Abstract
Guard cells are highly specialized cells that form tiny pores called stomata on the leaf surface. The opening and closing of stomata control leaf gas exchange and water transpiration as well as allow plants to quickly respond and adjust to new environmental conditions. Mesophyll cells are specialized for photosynthesis. Despite the phenotypic and obvious functional differences between the two types of cells, the full protein components and their functions have not been explored but are addressed here through a global comparative proteomics analysis of purified guard cells and mesophyll cells. With the use of isobaric tags for relative and absolute quantification (iTRAQ) tagging and two-dimensional liquid chromatography mass spectrometry, we identified 1458 non-redundant proteins in both guard cells and mesophyll cells of Brassica napus leaves. Based on stringent statistical criteria, a total of 427 proteins were quantified, and 74 proteins were found to be enriched in guard cells. Proteins involved in energy (respiration), transport, transcription (nucleosome), cell structure, and signaling are preferentially expressed in guard cells. We observed several well characterized guard cell proteins. By contrast, proteins involved in photosynthesis, starch synthesis, disease/defense/stress, and other metabolisms are preferentially represented in mesophyll cells. Of the identified proteins, 110 have corresponding microarray data obtained from Arabidopsis guard cells and mesophyll cells. About 72% of these proteins follow the same trend of expression at the transcript and protein levels. For the rest of proteins, the correlation between proteomics data and the microarray data is poor. This highlights the importance of quantitative profiling at the protein level. Collectively this work represents the most extensive proteomic description of B. napus guard cells and has improved our knowledge of the functional specification of guard cells and mesophyll cells.
Highlights
Guard cells are highly specialized cells that form tiny pores called stomata on the leaf surface
Our results revealed that proteins involved in energy, transport, transcription, cell structure, and signaling are preferentially expressed in Guard cells (GC), whereas proteins important to photosynthesis, starch synthesis, defense/disease/ stress, and other metabolisms are highly represented in Mesophyll cells (MC)
Isolation of Guard Cells and Mesophyll Cells from B. napus Leaves—The objective of this study was to compare the proteome of GC and MC in B. napus
Summary
Guard cells are highly specialized cells that form tiny pores called stomata on the leaf surface. Using microarrays covering just one-third of the Arabidopsis genome, Leonhardt et al [5] observed a differential abscisic acid (ABA) modulation of many guard cell ABA signaling components as well as key enzymes involved in carbon metabolism in GC and MC This only available large scale genomics study identified 1309 guard cell expressed genes of which 64 transcripts mainly involved in transcription, signaling, and cytoskeleton were preferentially expressed in GC compared with MC. The percentages of genes in all other categories such as protein turnover, defense, signaling, channels and transporters, and metabolism are similar between the two distinct cell types [5] These proteins are known to play specific roles in guard cell functions [6]. To the best of our knowledge isolation and purification of GC from B. napus has not been reported
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