The Arabidopsis NMD Factor UPF3 Is Feedback-Regulated at Multiple Levels and Plays a Role in Plant Response to Salt Stress

Nonsense-mediated mRNA decay (NMD) is a eukaryotic RNA surveillance mechanism that degrades aberrant transcripts and controls the levels of many normal mRNAs. It was shown that balanced expression of the NMD factor UPF3 is essential for the maintenance of proper NMD homeostasis in Arabidopsis. UPF3 expression is controlled by a negative feedback loop that exposes UPF3 transcript to NMD. It was shown that the long 3′ untranslated region (3′ UTR) of UPF3 exposes its transcript to NMD. Long 3′ UTRs that subject their transcripts to NMD were identified in several eukaryotic NMD factors. Interestingly, we show here that a construct that contains all the regulatory regions of the UPF3 gene except this long 3′ UTR is also feedback-regulated by NMD. This indicates that UPF3 expression is feedback-regulated at multiple levels. UPF3 is constitutively expressed in different plant tissues, and its expression is equal in leaves of plants of different ages. This finding is in agreement with the possibility that UPF3 is ubiquitously operative in the Arabidopsis NMD pathway. Expression mediated by the regulatory regions of UPF3 is significantly induced by salt stress. We found that both a deficiency and a strong excess of UPF3 expression are detrimental to plant resistance to salt stress. This indicates that UPF3 plays a role in plant response to salt stress, and that balanced expression of the UPF3 gene is essential for coping with this stress.