DNA replication and cell cycle in plants: learning from geminiviruses

Abstract

Plant cell growth and development depend on continuous cell proliferation which is restricted to small regions of the plant called meristems. Infection by geminiviruses, small DNA viruses whose replicative cycle relies on host cell factors, is excluded from those proliferating areas. Since most of the replicative factors are present, almost exclusively, in proliferating cells, geminivirus infection is believed to induce a cellular state permissive for viral DNA replication, e.g. S-phase or, at least, some specific S-phase functions. The molecular basis for this effect seems to be the interference that certain geminivirus proteins exert on the retinoblastoma-related (RBR) pathway, which analogously to that of animal cells, regulates plant cell cycle activation and G(1)-S transition. In some cases, geminiviruses induce cell proliferation and abnormal growth. Mechanisms other than sequestering plant RBR probably contribute to the multiple effects of geminivirus proteins on cellular gene expression, cell growth control and cellular DNA replication. Current efforts to understand the coupling of geminivirus DNA replication to cell cycle and growth control as well as the directions in which future research is aiming are reviewed.

Fig. 1. Genetic organization of geminiviruses. The Geminiviridae family is composed of three genera, distinguished according to their genetic organization, plant host and insect vector: mastrevirus (maize streak virus, MSV; wheat dwarf virus, WDV; bean yellow dwarf virus, BeYDV; tobacco yellow dwarf virus, TobYDV, among others), curtovirus (beet curly top virus; BCTV, among others) and begomovirus (bean golden mosaic virus, BGMV; tomato golden mosaic virus, TGMV; African cassava mosaic virus, ACMV; squash leaf curl virus, SqLCV; tomato yellow leaf curl virus, TYLCV, among others). The genetic organization of mastreviruses (the map corresponds to WDV) is unique in that (i) they contain a large (LIR) and a small (SIR) intergenic region, with which a small DNA molecule (presumed primer) is associated in the ssDNA form, (ii) the complementary sense major transcript (c-sense, toward the left) encodes the RepA protein (Dekker et al., 1991; Liu et al., 1998) and, after a splicing event, first observed in WDV (Schalk et al., 1989), the replication initiator protein Rep, and (iii) the virion sense transcripts (v-sense, toward the right), which may also undergo splicing (Wright et al., 1997), encode the movement protein (MP), also involved in nuclear transport of viral DNA (Liu et al., 1999a) and systemic infection, and the capsid protein (CP). Curtoviruses (the map corresponds to BCTV) encode, on the c-sense strand, Rep, the C2 protein, a DNA replication enhancer protein (REn) and the C4 protein, involved in symptom development and induction of cell division (Stanley and Latham, 1992; Stanley et al., 1992; Latham et al., 1997), and MP, the V2 protein, which may have some role in DNA replication (Hormuzdi and Bisaro, 1993), and the CP on the v-sense strand. Begomoviruses (the map corresponds to TGMV) have, apart from a few exceptions, two genomic components (A and B). The A component encodes, on the c-sense, Rep (Elmer et al., 1988; Etessami et al., 1991), a transcriptional activator protein (TrAP; Sunter and Bisaro, 1991; Hartitz et al., 1999), REn (Elmer et al., 1988) and a C4 protein, and the CP on the v-sense. The B component encodes proteins involved in intra- and intercellular viral movement (BC1 and BV1; Lazarowitz, 1999). The invariant TAATATT↓AC sequence, located in the intergenic regions, contains the initiation site (↓) of rolling circle DNA replication.

Fig. 2. A simplified version of the geminivirus DNA replication cycle and intercellular movement of viral DNA. Geminivirus DNA replication occurs in two stages. First, the ssDNA is converted into dsDNA with the participation of cellular factors. The dsDNA serves as template for viral gene expression. Secondly, the dsDNA initiates the rolling circle phase, with the participation of viral and cellular factors, to produce new ssDNA products. These can (i) re-enter the DNA replication pool, (ii) associate with CP or (iii) be transported outside the nucleus and to the neighbouring cell, most probably through plasmodesmata, with the help of viral MPs.

Fig. 3. Model proposed for the interference of geminivirus proteins with the retinoblastoma-related (RBR) pathway. The G₁–S transition in plants is controlled, most probably, by a pathway dependent on the retinoblastoma-related (RBR) protein which associates with the E2F–DP heterodimeric transcription factor(s). In cycling cells, phosphorylation of RBR by a CDK–cyclin complex (Nakagami et al., 1999) is thought to release active E2F–DP complexes required to activate G₁–S transition and S-phase progression. S-phase-specific gene expression also occurs through other mechanisms. Geminivirus RBR-binding proteins (RepA and Rep) would bypass this control by sequestering RBR from the ternary RBR–E2F–DP complex.

Fig. 4. Impact of geminivirus proteins on different host cell pathways. Viral proteins produced from the transcriptionally active dsDNA template have effects on a number of cellular pathways, some of which are still a matter of speculation. See text for details.