Agrobacterium LBA 4404 cells successfully attached to Chondrus cells Under the SEM, short, rod-shaped bacterial cells about 0.8 × 2 µm were seen attached in large numbers (an estimated average of one bacterial cell per µm 2) to the Chondrus thallus surface at the wound sites. Whole colonies were observed in a fibrillar mesh (Figure 2). Protocol for eliminating Agrobacterium using cefotaxime The results of testing cefotaxime concentrations of 500 and 800 mg l−1 confirmed that this cefotaxime-supplemented wash protocol ensured that residual Agrobacterium and other culturable bacteria were eliminated on the thalli within 6 days (Table 1). Agrobacterium successfully delivered pCAMBIA 1301 into Chondrus thalli The methodology for transformation developed using pCAMBIA 1301 followed by the established wash protocol included controls to detect any false positives occurring from indigenous activity of beta-glucoronidase in Chondrus (unwounded and untransformed thalli, Figure 3C), indigenous activity from LBA4404 bacterial cells (pin-pricked thalli co-cultivated with untransformed Agrobacterium LBA 4404 bacteria, Figure 3B), physiological response of algal cells to wounding (pin-pricked thalli cultivated without bacteria, Figure 3A) and vector components (pin-pricked thalli co-cultivated with Agrobacterium LBA 4404 bearing a construct where the GUS gene is absent; pRI 910 binary vector, Figure 3D). None of these controls were stained. However, thalli transformed with pCAMBIA 1301 were stained blue (Figure 3E). Manipulating transformation conditions can enhance transformation efficiency As shown in Figure 4, among the pin-pricked thalli, a significant increase in transformation efficiency (higher percentage of thallus segments stained blue) was observed when seawater was used in preference to Induction Medium (Mann-Whitney test, p = 0.029) in the presence of acetosyringone. However, no transformation was detected in the unwounded thallus segments or those wounded by bombardment when co-cultivation was conducted using seawater. In addition, without the presence of acetosyringone, transformation occurred at a significantly lower efficiency of less than 10% (Mann-Whitney, p = 0.004). Agrobacterium co-cultured with thalli was eliminated by consecutive washes and incubated in fresh antibiotic. Every 2 days, the thallus segments were rinsed for 4 min with sterile seawater six times before incubation for 48 h in fresh seawater with replenished cefotaxime. A thallus segment was streaked on a LBA plate which was incubated at 28 ° C for up to 4 days to check for the persistence of Agrobacterium. This was conducted in triplicates (n = 3). Colonies appearing from periodic streaks of thalli on LBA plates were counted. Successful expression obtained with the Chondrus -specific expression cassette Using the Agrobacterium transformation protocol developed using pCAMBIA 1301, the constructed pRI 910 (Ac- GUS) was delivered into thallus cells using seawater as the co-cultivation medium and with the same controls as for pCAMBIA 1301. Blue colouration was apparent at the sites of tear, cut or wounding on Agrobacterium -mediated transformed thalli when compared to controls (Figure 5). Transverse sections through the tissue revealed blue localization of the GUS precipitate within the peripheral cortical cells and central medullary filaments. In general, a darker staining intensity was observed with pRI 910 (Ac- GUS). Furthermore, a higher transformation efficiency of 85% was obtained with this Chondrus -specific construct as opposed to 52% with pCAMBIA 1301.

Published as part of Ramessur, Anusha Devi, Bothwell, John H., Maggs, Christine A., Gan, Sook Yee & Phang, Siew Moi, 2018, Agrobacterium-mediated gene delivery and transient expression in the red macroalga Chondrus crispus, pp. 499-510 in Botanica Marina (Warsaw, Poland) (Warsaw, Poland) 61 (5) on pages 502-503, DOI: 10.1515/bot-2018-0028, http://zenodo.org/record/11360085