In plants, the process of senescence advances an individual cell, organ, or plant toward developmentally programmed, or environmentally induced, death. This requires the coordination of distinct, but functionally related, pathways that integrate external cues from biotic and abiotic stress factors with internal age-related cues. This allows for drastic alteration of cellular metabolism and organelle functions. Once initiated, senescence allows for degradation of intracellular components and nutrient recycling to healthy developing areas of the plant. The stress-related autophagy pathway is active during senescence, and mediates cellular degradation by delivering selective and non-selective cargo to the vacuole for hydrolysis. Through the study of three proteins, AtATG6, AtFDC, and AtNHL57, my research provides an integrated perspective on plant stress biology.
Examination of Arabidopsis Autophagy-related Protein 6 (AtATG6) established a link between autophagy and reproductive development. Homozygous atg6 plants were never recovered, suggesting a gamete transmission defect. Pollen test crosses determined that a male gametophyte defect was responsible and germination assays revealed that atg6(-) pollen grains germinated at very low efficiencies in vitro. This highlighted a new role for AtATG6 during pollen germination.
Our understanding of AtATG6 function was broadened through characterization of a novel interacting partner, a FYVE-domain-containing protein (AtFDC). AtFDC bound AtATG6 directly, was zinc coordinated, and peripherally associated with a punctate membrane structure. These traits are all consistent with its putative role as PI3P-binding protein. AtFDC is the first of the non-PRAF Arabidopsis FYVE-domain proteins to be characterized, and the first novel ATG6 interaction partner identified in plants.
Finally, we identified AtNHL57, a senescence-associated protein with a novel Peroxisomal Targeting Signal 1 (PTS1), an SFL tripeptide. NHL57 localized to peroxisomes, and in vitro protein import experiments revealed that the SFL tripeptide was necessary, but not sufficient, for targeting. In addition, nhl57 homozygous mutant plants exhibited retarded degradation of chlorophyll during dark-induced senescence. This furthered our knowledge of PTS1 tripeptides and provided an intriguing link between chlorophyll degradation and peroxisomes during senescence.
In conclusion, these data provide a unique perspective on plant stress biology and provide a foundation for future work in pollen biology, lipid signaling and the physiology of senescent peroxisomes.