. therefore established that SLs have a dual role in cotton: they not only promote fiber cell elongation, but also enhance secondary cell wall thickness. The authors did not stop here but investigated what transmitted the phytohormone signals to the KCS and CesA genes. They identified two transcription factors binding to the CesA promoters, and three to the KCS promoters. Silencing the genes encoding these transcription factors led to diminished cell wall thickness and to altered fatty acid composition. Next, they looked at a possible interaction between SL-driven cotton fiber development and similar effects that are known to be caused by GAs (Shan et al., 2014). GR24 was able to reverse the inhibitory action of the GA biosynthesis inhibitor paclobutrazol (PAC) on cotton fiber development; however, GA treatment did not reverse the effect of Tis108 application. These results suggested that SLs act downstream of GAs. Finally, a transcriptome analysis of GA-treated fibers and systematic yeast one-hybrid assays, revealed that the transcription factor GROWTH-REGULATING FACTOR4 (GRF4), the expression of whose encoding gene is upregulated after GA treatment and downregulated after PAC application, binds to the D27 promoter. GA, therefore, through GRF4, directly activates the expression of the SL biosynthesis gene D27 to trigger the SL-mediated increase in fiber cell elongation and cell wall thickness (see Figure).

Figure.

GAs activate SL signaling to promote cotton fiber development. The GA-controlled transcription factor GRF4 activates the SL biosynthesis gene DWARF27 (D27); SLs then upregulate the expression of transcription factor genes that promote both cellulose and fatty acid accumulation. Adapted from Zhang et al. (2022), Figure 11.

This work provides new insights into how GAs and SLs act together in plant development. It is fascinating that SLs manage to promote both cell elongation and secondary wall thickening because these two processes are generally antagonistic. Another interesting aspect is the variation in crosstalk between the SL and GA pathways in different plant species. Indeed, GA promotes SL signaling in cotton, but inhibits SL signaling in rice (Oryza sativa). These open questions remain exciting points for future research.