Morphological, Physiological and Transcriptional Analyses Provide New Insights into the Abnormal Phenotype of the Macroalga Agarophyton Vermiculophyllum (Rhodophyta)

Agarophyton vermiculophyllum is an important macroalga. However, a strain of this species with a phenotype exhibiting malformed branch (MB), located at the third-generation branch was discovered in Zhanshan Bay, China. This phenotype was visibly different from the normal one with regular branch (RB). A. vermiculophyllum with MB was cultivated in nutrient-rich seawater (Provasoli medium) and it gradually returned to the middle state (MS) displaying bent branches and to the recovered state (RS), in which branch morphology was nearly the same as that of RB. Stereoscopic, microscopic, and ultramicroscopic observations revealed that the MB and MS presented irregular edges. The epidermal cells in the MB were fewer and loosely distributed. The thylakoid lamellae in the MB and RB were indistinct. Many starch particles were detected in the RB. The pigment contents and photosynthetic efficiency in both these branch types were significantly reduced compared with those of branches in the MS and RS. Accordingly, the genes associated with photosynthesis and thylakoid membrane were downregulated in both the MB and RB, while those correlated with polysaccharide binding, starch binding, and carbohydrate binding were upregulated in the latter. The phosphate concentration in seawater where A. vermiculophyllum with MB was collected was below the average level. The decreased pigment contents and photosynthetic efficiency in both MB and RB may be related to phosphorus limiting conditions. Under these conditions, genes related to phosphorus metabolism were downregulated in the MB, which was not observed in the RB. Gene co-expression analyses revealed that 35 genes, associated with stress responses were not expressed in the MB, suggesting that these branches may not be able to effectively respond to low phosphorus stress. The mitogen-activated protein kinase (MAPK) signaling pathway was the most significantly enriched between the MB and other branch types. In addition, the MB exhibited the lowest the number of upregulated MAPK genes. The MAPK signaling pathway may be involved in branch development and the response to low phosphorus stress. The strains of A. vermiculophyllum and Gracilariopsis lemaneiformis showing irregular branches showed similarities as well as differences. The results of this study will contribute to elucidating the characteristics of irregular branches in algal species and to the development of the macroalgal germplasm, assisting in breeding programs that aim to achieve specific macroalgal morphologies.