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Two isolates of Turnip mosaic virus (UK 1 and JPN 1), representative of two different viral strains, induced differential alterations on secondary cell wall (SCW) development in Arabidopsis thaliana , suggesting cell-type specific effects of these viral infections. These potential effects were analyzed in inflorescence stems and flowers of infected plants, together with other possible cellular effects of the infections. Results obtained from macroscopic and histochemical analyses showed that infection with either virus significantly narrowed stem area, but defects in SCW were only found in JPN 1 infections. In flowers, reduced endothecium lignification was also found for JPN 1, while UK 1 infections induced severe floral cell and organ development alterations. A transcriptomic analysis focused on genes controlling and regulating SCW formation also showed notable differences between both viral isolates. UK 1 infections induced a general transcriptional decrease of most regulatory genes, whereas a more complex pattern of alterations was found in JPN 1 infections. The role of the previously identified viral determinant of most developmental alterations, the P3 protein, was also studied through the use of viral chimeras. No SCW alterations or creeping habit growth were found in infections by the chimeras, indicating that if the P3 viral protein is involved in the determination of these symptoms, it is not the only determinant. Finally, considerations as to the possibility of a taxonomical reappraisal of these TuMV viral strains are provided. [ABSTRACT FROM AUTHOR]

Summary: The transcription factor KNOTTED ARABIDOPSIS THALIANA7 (KNAT7) is a Class II KNOTTED1‐like homeobox (KNOX2) gene that, in interfascicular fibres, acts as a negative regulator of secondary cell wall biosynthesis. In addition, knat7 loss‐of‐function mutants display an irregular xylem (irx) phenotype, suggesting a potential positive regulatory role in xylem vessel secondary cell wall deposition. Although our understanding of the role of KNAT7 is evolving, the function(s) of the closely related KNOX2 genes, KNAT3, KNAT4, and KNAT5, in secondary wall formation still remain unclear. We found that all four Arabidopsis KNOX2 genes were expressed in the inflorescence stems. However, only the knat3 knat7 double mutants showed a phenotype, displaying an enhanced irx phenotypes relative to the single mutants, as well as decreased interfascicular fibre cell wall thickness. Moreover, knat3 knat7 double mutants had reduced stem tensile and flexural strength compared with wild‐type and single mutants. In contrast, KNAT3 overexpression resulted in thicker interfascicular fibre secondary cell walls in inflorescence stems, suggesting a potential positive regulation in interfascicular fibre secondary wall development. This work identifies KNAT3 as a potential transcriptional activator working together with KNAT7 to promote secondary cell wall biosynthesis in xylem vessels, while concurrently acting antagonistically with KNAT7 to influence secondary wall formation in interfascicular fibres. Significance Statement: This work identifies KNAT3 as a potential transcriptional activator working together with KNAT7 to promote secondary cell wall biosynthesis in xylem vessels, while concurrently acting antagonistically with KNAT7 to influence secondary wall formation in interfascicular fibres. [ABSTRACT FROM AUTHOR]

In plants, plasma membrane-embedded CELLULOSE SYNTHASE (CESA) enzyme complexes deposit cellulose polymers into the developing cell wall. Cellulose synthesis requires two different sets of CESA complexes that are active during cell expansion and secondary cell wall thickening, respectively. Hence, developing xylem cells, which first undergo cell expansion and subsequently deposit thick secondary walls, need to completely reorganize their CESA complexes from primary wall- to secondary wall-specific CESAs. Using live-cell imaging, we analyzed the principles underlying this remodeling. At the onset of secondary wall synthesis, the primary wall CESAs ceased to be delivered to the plasma membrane and were gradually removed from both the plasma membrane and the Golgi. For a brief transition period, both primary wall- and secondary wall-specific CESAs coexisted in banded domains of the plasma membrane where secondary wall synthesis is concentrated. During this transition, primary and secondary wall CESAs displayed discrete dynamic behaviors and sensitivities to the inhibitor isoxaben. As secondary wall-specific CESAs were delivered and inserted into the plasma membrane, the primary wall CESAs became concentrated in prevacuolar compartments and lytic vacuoles. This adjustment in localization between the two CESAs was accompanied by concurrent decreased primary wall CESA and increased secondary wall CESA protein abundance. Our data reveal distinct and dynamic subcellular trafficking patterns that underpin the remodeling of the cellulose biosynthetic machinery, resulting in the removal and degradation of the primary wall CESA complex with concurrent production and recycling of the secondary wall CESAs. [ABSTRACT FROM AUTHOR]

Conifers possess an array of physical and chemical defences against stem-boring insects. Stone cells provide a physical defence associated with resistance against bark beetles and weevils. In Sitka spruce ( Picea sitchensis), abundance of stone cells in the cortex of apical shoots is positively correlated with resistance to white pine weevil ( Pissodes strobi). We identified histological, biochemical and molecular differences in the stone cell phenotype of weevil resistant (R) or susceptible (S) Sitka spruce genotypes. R trees displayed significantly higher quantities of cortical stone cells near the apical shoot node, the primary site for weevil feeding. Lignin, cellulose, xylan and mannan were the most abundant components of stone cell secondary walls, respectively. Lignin composition of stone cells isolated from R trees contained a higher percentage of G-lignin compared with S trees. Transcript profiling revealed higher transcript abundance in the R genotype of coumarate 3-hydroxylase, a key monolignol biosynthetic gene. Developing stone cells in current year apical shoots incorporated fluorescent-tagged monolignol into the secondary cell wall, while mature stone cells of previous year apical shoots did not. Stone cell development is an ephemeral process, and fortification of shoot tips in R trees is an effective strategy against insect feeding. [ABSTRACT FROM AUTHOR]