On the Inside: Serine Hydrolases and Lipid Mobilization During Seed Germination

During seed germination, lipolytic enzymes are involved in the mobilization of lipid reserves. In particular, the lipolytic activities of lipases and esterases play crucial roles in proper seedling growth and development. Lipases and  esterases belong to the serine hydrolase (SH) superfamily. Established  genomic and proteomic approaches have provided enormous information about the presence and expressional profile of rice (Oryza sativa) enzymes. However, such studies fail to provide insights into the functional status of enzymes. The major limitation of abundance-based methods is that the level of mRNA is often only weakly correlated with protein function. To fill such gaps in our knowledge, activity-based protein profiling (ABPP) can be used. ABPP uses an active site-directed probe to monitor the functional state of enzymes, and it enables the detection as well as affinity purification of the target enzymes. The application  of ABPP, in combination with mass spectrometry, provides insights into the activity of enzyme species irrespective of their abundance. Dolui & Vijayaraj (10.1104/pp.20.00268) have applied an ABPP approach to reveal the activity of lipases during seed germination in rice (Oryza sativa). They successfully mapped the active sites of 43 active SHs.  The mRNA expression levels of the genes encoding the identified SHs were monitored using microarray analysis. The lipidome analyses revealed distinct patterns of molecular species distribution in individual lipid classes and shed light on the metabolic connections between lipid mobilization and rice seedling growth. Changes in the mobilization of storage lipids and their molecular species remodeling were correlated with the expression of the identified lipases and their lipase activity in a time-dependent manner. The physiological significance of the identified SHs was also explored during  Fusarium verticilloides infection. The fungal infection significantly reduced lipase activity and lipid mobilization, thus impairing proper seedling development.