Leaves of Field-Grown Mastic Trees Suffer Oxidative Stress at the Two Extremes of their Lifespan

Leaf senescence is a complex phenomenon occurring in all plant species, but it is still poorly understood in plants grown in Mediterranean field conditions and well-adapted to harsh climatic conditions. To better understand the physiological processes underlying leaf senescence in mastic trees （Pistacia lentiscus L.）, we evaluated leaf growth, water and N content, photosystem II （PSII） photochemistry, lipid peroxidation and levels of photosynthetic pigments, antioxidants, abscisic acid, and salicylic acid and jasmonic acid during the complete leaf lifespan, from early expansion to late senescence in relation to natural climatic conditions in the field. While mature leaves suffered from water and N deficit during late spring and summer, both young （emerging） and old （senescing） leaves were most sensitive to photo- oxidative stress, as indicated by reductions in the Fv/Fm ratio and enhanced lipid peroxidation during late autumn and winter. Reductions in the FvlFm ratio were associated with low ^-tocopherol （vitamin E） levels, while very old, senescing leaves additionally showed severe anthocyanin losses. We have concluded that both young （emerging） and old （senescing） leaves suffer oxidative stress in mastic trees, which may be linked in part to suboptimal temperatures during late autumn and winter as well as to low vitamin E levels.

Abscisic Acid Connects Phytohormone Signaling with RNA Metabolic Pathways and Promotes an Antiviral Response that Is Evaded by a Self- Controlled RNA Virus

A complex network of cellular receptors,RNA targeting pathways,and small-molecule signaling provides robust plant immunity and tolerance to viruses.To maximize their fitness,viruses must evolve control mechanisms to balance host immune evasion and plant-damaging effects.The genus Potyvirus comprises plant viruses characterized by RNA genomes that encode large polyproteins led by the P1 protease.A P1 autoinhibitory domain controls polyprotein processing,the release of a downstream functional RNAsilencing suppressor,and viral replication.Here,we show that P1Pro,a plum pox virus clone that lacks the P1 autoinhibitory domain,triggers complex reprogramming of the host transcriptome and high levels of abscisic acid(ABA)accumulation.A meta-analysis highlighted ABA connections with host pathways known to control RNA stability,turnover,maturation,and translation.Transcriptomic changes triggered by P1Pro infection or ABA showed similarities in host RNA abundance and diversity.Genetic and hormone treatment assays showed that ABA promotes plant resistance to potyviral infection.Finally,quantitative mathematical modeling of viral replication in the presence of defense pathways supported self-control of polyprotein processing kinetics as a viral mechanism that attenuates the magnitude of the host antiviral response.Overall,our findings indicate that ABA is an active player in plant antiviral immunity,which is nonetheless evaded by a self-controlled RNA virus.