Abstract
The plant hormone salicylic acid (SA) plays critical roles in plant
defense, stress responses, and senescence. Although SA biosynthesis
is well understood, the pathways by which SA is catabolized
remain elusive. Here we report the identification and characterization
of an SA 3-hydroxylase (S3H) involved in SA catabolism
during leaf senescence. S3H is associated with senescence and is
inducible by SA and is thus a key part of a negative feedback
regulation system of SA levels during senescence. The enzyme
converts SA (with a Km of 58.29 μM) to both 2,3-dihydroxybenzoic
acid (2,3-DHBA) and 2,5-DHBA in vitro but only 2,3-DHBA in vivo.
The s3h knockout mutants fail to produce 2,3-DHBA sugar conjugates,
accumulate very high levels of SA and its sugar conjugates,
and exhibit a precocious senescence phenotype. Conversely, the
gain-of-function lines contain high levels of 2,3-DHBA sugar conjugates
and extremely low levels of SA and its sugar conjugates
and display a significantly extended leaf longevity. This research
reveals an elegant SA catabolic mechanism by which plants regulate
SA levels by converting it to 2,3-DHBA to prevent SA overaccumulation.
The research also provides strong molecular genetic
evidence for an important role of SA in regulating the onset and
rate of leaf senescence.
