Reproducti ve Biology of Plants Reproducti ve Biology of Plants Editors B.M. lohri 0 P.S. Srivastava Springer-Verlag Narosa Publishing House EDITORS Dr. B.M. Johri Central Reference Library University of Delhi Delhi-l \0 007, India Dr. P.S. Srivastava Centre for Biotechnology, Faculty of Science lamia Hamdard, New Delhi-l \0 062, India Copyright © 2001 Narosa Publishing House Softcover reprint of the hardcover 1s t edition 200 I All rights reserved. No part of this publicatioJl may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publishers. Exclusive distribution in North America (including Canada and Mexico), Europe and Japan by Springer-Verlag Berlin Heidelberg New York. All export rights for this book vest exclusively with Narosa Publishing House. Unauthorised export is a violation of Copyright Law and is subject to legal action. ISBN 978-3-642-50135-7 ISBN 978-3-642-50\33-3 (eBook) DOl 10.1007/978-3-642-50133-3 Preface The Reproductive Biology of Plants is a multiauthored edited book meant to be studied by the undergraduate and postgraduate students, and even the research scholars. The plant viruses, bacteria, cyanobacteria, some algae and some fungi (Chapters 2 to 6) do not have reproduction comparable to other groups of plants. Therefore, especialized (and highly especialised) mode of reproduction in the above groups is discussed on the basis of their physiology, biochemistry and cell biology. The archegoniates (bryophytes, pteridophytes and gymnosperms) have several common features, as have the seed plants (gymnosperms and angiosperms). These features are amply brought out in Chapters 8 to II. Reproductive biology is fully illustrated (Chapters 2 to 10), and theoretical aHd evolutionary tendencies are discussed. A glossary to help the students, teachers and other readers to follow the description more meaningfully, and have a better understanding of the subject is provided. There is plant index which should be very helpful in locating specific information. All the contributors are experienced teachers and researchers, and have made every possible effort to present a comparative account of reproductive processes in plants. January 2001 B.M. JOHRI P.S. SRIVASTAVA Acknowledgements We wish to thank, and express our gratitude to: The contributors who accepted our invitation and prepared various chapters. Professor S.S. Bhojwani, Department of Botany, University of Delhi, for critically examining the final proofs. Narosa Publishing House, New Delhi, especially Mr. N.K. Mehra and Mr. M.S. Sejwal, and Editors of Springer-Verlag, Heidelberg, for processing and publishing this book. Mr. R.K. Gupta, University of Delhi, for typing. Mr. Vishwanath (Raj pal & Sons, Delhi) for advice and encouragement. Mr. M.L. Saini and Mr. V.N. Vashishta of Delhi University Library for facilities to complete this work. B.M. JOHRI P.S. SRIVASTAVA List of Contributors 1. Asthana, A.K. 10. Sambali, Geeta Bryology Lab. Department of Botany National Botanical Research Institute Jammu University Lucknow 226001. India Jammu 180 00 I. India 2. Biswas, Chhaya 11. Sen, Sumitra J-1931 Chittaranjan Park Department of Botany New Delhi 110 019, India Calcutta University 35 Ballygunge Circular Road 3. Johri, B.M. Calcutta 700 019, India Central Reference Library University of Delhi 12. Singh, Nandita Delhi 110 007, India Department of Biotechnology Hamdard University 4. Krishnamurthy, K.V. Hamdard Nagar, New Delhi 110062 Department of Life Sciences India Bharathidasan University Tiruchirapalli 620 024, India 13. Srivastava, P.S. Department of Biotechnology 5. Krishnamurthy, V Hamdard University 15 Ramanathan Street Hammdard Nagar, New Delhi 110 062 T. Nagar, Chennai 600 017, India India 6. Koul,A.K. 14. Srivastava, Sheela Department of Biosciences Department of Genetics Jammu University University of Delhi, South Campus Jammu 180001, India New Delhi IlO 021, India 7. Kumar, Krishna 15. Upreti, D.K. Department of Botany Lichenology Lab Gargi College Cryptogamic Botany University of Delhi South Campus National Botanical Research Institute Siri Fort Road, New Delhi IlO 021, India Rana Pratap Marg. P.O. 436 8. Nath, Virendra Lucknow 226 00 1. India Bryology lab. 16. Verma, Anupam National Botanical Research Institute CAS Virology Lucknow 226 001. India Division of Mycology and Plant Pathology 9. Ramachandran, Padma Indian Agricultural Research Institute CAS Virology New Delhi 110 012, India Division of Mycology and Plant Pathology Indian Agricultural Research Institute New Delhi 110012, India Contents PREFACE v ACKNOWLEDGEMENTS Vll LIST OF CONTRIBUTORS ix I. Reproductive Biology of Plants 1 B.M.lohri 2. Replication of Plant Viruses 2 Anupam Varma and Padma Ramachandran 3. Reproductive Biology of Bacteria 22 Sheela Srivastava 4. Reproductive Biology of Cyanophycota 45 V. Krishnamurthy 5. Reproductive Biology of Eukaryotic Algae 57 