A Proteomic Analysis of the Upper and Lower Flanks of the Base of Rice Shoot in the Gravitropism

Abstract

Due to gravitational stimulation, the lower part of a shoot base grows faster than the upper part, leading the shoot to curve upward. Though much research has been done on the mechanism of plant gravitropism, it still requires extensive elucidation. Recently, functional genomic strategies have been applied to study this mechanism in plants. The present study carried out a proteomic analysis to gain a better understanding of gravity stimulation in rice. Three-week-old rice seedlings were gravitropically stimulated and samples were harvested at 4 different time points: 0.5, 3, 6, and 9 h. Then, the total crude proteins were extracted from the lower and upper parts of the shoot base, separated by 2-DE, and silver stained. At each time point, proteins in the lower and upper parts were compared, and the differently expressed proteins were identified using MALDI TOF or ESI-MS/MS. After gravity stimulation, proteins involved in nine different functional categories were either up-regulated or down-regulated. Sugar metabolism, glycolysis, the tricarboxylic acid (TCA/citric) cycle, pyruvate metabolism, and transcription regulation-related proteins were regulated. Although the initiation of defense reactions mainly occurred in roots, some different defense mechanisms were also evoked in the aerial tissues. Interestingly, the abundance of some proteins changed drastically at only 0.5 h after reorientation: inosine monophosphate dehydrogenase (up to 6.49-fold higher in lower flanks at 0.5 h), ATP synthase D (4.25-fold), and ribulose-1,5 -bisphosphate carboxylase oxygenase (3.62-fold). These findings may aid in understanding the mechanism of the gravitropism.