Comparative profiling of primary metabolites and endogenous hormones in different macadamia infructescences during its early development

Abstract

Macadamia (Macadamia integrifolia and M. tetraphylla) is a typical evergreen nut crop with a large amount of racemes. During the development of its racemes, two infructescences of different growth potential, bearing the distinctly different fruit-setting ability and rachis size, can often be found in the trees. However, knowledge about the formation mechanism of these two types of infructescences is still unknown. In the present research, two different sized macadamia infructescences, namely the strong and the weak infructescence, were subjected to analyze the primary metabolites and endogenous hormones in their fruit and rachis by an ultra-high-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Result showed that the strong infructescences presented a significantly greater fruit set and thicker rachis than the weak ones. Based on widely targeted metabolomics analysis, a total of 151 and 122 primary metabolites with differential accumulation were identified in the fruit and the rachis, respectively. Among these, the lipids were the largest group, followed by the amino acids and derivatives. Accompanied by the infructescence growth, a large proportion of the differential accumulated metabolites (DAMs), including lipids, saccharides and alcohols, amino acids, organic acids and nucleotides, were significantly decreased in the fruit, while most of the DAMs were significantly increased in the rachis except the amino acids. Compared with the weak infructescence, some DAMs, mainly the lipids, showed a greater accumulation in the fruit of the strong infructescence, as well as the lipids, nucleotides and sugars in the rachis. Furthermore, the higher levels of IAA, Me-IAA, IP, tZ, and GA1 were invested to the fruit and rachis of the strong infructescence, while the contents of the JA, Me-JA, SA, and ABA in the fruit and of the JA, JA-Ile, and H2JA in the rachis were opposite. Notably, the ABA was observed to increase and decrease significantly in the rachis and the fruit of the strong infructescence, respectively. Jointly, these are the first results showing the essential role of these significantly accumulated primary metabolites and endogenous hormones during the formation of the strong infructescence in macadamia.