V. Krishnamurthy 6. Reproductive Biology of Fungi 96 A. K. Koul and G. Sumbali 7. Reproductive Biology of Lichens 127 K. V. Krishnamurthy and D.K. Upreti 8. Reproductive Biology of Bryophytes 148 Virendra Nath and AK. Asthana 9. Reproductive Biology of Pteridophytes 175 Krishna Kumar 10. Reproductive Biology of Gymnosperms 215 Chhaya Biswas and B.M. lohri II. Reproductive Biology of Angiosperms 237 B.M. lohri, P.S. Srivastava and Nandita Singh 12. Cytology and Genetics of Reproduction 273 Sumi(ra Sen 13. Concluding Remarks 304 B.M. lohri 14. Future Studies 306 B.M. lohri and P.s. Srivastava PLANT INDEX 309 1 Reproductive Biology of Plants B.M. Johri The reproductive phase in the life cycle of plants is very significant and absolutely essential for diversity and the evolution of plants from simpler to more complex forms and structures. Knowledge of the origins and diversification of land plants is based on dispersal of spores and megafossils. The megafossils are first recognized roughly 50 million years (Myr) after the appearance of land plants. Three plant-based epochs are recognized (see Crane and Kenrick 1997). Eoemhryophytic (432 Myr). Spore tetrads appear. The decay-resistant wall (sporopollenin) and tetrahedral configuration (implies meiosis) are diagnostic features of land plants. Eotracheophytic (432-402 Myr). There is decline in diversity of tetrads, and there is dominance of individually-dispersed simple spores (homworts, some mosses, and early vascular plants). Eutracheophytic (398-256 Myr). Diversity of spores and megaphylls increases, and there is a substantial increase in diversity of land plants, including the early diversification of many important living groups. Phylogenetic studies favour a single origin of land plants from charophycean green algae. A fresh water origin is likely. The absence of well-developed sporophytes, gametophytes with sexual organs of land plant-type, cuticle and sporopollenin-walled spores suggest that these innovations evolved during the transition to land. Morphological differentiation occurred both in gametophytic and sporophytic phases (Crane and Kenrich 1997). Subsequently, there was significant reduction in gametophytes and an increase in sporophytes. This will be clearly borne out by the facts mentioned in the various chapters. Reference Crane PR, Kenrick P (1997), Diverted development of reproductive organs: A source of morphological innovation in land plants. PI Syst Evol 206: 161-174 2 Replication of Plant Viruses Anupam Varma and Padma Ramachandran The year 1998 is the centenary year of the discovery of viruses. Critical experiments by Beijerinck (1898) proved for the first time that tobacco mosaic disease was not caused by a bacterium or any corpuscular organism. He called this agent 'contagium vivum fluidum'. Since then, the science of virology has come a long way and has played an important role in our understanding of modem biology. The isolation and crystallization of tobacco mosaic virus (TMV) by Stanley (1935), demonstration that TMV particles contain nucleic acid of the ribose type by Bawden and Pirie (1937), the findings of Hershey and Chase (1952) that the protein of T2 bacteriophage does not enter bacterial cells during infection, and those of Fraenkel-Conrat (1956) and Gierer and Schramm (1956) that the nucleic acid ofTMV is the main infective component laid the foundations of molecular biology and biotechnology. Viruses are nucleoproteins which contain either DNA or RNA as the genetic material. The proportion of nucleic acid and protein varies from virus to virus. Generally anisometric viruses like TMV contain 5% nucleic acid and 95% protein whereas isometric viruses like cucumber mosaic virus (CMV) have 15 to 45% nucleic acid and the remaining protein. Some viruses also contain lipids, mainly forming an envelope, as in tomato spotted wilt virus (TSWV). The other constituents are water and metallic ions. The most important component of viruses is nucleic acid, which is required for virus replication. It is, therefore, natural that the nucleic acid forms the base for defining viruses as "transmissible parasites whose nacleic acid genome is less than 3 x 108 Da in weight and that need ribosome and other components of their host cells for multiplication" (Gibbs and Harrison, 1976). This definition not only includes the large viruses belonging to Poxviridae but also the smallest known pathogens, the viroids, which are naked single stranded RNA of about 375 nucleotides capable of causing severe diseases in plants. In this chapter, we briefly discuss the process of replication of plant viruses which form the largest group of all the known viruses infecting bacteria, fungi, plants, invertebrates and vertebrates. 