PEROXIDASES AND WALNUT (Juglans nigra L.) WOOD COLOURING

Roles of JnRAP2.6-like from the Transition Zone of Black Walnut in Hormone Signaling

“…Expression of JnRAP2-like was similar in the TZ and inner sapwood of the tree harvested in fall. Because heartwood formation is thought to occur in autumn [ 4 , 9 , 10 , 15 ], the observed increase in expression of JnRAP2-like in xylem tissues harvested in fall relative to other tissues and other seasons may indicate it is associated with heartwood formation, possibly by influencing the transcription of genes in response to environmental signals such as shorter days or cold temperatures.…”

“…After returning to the lab, the cookies were transferred to an ultra-low freezer (-80 °C) for storage. TZs were identified under UV light [ 4 ] and carefully chiseled out of the cookies from each tree. Multiple cookies from a single tree were used to produce RNA; multiple extractions were required to obtain sufficient RNA for use as a single replicate.…”

“…Despite its economic importance, heartwood formation is poorly understood at both the physiological and molecular level [ 2 ]. Most research on heartwood formation has focused on a narrow transition zone (TZ) located between the sapwood and heartwood [ 1 , 3 , 4 ] because the death of all parenchyma cells at the TZ of angiosperms results in the formation of heartwood [ 5 - 7 ]. Heartwood formation in black walnut probably occurs at the time of cambial dormancy in the fall [ 4 , 8 - 10 ], but it can also be understood as a developmental process involving a complex series of events that take place throughout the year [ 11 ].…”

“…Most research on heartwood formation has focused on a narrow transition zone (TZ) located between the sapwood and heartwood [ 1 , 3 , 4 ] because the death of all parenchyma cells at the TZ of angiosperms results in the formation of heartwood [ 5 - 7 ]. Heartwood formation in black walnut probably occurs at the time of cambial dormancy in the fall [ 4 , 8 - 10 ], but it can also be understood as a developmental process involving a complex series of events that take place throughout the year [ 11 ]. Heartwood formation is strongly spatially regulated; in many ways it resembles programmed cell death (PCD) [ 5 , 12 - 14 ], but it is not clear how cell death in the TZ is related to the formation and deposition of extractives that produce the familiar colors of hardwood lumber [ 2 , 15 , 16 ], since not all tree species have darkly-colored heartwood.…”

“…Extractives are primarily phenolic compounds polymerized to varying degrees; most colored heartwood is associated with the accumulation of polyphenols, flavonoids and isoprenoids [ 17 ]. In black walnut, the TZ can be identified macroscopically because it fluoresces blue under UV light [ 4 ], perhaps because of the accumulation of sinapic acid esters [ 18 ]. Endogenous and exogenous factors can affect heartwood formation, including phytohormones, photoperiod and temperature [ 17 , 19 ].…”