Fig. 8

Model of retro-anterograde communication proposed for albino plantlets of A. angustifolia. In green plantlets, environmental factors such as light trigger chloroplast biogenesis through the activation of nuclear genes encoding chloroplastic proteins, among which are the PPRs (1). This interorganellar nucleus-plastid communication is part of anterograde signaling. PPR transcripts must be translated in cytoplasmic ribosomes and subsequently translocated to plastids, where they participate in RNA processing and in the maturation of the proplastids into mature and functional chloroplasts. On the other hand, in the numerous proplastids of the albino Agave plantlets, a possible signal of unknown origin (2) seems to activate a retrograde response (plastid-to-nucleus). We propose that when this signal reaches the nucleus, it activates the overexpression of PPRs, which is part of a compensatory retro-anterograde response (3) whose objective is to correct the blockage in plastid biogenesis. It is possible that the overexpression of PPRs whose targets are RNAs of genes related to translation and the ribosome could play a central role in this corrective action. However, this compensatory response is unable to remove the proplastid from this immature and undifferentiated stage and, therefore, does not reverse the albino phenotype