Anaerobic metabolism in Hevea brasiliensis laticifers is relevant to rubber synthesis when tapping is initiated

Abstract

In Hevea brasiliensis, the laticifers, a green factory of natural rubber, are specialised vessels in which rubber biosynthesis is the main metabolism, and fresh rubber occupies 35–45% volume of latex (the cytoplasm of laticifers). Intriguingly, Latex can be reproduced, drawing on three essential components (i.e., ATP, Acetyl-CoA and NADPH) from glycolysis in laticifers during tapping (a simple collection method). Some related key genes of sucrose loading and cleavage have been identified, such as HbSUT3/HbSUT1a, HbNIN2, and surely more genes in TCA cycle will be exploited. Besides the aerobic respiration, the anaerobic fermentation is found highly active in laticifers, however, the role of anaerobic metabolism in laticifers remains unclear. In view of the essential role of anaerobic respiration in helping plant survive adverse conditions by supplementing ATP and intermediate metabolites, we hypothesised that it would be involved in rubber biosynthesis in similar way. To assess the role of anaerobic metabolism in laticifers, two main fermentation genes, lactate dehydrogenase (LDH) and pyruvate decarboxylase (PDC) were analysed and characterised on their gene transcription, protein expression, enzyme activities, and the corresponding metabolites flux in response to tapping treatments. Our comprehensive results revealed that the available ATP decreased rapidly with tapping, accompanying by the increasement of dry rubber yield. Moreover, HbLDH1 and HbPDC4 were identified as the key genes in anaerobic metabolism of laticifers, and they were activated by tapping，indicated by a 3.4 - fold decrease in lactate content and a 4.6 - fold increase of acetaldehyde. In addition, the levels of both acetate and acetyl-CoA decreased rapidly. These results suggested that anaerobic metabolism is indispensable to laticifers, and furthermore the synergistic regulation between ethanol and lactate fermentation has been predicted to be involved in rubber biosynthesis by providing ATP or possible intermediate material acetate. A schematic diagram of anaerobic fermentation that contributes to rubber synthesis after tapping was hereby established.