Application of NPA Restrained Leaf Expansion by Reduced Cell Division in Soybean Under Shade Stress

Auxin is the major hormone involved in the shade-avoidance response in plants. Auxin-induced elongation of the stem is accompanied by the restraint of leaf blade expansion. The auxin transportation inhibitor, 1-N-naphthylphthalamic acid (NPA), is widely used in auxin functional studies, while the effects of NPA on soybean leaf expansion under shade conditions is still not known. This study assessed the effect of NPA on hormone content and cell dynamics variations during leaf expansion of soybeans under shade conditions. The results revealed that the inhibition of soybean leaf area under shade conditions was caused by a reduction in the cell number at the early stages of leaf expansion. The application of NPA under shade conditions inhibited leaf expansion by reducing the cell number. Auxin levels showed an increasing trend during the leaf expansion under shade, while cytokinin was significantly reduced. The application of NPA also reduced cytokinin levels. Cell flow cytometry revealed that reduction of the cell mitotic index by shade and NPA were caused by decreasing cell division at the early stages of leaf expansion. The shade-tolerant variety had a relative larger leaf area than the shade-sensitive variety due to higher cell division. In conclusion, the application of NPA caused a cytokinin reduction at the early stages of leaf expansion, leading to a lower cell division and cell number and inhibiting the leaf area in soybeans under shade. Future works of the function of cytokinin on cell division for selecting a higher leaf area may increase shade tolerance.