ABSTRACT: Cotton fiber is actually unicellular trichome, therefor its length is really hard to be modified but very meaningful to fiber quality and yield. We have reported the function of the second RRM domain of Oryza sativa FCA in rice cell size regulation. Data shows it is highly conserved across dicotyledonous and monocotyledonous plants. Here we provide evidence showing that the second RRM domain of Brassica napus FCA worked in Gossypium hirsutum, leading to the enlargement of multiple types of cells, such as pollen, cotyledon petiole and cotton fiber. In the transgenic cotton, the length of unicellular cotton fiber increased by about 10% and fiber yield per plant also showed a dramatic increase, ranging from 35% to 66%, over the control. Thus, this RRM domain may be an ancient and common cell size regulator and has great economic value on cotton industry. FCA encodes a strong promoter of the transition to flowering in Arabidopsis thaliana, which contains two RRM (RNA recognition motif) domain and a WW protein interaction domain (Macknight et al., 1997). We have previously found that cell size and yield of rice (Oryza sativa) can be increased by ectopic expression of the first RRM domain of OsFCA (Hong et al., 2007). The second RRM domain of OsFCA can also increase cell size (Attia et al., 2005), suggesting OsFCA-RRMs each play a role in homeostatic cell size regulation. We designate them as Oryza sativa cell size RRM 1 (Os-csRRM1) and Oryza sativa cell size RRM 2 (Os-csRRM2), respectively. Both of them exhibit a high degree of evolutionary conservation in plant. For Os-csRRM2, significant homology was observed in Triticum aestivum (90% identity), Hordeum vulgare (90% identity), Lolium perenne (82% identity), Zea mays (81% identity),Ricinus communis (76% identity), Vitis vinifera (68% identity), Arabidopsis thaliana (68% identity) and Brassica napus (64% identity) (Fig. 1). The high degree of conservation suggests that this RRM domain might have similar function in different plants. Indeed, we observed that overexpression of Bn-csRRM2 also increased the cell size of B. napus (unpublished data). As cotton fiber length is a key factor in cotton yield and quality, we investigated whether this attribute could be enhanced by constitutive expression of Bn-csRRM2. Transgenic and wild-type cotton were grown in same condition. The leaves of 25 day and 45 day plants were harvested for microarray analysis. RNA samples were isolated from 3 biological replications using TRIzol (Invitrogen) as described by the manufacturer. Microarray analyses were carried out using Agilent Cotton Gene Expression Microarray (G2519F-022523). Microarrays were scanned on Agilent Technologies Scanner G2505C and data points were extracted using Agilent Feature Extraction software (Version 10.7.1.1). Comparisons were made between transgenic samples and their corresponding wild-type samples.
