Genetic and physiological analysis of early drought response in Manihot esculenta and its wild relative

Abstract

Cassava (Manihot esculenta) is a staple food crop mostly grown in the tropics. Successful cultivation in marginal areas derives from its ability to withstand difficult environmental conditions. Aiming at providing new insights into drought tolerance in Manihot spp., we performed physiological and molecular analyses of early drought response in three cassava varieties and in the wild species, Manihot glaziovii (manicoba). Plants grown in pots were subjected to three water regimes for 5 days, based on soil field capacity (FC): 75% (well-watered plants); 45% (moderately stressed plants), and 20% (severely stressed plants), under greenhouse condition. Analysis of leaf gas exchange showed a downward trend in photosynthesis, stomatal conductance, and transpiration, with intensification of the stress, in all genotypes, being significantly reduced only at 20% FC. Manicoba stood out for maintaining a positive carbon balance in severe stress condition via stomatal aperture control. Photoinhibition of the photosystem II by drought was also evident only at 20% FC. There was no clear association between proline accumulation and drought stress tolerance. Expression analysis of nine genes encoding heat-shock proteins, transcription factors, a cell redox homeostasis protein, and a no-hit protein confirmed the activation of classical stress-responsive pathways, especially those involved in oxidative damage avoidance. These results reinforce the intrinsic drought tolerance of cassava, highlight the superior performance of manicoba under water deficit conditions, and give insights into drought phenotyping in cassava and contribute to further development of functional molecular markers to be used in assisted breeding.