In cereal crops, when a leaf or even the entire vegetative part of the plant senesces, the amino acids in those tissues can be recycled – that is, moved to developing tissues such as grains, to serve as a nitrogen source. In order for this to happen, proteases must drive a massive breakdown of complex molecules such as proteins and redirect them out of the dying tissues. It’s been found that sulphur deficiency can lead to a delay in senescence, in turn leading to potential decrease in quality and quantity of harvestable grain. This is of particular interest since the past few decades have seen sulphur become a limiting nutrient for crops in several parts of the globe.
In a new paper published in Annals of Botany, lead author Cintia G. Veliz and colleagues examine how sulphur deficiency affects eight different proteases from all four major classes: cysteine-proteases, serine-proteases, aspartic-proteases, and metallo-proteases. The authors also treated sulphur-deficient plants with the senescence-inducer methyl-jasmonate (MJ) in darkness (itself a senescence inducer) to investigate whether the proteases could be induced even with sulphur deficiency.
Seven of the eight proteases tested showed reduced or even no induction under sulphur-limited conditions. Treatment with MJ and darkness reversed the inhibition after a delay, allowing proteases to reach similar levels to those in the control group, showing that sulphur deficiency causes a delay in the progression of senescence, but does not prevent it. The nutrient deficiency also caused an altered protease gene expression pattern with respect to leaf age and stage of development. Three of the genes coding proteases in two different classes were induced in green leaves during the plant’s vegetative stage.
“The results obtained here show that the shortage of sulphur and the senescence induction does not exert the same effect on all the proteases analysed, even within the same catalytic group,” write the authors, demonstrating the complexity of the regulatory network involved and suggesting “the involvement of different transduction pathways triggering the induction of each one.” The downstream effect on nitrogen remobilization efficiency that is caused by sulphur limitation will have an impact on both seed yield and quality, making its further study of significant interest.