1. Types of Plant Viruses The majority of plant viruses are RNA viruses. Until the late 1960s it-was thought that all plant viruses were RNA viruses. It made Bawden (1964) observe: "It would be premature yet to assume this is generally true, because most viruses that infect bacteria, and some that infect animals, contain deoxy nucleic acid, and there seems to be no a priori reason why viruses containing deoxynucleic acid should not infect flowering plants. However, if there are any, they await discovery". He was right, the discovery came within a few years of his observations, that cauliflower mosaic virus (CaMV) is a DNA virus (Shephard et al. 1970); later, CaMV became an important tool in the hands of biotechnologists. Now we know that several DNA viruses infect plants, although the relative number of DNA viruses infecting plants compared with Replication of Plant Viruses 3 the RNA viruses is small (Fig. 1). On the basis of nucleic acid, plant viruses can be grouped as dsDNA, ssDNA, dsRNA and ssRNA viruses. The ssRNA viruses can be further subgrouped as ssRNA(+) and ssRNA(-). Some of the ssRNA(-) are enveloped and some naked-lacking even the coat protein (Fig. 1). The most preferred form of the genome is ssRNA( +) which must have been selected for efficient replication utilizing host machinery. The two groups of enveloped ssRNA viruses are similar to those found infecting invertebrates and vertebrates. These appear to be animal viruses which moved and established in plants during evolution. So far, no enveloped DNA virus has been found to infect plants. It is a matter of time before such viruses are also found. Plant viruses not only vary in the nature of their genome but also in shape and size. Basically, they either have spiral symmetry, forming anisometric particles, or icosahedral symmetry, forming isometric particles; virus particles are also referred to as 'virions'. Many plant viruses have split genomes consisting of two or more distinct nucleic acid molecules encapsidated in different or similar sized particles made of the same coat-protein subunits. Among the anisometric viruses hordei-, tobra- and furoviruses have split genomes encapsidated in particles of different sizes (Fig. 1). The alfamovirus has four genomic molecules encased in particles of four different sizes 19,36,48 and 58 nm long and 19 nm wide. Among the isometric viruses, ilarviruses and the viruses belonging to Bromoviridae, Comoviridae, Geminiviridae and Partitiviridae have divided genomes. The total mass of nucleoprotein of different virus particles varies from 4.6 to 73 million Da. The weight of nucleic acid alone is between 1-3 million Da per virus particle for most of the viruses; sometimes it may be as high as 16 x 106 Da, as in the case of wound tumor virus, of the reovirus group. The protein shell which constitutes the rest of the mass of the virus particle, is made up of one kind of coat protein subunit. The subunits of coat-protein are packed in regular arrays either to give icosahedral or spiral symmetry. The amino acid sequence is identical in the coat-protein subunits of a given virus, but it varies from virus to virus and even strains of the same virus. The interaction between the protein and viral nucleic acid determines the stability of the virus particle. 2. Strategies for Replication Plant viruses, like all other viruses, differ greatly from other microorganisms, as the viruses completely depend on the host machinery for replication. The replication of a virus starts the moment an infective particle enters a susceptible cell. A general course of events in the life cycle of a virus is: (1) entry of the virus particle into living susceptible cell, (2) disassembly or 'unwinding of the virus particle' , (3) transcription or synthesis of "mRNA" of viruses other than those with positive sense ssRNA, (4) translation or synthesis of proteins required for virus replication, and (5) maturation of virus particles and their movement to the neighbouring cell (Fig. 2). The main strategies which the viruses use for replication are: 1. Subgenomic (sg) nucleic acid. The synthesis of one or more subgenomic nucleic acid molecules enables the production of proteins in desired amounts. 2. Polyprotein. The viral genome may code for one polyprotein from the whole genome using single open reading frame (ORF). The polyprotein is then cleaved at specific sites by a viral-coded protease/s to give a final gene product. 3. Multipartite genome. In a large number of viruses the genome is in the form of two or more molecules which may be encapsidated in one particle or different particles. 4. Readthrough proteins. The termination codon of the 5' gene may be 'leaky' and allow a proportion of ribosomes to carryon translation to another stop codon downstream from the first, giving rise to a second larger functional polypeptide. 5. Transframe proteins. Two proteins may commence at the same 5' AUG codon by a switch of reading frame to give a second longer transframe protein.