Phytohormonal Networks Promote Differentiation of Fiber Initials on Pre-Anthesis Cotton Ovules Grown <i>In Vitro</i> and <i>In Planta</i>

The number of cotton (Gossypium sp.) ovule epidermal cells differentiating into fiber initials is an important factor affecting cotton yield and fiber quality. Despite extensive efforts in determining the molecular mechanisms regulating fiber initial differentiation, only a few genes responsible for fiber initial differentiation have been discovered. To identify putative genes directly involved in the fiber initiation process, we used a cotton ovule culture technique that controls the timing of fiber initial differentiation by exogenous phytohormone application in combination with comparative expression analyses between wild type and three fiberless mutants. The addition of exogenous auxin and gibberellins to pre-anthesis wild type ovules that did not have visible fiber initials increased the expression of genes affecting auxin, ethylene, ABA and jasmonic acid signaling pathways within 1 h after treatment. Most transcripts expressed differentially by the phytohormone treatment in vitro were also differentially expressed in the ovules of wild type and fiberless mutants that were grown in planta. In addition to MYB25-like, a gene that was previously shown to be associated with the differentiation of fiber initials, several other differentially expressed genes, including auxin/indole-3-acetic acid (AUX/IAA) involved in auxin signaling, ACC oxidase involved in ethylene biosynthesis, and abscisic acid (ABA) 8'-hydroxylase an enzyme that controls the rate of ABA catabolism, were co-regulated in the pre-anthesis ovules of both wild type and fiberless mutants. These results support the hypothesis that phytohormonal signaling networks regulate the temporal expression of genes responsible for differentiation of cotton fiber initials in vitro and in planta.棉花（Gossypium sp.）胚珠表皮细胞分化为纤维原始细胞的数量，是影响棉花产量与纤维品质的重要因素。尽管学界已在解析调控纤维原始细胞分化的分子机制方面开展了大量研究，但目前仅发现少量参与纤维原始细胞分化的功能基因。为直接鉴定参与纤维起始过程的候选基因，本研究采用棉花胚珠体外培养技术，通过施加外源植物激素调控纤维原始细胞的分化时序，并结合野生型与三个无纤维突变体的比较表达分析。对尚未出现可见纤维原始细胞的花前野生型胚珠施加外源生长素与赤霉素后，处理后1小时内，参与生长素、乙烯、脱落酸（ABA）以及茉莉酸信号通路的基因表达水平显著上调。体外激素处理诱导的差异表达转录本，大多也在田间种植的野生型与无纤维突变体胚珠中呈现差异表达。除了此前已被证实与纤维原始细胞分化相关的MYB25-like基因外，在野生型与无纤维突变体的花前胚珠中，还有多个差异表达基因受到共同调控，包括参与生长素信号通路的生长素/吲哚-3-乙酸（AUX/IAA）家族基因、参与乙烯生物合成的ACC氧化酶，以及调控脱落酸（ABA）代谢速率的ABA 8'-羟化酶。上述研究结果支持了这一假说：植物激素信号网络可在体外与田间条件下，调控棉花纤维原始细胞分化相关基因的时序性表达。