The Application of Nitrogen Source in Regulating Lignin Biosynthesis, Storage Root Development and Yield of Sweet Potato

Abstract

The understanding of the effects of nitrogen sources on lignin synthesis in sweet potato during storage root formation is limited. In this study, we investigated the effects of different nitrogen source on sweet potato storage root formation and development, as well as lignin synthesis in potential storage roots. The sweet potato cultivars Shangshu 19 and Jixu 23 were used in field experiments in 2019 and 2020. Three treatments were tested: (a) no nitrogen fertilizer application (control); (b) 60 kg hm−2 ammonium nitrogen; and (c) 60 kg hm−2 amide nitrogen. The results indicate that during sweet potato storage root formation, ammonium nitrogen significantly enhanced root activity compared to that of the control. The ammonium nitrogen treatment promoted IbEXP1 and inhibited Ibkn1 and Ibkn2 expression during the early stages of storage root formation, then increased gibberellic acid and decreased zeatin riboside content, enhanced phenylalanine ammonia lyase and peroxidase activities, and promoted lignin synthesis in potential storage roots. The opposite effects of ammonium nitrogen treatment on gene expression, hormone contents, and enzyme activity were observed in the late stages of storage root formation. Relative to the control, the ammonium nitrogen treatment significantly increased the number of storage roots during canopy closure. The ammonium nitrogen treatment produced the highest storage root yield and number of storage roots per plant. These results indicated that the ammonium nitrogen can inhibit root lignin synthesis, then promote storage root formation and increase the yield of sweet potato.

Influence of Planting Density on Sweet Potato Storage Root Formation by Regulating Carbohydrate and Lignin Metabolism

“…The upregulation of starch biosynthesis and downregulation of lignin biosynthesis in potential storage roots are the main events during the storage root formation period [ 8 , 15 , 22 ], probably because the activity of carbon flow towards carbohydrate metabolism is stronger than the delivery into phenylpropanoid biosynthesis during the storage root formation period [ 14 , 30 ]. Previous findings have shown that low sucrose and high hexose content in the root system could facilitate storage root formation [ 14 , 31 ].…”

“…Sucrose is decomposed into hexoses by the SuSy and sucrose invertase pathways, and the former is the main source for fructose and UDP-glucose biosynthesis, which provides the substrate for starch biosynthesis [ 14 , 31 ]. Crop root development is adversely correlated with lignin deposition, and lignin accumulation has a negative effect on root development [ 8 , 32 ].…”

“…Simultaneously, carbohydrates provide energy for cambium cell development. The lignin content in potential storage roots is a common physiological indicator of sweet potato for the calculation of lignification [ 8 ]. The upregulation of carbohydrate metabolism and down-regulation of lignin biosynthesis usually facilitate sweet potato storage root formation [ 9 ].…”

“…CCoAOMT further catalyzes the reaction by methylating caffeoyl CoA to feruloyl CoA, and CAD catalyzes the final step of lignin biosynthesis by converting the corresponding cinnamyl aldehydes into cinnamyl alcohols [ 18 , 19 , 20 ]. The downregulation of gene expression in lignin biosynthesis could reduce the lignin content in the plant tissue, including the root and stem [ 8 , 21 ]. Ibkn1, Ibkn2, and Ibkn3 are members of the class I knotted 1-like (KNOX-box) gene family, and they are expressed in primary cambium cells [ 8 , 15 ].…”

“…The downregulation of gene expression in lignin biosynthesis could reduce the lignin content in the plant tissue, including the root and stem [ 8 , 21 ]. Ibkn1, Ibkn2, and Ibkn3 are members of the class I knotted 1-like (KNOX-box) gene family, and they are expressed in primary cambium cells [ 8 , 15 ]. They regulate cell proliferation and differentiation in potential storage roots [ 22 , 23 ], and their expression in storage roots was two-fold higher than that in fiber roots [ 8 , 15 ].…”