FIGURE 1.

Proposed model for the selective movement of mRNA and protein from companion cells to sieve elements in grafted cucurbits. When grafted plants experience stress, the source tissues produce poly(A)-transcripts that are transcribed in companion cells, can be recognized by RNA-binding proteins, and selectively move through plasmodesmata into sieve elements. These mobile transcripts then bind with phloem proteins to form RNP complexes with specific motifs such as poly-CU, TLS, or 5-methylcytosine methylation sites. Furthermore, they are targeted to sink cells in the phloem flow and are unloaded into destination cells, where they are translated and function.