Abstract
BackgroundThe walnut shell, which is composed of a large number of sclereids originating from the lignified parenchyma of the endocarp, plays an important role in fruit development and during harvesting and storage. The physical and chemical properties of walnut shells are closely related to the lignin content. Laccase is the key enzyme responsible for lignin biosynthesis by the polymerization of monolignols and plays crucial roles in secondary cell wall formation in plants. In this study, we screened and identified laccase family genes from the walnut genome and investigated the expression of laccase during endocarp lignification in walnut.ResultsA total of 37 laccase genes were screened from the walnut genome and distributed on nine chromosomes and classified into 6 subfamilies, among which subfamily IV showed distinct expansion. We observed that endocarp lignification started 44 days after flowering (DAF), and at later periods, the lignin content increased rapidly, with growth peaks at 44–50 DAF and 100–115 DAF. The lignification of the endocarp proceeded from the outside to the inside, as demonstrated by section staining in combination with endocarp staining. Furthermore, the changes in the expression of laccase family genes in the endocarp at different developmental stages were studied, and JrLACs showed different expression trends. The expression of nine genes showed significant increase after 44 DAF, and among these, JrLAC12–1, JrLAC12–2 and JrLAC16 showed a significant change in expression at the lignification stage. A study of the expression of JrLACs in different tissues and at various endocarp developmental stages revealed, that most JrLACs were expressed at low levels in mature tissues and at high levels in young tissues, in particular, JrLAC12–1 showed high expression in the young stems. A significant positive correlation was found between the expression of JrLAC12–1 and the variation in the lignin content in the endocarp.ConclusionLaccase genes play an important role in the lignification of the walnut endocarp, and JrLACs play different roles during fruit development. This study shows that JrLAC12–1 may play a key role in the lignification of endocarp.
Highlights
The walnut shell, which is composed of a large number of sclereids originating from the lignified parenchyma of the endocarp, plays an important role in fruit development and during harvesting and storage
Laccase genes play an important role in the lignification of the walnut endocarp, and JrLACs play different roles during fruit development
This study shows that JrLAC12–1 may play a key role in the lignification of endocarp
Summary
The walnut shell, which is composed of a large number of sclereids originating from the lignified parenchyma of the endocarp, plays an important role in fruit development and during harvesting and storage. Much attention was given to the quality and nutrition of walnut kernels, and the crucial role of the shell was often ignored. The shell plays an important role in fruit development [3], during harvest and transport [4, 5], and even during the process of storage [6]. The shell is the endocarp of the walnut fruit and consists of sclereids derived from lignification of the parenchyma originating from the primary meristem [13]. Zhao et al [16] confirmed that the development of walnut shells is closely related to lignin biosynthesis, and lignin is the major factor affecting the shell structure [13]
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