CP's differential role in mono- and bipartite begomovirus movement and its role in phloem access for monopartite begomoviruses

Begomoviruses (family Geminiviridae), the most wide-spread and devastating single-stranded DNA viruses, rely heavily on their coat protein (CP) for essential functions including encapsidation, DNA binding (both single-stranded and double-stranded), in planta movement, insect transmission, and virus transport inside the insect. This study aimed to assess whether the continuous provision of CP is an essentiality in the begomovirus movement. To do so, the CP genes of two monopartite begomoviruses (pedilanthus leaf curl virus and cotton leaf curl Kokhran virus), and a bipartite begomovirus (tomato leaf curl New Delhi virus), were mutated by introducing a stop codon in the N-terminal region, non-overlapping region to the (A)V2 gene, to disrupt CP function. To ensure continuous CP provision (expression) at the site of inoculation and within the phloem, the CPs of these three begomoviruses were cloned under the cauliflower mosaic virus 35S promoter and within potato virus X vector. The CP mutant viral clones were agro-infiltrated into Nicotiana benthamiana plants to verify the objectives. The results demonstrated that CP mutation abolished the viral infectivity and spread in monopartite begomoviruses but not in bipartite begomoviruses. Notably, transient expression of CP, particularly through PVX and 35S promoter, significantly increased the presence of viral DNA in newly developing leaves, suggesting improved viral movement. This suggests that the continuous presence of CP is necessary for efficient spread, particularly in monopartite begomoviruses, likely by facilitating plasmodesmatal access to phloem cells for systemic infection. However, these findings are preliminary and require further investigation to draw definitive conclusions. Nevertheless, they have significant implications for understanding begomovirus biology and developing novel control strategies.