POPLAR BARK STORAGE PROTEIN AND A RELATED WOUND-INDUCED GENE ARE DIFFERENTIALLY INDUCED BY NITROGEN

Poplars (Populus deltoides Bartr. ex Marsh) accumulate a 32-kD bark st orage protein (BSP) in phloem parenchyma and xylem ray cells during au tumn and winter. Accumulation of poplar BSP is associated with short-d ay (SD) photoperiods. Poplar BSP shares sequence similarity with the p roduct of the wound-inducible poplar gene win4. The influence of nitro gen availability and photoperiod on the levels of BSP, BSP mRNA, and w in4 mRNA was investigated. In long-day (LD) plants BSP, BSP mRNA, and win4 mRNA levels were correlated with the amount of NH4NO3 provided to the plant. BSP mRNA and BSP were detected only in bark, whereas win4 mRNA was detected only in leaves. In LD plants treated with NH4NO3, BS P mRNA levels were significantly greater than those of win4. In nitrog en-deficient plants exposed to SD conditions, the accumulation of BSP mRNA and BSP was delayed for 2 weeks. This delay was eliminated by fur ther SD exposure, and after 6 weeks of SD treatment similar levels of BSP and BSP mRNA were detected in the bark of SD plants regardless of the level of NH4NO3 treatment. win4 mRNA levels declined to undetectab le levels in young leaves of SD plants but increased in mature leaves. These results indicate that BSP accumulation in both LD and SD plants is influenced by nitrogen availability. Although both BSP and win4 ap pear to be involved in nitrogen storage, our data suggest that BSP is probably the primary protein involved in both seasonal and short-term nitrogen storage in poplar. These results also suggest that nitrogen c ycling and storage in poplar could involve a two-component system. In this system the win4 gene product may modulate accumulation and mobili zation of leaf nitrogen, whereas BSP is involved in seasonal and short -term nitrogen storage during periods of excess nitrogen availability.