Identification of XTH Family Genes and Expression Analysis of Endosperm Weakening in Lettuce (Lactuca sativa L.)

Seed germination requires the relaxation of endosperm cap and radicle cell walls, with cell wall hydrolases playing a significant role in this process. Our study revealed that a type of cell wall hydrolase, xyloglucan endotransglucosylase, may significantly contribute to endosperm weakening during lettuce seed germination. Through bioinformatics analysis, the XTH gene family in lettuce was divided into five subfamilies localized on nine chromosomes. Notably, there were significant differences in gene structure among the members of the LsXTHs family containing 1–4 exons and 20 conserved motifs. Among these motifs, motif1, motif2, and motif3 encoded the XTH structural domain. The promoter regions of LsXTHs contained a large number of cis-acting elements responsive to various abiotic stresses, such as drought, anaerobiosis, low temperature, high temperature, and salt stress. Germination experiments showed that seeds imbibed in water and 5 μmol/L abscisic acid (ABA) were able to achieve typical germination with radicle protrusion from the endosperm cap, achieving germination of 100% and 36%, respectively. Conversely, in 0.3% sodium dichloroisocyanurate (SDIC), the swollen seeds were unable to germinate or complete atypical germination, resulting in a germination rate of 30%. Compared to the control, the mechanical strength of the endosperm cap of seeds imbibed in 0.3% SDIC for 8 h increased by 14%, indicating that SDIC may inhibit seed germination by enhancing the mechanical strength of the endosperm cap. Enzyme activity analysis revealed that during lettuce seed germination, XTH enzyme activity in the endosperm cap was significantly higher than in other tissues and increased gradually with imbibition. Transcriptome analysis of the endosperm cap detected the expression of 10 LsXTH genes. Among these, LsXTH43 exhibited the highest expression during germination and was significantly upregulated two-fold by high temperatures, suggesting a potential role in the high-temperature germination of lettuce seeds. Additionally, SDIC downregulated the expression of LsXTHs to varying degrees, with the expression of LsXTH15 reduced to only 6% of its original level. Low temperature, high temperature, drought, and salt stress all reduced the expression of most LsXTHs to different degrees; when seeds germinated under waterlogging and cadmium stress, LsXTH6, LsXTH7, LsXTH8, LsXTH32, and LsXTH33 were all upregulated to some extent.