SAG12, a Major Cysteine Protease Involved in Nitrogen Allocation during Senescence for Seed Production in Arabidopsis thaliana.

SAG12 is the most widely used senescence-associated reference gene for characterizing leaf senescence, and the increase in SAG12 protein during leaf senescence is remarkable. However, the role of this cysteine protease in N remobilization and the leaf senescence process remains unclear. The role of SAG12 has been poorly investigated and the few reports dealing with this are somewhat controversial. Indeed, sag12 Arabidopsis mutants have not shown any phenotype, while OsSAG12-1 and OsSAG12-2 overexpression in rice moderates senescence progression. Therefore, this study aims at clarifying the role of the SAG12 cysteine protease during the entire plant life span and during leaf senescence. Arabidopsis thaliana plants knocked-out for the SAG12 gene (sag12) did not exhibit any special phenotypic traits when grown under optimal nitrogen supply (HN), suggesting that other cysteine proteases could provide compensatory effects. Moreover, for the first time, this study shows that aspartate protease activity is significantly increased in sag12. Among the putative aspartate proteases involved, a CND41-like aspartate protease has been identified. Under low nitrogen (LN) availability, when inducible proteolytic systems are not sufficient to cope with SAG12 depletion, a decrease in yield is observed. Altogether, these results show that SAG12 (and perhaps also aspartate proteases) could be involved in RuBisCO degradation during the leaf senescence associated with seed filling.

A New Role for SAG12 Cysteine Protease in Roots of Arabidopsis thaliana.

Senescence associated gene (SAG) 12, which encodes a cysteine protease is considered to be important in nitrogen (N) allocation to Arabidopsis thaliana seeds. A decrease in the yield and N content of the seeds was observed in the Arabidopsis SAG12 knockout mutants (sag12) relative to the wild type (Col0) under limited nitrogen nutrition. However, leaf senescence was similar in both lines. To test whether SAG12 is involved in N remobilization from organs other than the leaves, we tested whether root N could be used in N mobilization to the seeds. Root architecture, N uptake capacity and 15N partitioning were compared in the wild type and sag12 under either high nitrogen (HN) or low nitrogen (LN) conditions. No differences in root architecture or root N uptake capacity were observed between the lines under HN or LN. However, under LN conditions, there was an accumulation of 15N in the sag12 roots compared to the wild type with lower allocation of 15N to the seeds. This was accompanied by an increase in root N protein contents and a significant decrease in root cysteine protease activity. SAG12 is expressed in the root stele of the plants at the reproductive stage, particularly under conditions of LN nutrition. Taken together, these results suggest a new role for SAG12. This cysteine protease plays a crucial role in root N remobilization that ensures seed filling and sustains yields when nitrogen availability is low.