Root Development PDF

Republic of the Philippines Benguet State University COLLEGE OF NATURAL SCIENCES DEPARTMENT OF BIOLOGY La Trinidad, Benguet ROOT DEVELOPMENT Bio 50. Developmenta...

Republic of the Philippines Benguet State University COLLEGE OF NATURAL SCIENCES DEPARTMENT OF BIOLOGY La Trinidad, Benguet ROOT DEVELOPMENT Bio 50. Developmental Biology CHAPTER ❷: Plant Developmental Biology Function of Roots absorption of H2O and nutrients (dissolved salts and minerals) conduction of absorbed materials into the plant body anchorage of plant in soil food storage  many roots have relationships with bacteria and fungi in the soil in the rhizosphere – contact zone between soil and root surface, often a few mm thick The radicle is the first root that is initiated. It penetrates into the soil and forms branches. Plants have different types of root. In a taproot root system, the radicle forms the primary root and add the lateral roots (branch roots.) In a fibrous root system, a  single radicle forms but, in addition, several other embryonic roots (seminal roots) forms just above the radicle. All roots then branch and form a fibrous system. Bio 50. Developmental Biology CHAPTER ❷: Plant Developmental Biology Development of  Roots The root tip is organized into regions and protected by a root cap The root cap protects the RAM; site of gravity perception, which controls the direction of root growth; constantly sloughed off  at the very tip, but new cells are added by RAM (sloughed-off cells can remain alive and provide nutrients in the rhizosphere) The root tip is organized into three regions RAM – derives the cells and tissues of the root Region of Elongation – cells are elongating in the roots Region of Maturation – basal to the regions of elongation; site of root hair formation and the maturation of other cell types A small, centrally located part of the RAM is called the quiescent center (QC) because its cells divide at an extremely slow rate. The function of the QC is not exactly known, but it seems to be activated during times of acute stress. It may be a site for the synthesis of plant hormones important for controlling root development. The RAM forms three primary meristems: protoderm, ground meristem, and procambium; these then go on to become the primary tissues of the root Bio 50. Developmental Biology CHAPTER ❷: Plant Developmental Biology Root Nodule Development NITROGEN FIXATION  the process by which nitrogen is taken from its molecular form (N2) in the atmosphere and converted into nitrogen compounds useful for other biochemical processes Biological nitrogen fixation is an extremely important process in the soil, carried out by several groups of bacteria able to absorb elemental nitrogen from the atmosphere and combine it into compounds that serve as nutrients from plants. NODULE a swelling on the root of a leguminous plant that contains bacteria of the  genus Rhizobium, capable of nitrogen fixation NITROGENASE a complex, bacterial enzyme that catalyzes the ATP dependent reduction of dinitrogen (N2) to ammonia (NH3) sensitive to inactivation by O2; low conc. of free O2 in nodules LEGHEMOGLOBIN oxygen-binding heme proteins to which legumes bind O2 most abundant proteins in nodules, giving a heme-pink color Symbiosis between legumes and Rhizobium is not obligatory. Legume remains unassociated throughout life cycle. Rhizobium occurs as free-living organisms in the soil. Nitrogen limited conditions – Symbionts seek each other out through  signaling ⭢ infection ⭢ development of nitrogen-fixing nodules; These involve specific genes: a. Nodulin genes plant genes specific to nodules b. Nodulation (nod) genes Rhizobium genes that participate in nodule formation The nod genes are classifies as common nod genes or host specific nod genes. COMMON NOD GENES (nodA, nodB, nodC) present in all rhizobial strains code for lipooligosaccharide signaling molecule – root hair curling, formation of infection threads, cortical cell divisions for nodulation HOST-SPECIFIC NOD GENES (nodP, nodQ, nodH, nodF, nodE, and nodL) differ among rhizobial species and determine the host  range nodD constitutively expressed protein product – NodD – regulates transcription of other nod genes SYMBIOSIS ❶ Recognition !"#$%&"'( (*+,!'&%*&"* $,-.'(-,) Bacteria roots secretes ESTABLISHMENT of the chemical attractants: (iso)flavonoids and betaines Bacteria0"(1-lectins (glycoprotein) of the root hair cell wall Bacteria.$'12*,- factors ⭢ root hair curling ⭢ Rhizobium proliferate within the curl Cell wall of root hair degrades allowing bacteria direct access to outer surface of the plant plasma membrane  Chemical attractants%*&"3%&,-NodD"(12*,-other nod genes transcription ESTABLISHMENT of the SYMBIOSIS  ❷ Infection Bacteria penetrates the plant. During the recognition stage, bacteria binding causes the root hairs to curl so that the tip bends back on the hair shaft, trapping some of the bacteria. The root hair stops growing in length but cellulose deposition continues inside hair to form the infection thread. Infection thread begins as an internal tubular extension of plasma membrane caused by fusion of Golgi derived vesicles at site of infection. Thread grows at its tip by fusion at end of tube.  Bacteria secrete enzymes that digests the plant cell wall and this allow the bacteria to get across the root hair cells. Bacteria becomes embedded inside infection thread and as it grows carries bacteria through the epidermal cells and root  cortex. The infection thread reaches the end of the cell and its membrane fuses with the membrane of root hair cell and releases free bacteria into the apoplast. Rhizobium are released into apoplasts and degrade the cell wall to gain access to the plasma membrane and infection process repeats itself. Infection threads extend and branch until they reach cortical cells where bacterial cells released into the cytosol. The bacterial then become enveloped by a membrane produced by the cortical cells, the peribacteroid membrane. ESTABLISHMENT of the SYMBIOSIS ❸ Differentiation Once inside the root cortical cells, bacteria differentiate into structures called bacteroids. After differentiation, bacteria become competent to fix nitrogen: they now can synthesize nitrogenase. Infected cortical cells form a meristem, the nodule meristem growth out through the cortex as it  produces the tissue of the nodule. When bacteria stops dividing, they enlarge and differentiate into nitrogen-fixing, endosymbiotic organelles called bacteroids enclosed in peribacteroid membranes.