Drought-induced senescence of Medicago truncatula nodules involves serpin and ferritin to control proteolytic activity and iron levels

Dhanushkodi R1, Matthew C2, McManus MT1 and Dijkwel PP1

  1. Institute of Fundamental Sciences, Massey University, Palmerston North, New Zealand.
  2. Institute of Agriculture and Environment, Massey University, Palmerston North, New Zealand.

Drought is a major constraint for legume growth and yield. Senescence of nitrogen-fixing nodules is one of the early drought responses which may cause nutrient stress in addition to water stress in legumes. For nodule senescence to function as part of a drought-survival strategy, we propose that the intrinsically destructive senescence process must be tightly regulated. M. truncatulaprotease inhibitor and iron scavenger-encoding genes, possibly involved in controlling nodule senescence, were identified. RNAi lines were constructed in which expression of a serpin or ferritins was knocked down. Both wild-type and RNAi lines were subjected to drought stress and nodule activity and plant physiological responses were measured. Drought caused M. truncatulato initiate nodule senescence before plant growth was affected and before an increase in papain-like proteolytic activity and free iron levels was apparent. Knock-down expression of serpin6 and ferritins caused increased protease activity, free iron levels, early nodule senescence and reduced plant growth. The results suggest that M. truncatula nodule-expressed serpin6 and ferritins mediate ordered drought-induced senescence by regulating papain-like cysteine protease activity and free iron levels. This strategy may allow the drought-stressed plants to benefit maximally from residual nitrogen fixation and nutrient recovery resulting from break-down of macromolecules.