diff --git a/tuf/api/metadata.py b/tuf/api/metadata.py
index 873ab85f..e7cc9a6d 100644
--- a/tuf/api/metadata.py
+++ b/tuf/api/metadata.py
@@ -32,7 +32,7 @@
 import io
 import logging
 import tempfile
-from datetime import datetime
+from datetime import datetime, timedelta
 from typing import (
     IO,
     Any,
@@ -120,11 +120,11 @@ class Metadata(Generic[T]):
     def __init__(
         self,
         signed: T,
-        signatures: Dict[str, Signature],
+        signatures: Optional[Dict[str, Signature]] = None,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ):
         self.signed: T = signed
-        self.signatures = signatures
+        self.signatures = signatures or {}
         self.unrecognized_fields: Mapping[str, Any] = unrecognized_fields or {}
 
     def __eq__(self, other: Any) -> bool:
@@ -480,11 +480,13 @@ def expires(self, value: datetime) -> None:
     # or "inner metadata")
     def __init__(
         self,
-        version: int,
-        spec_version: str,
-        expires: datetime,
-        unrecognized_fields: Optional[Mapping[str, Any]] = None,
+        version: Optional[int],
+        spec_version: Optional[str],
+        expires: Optional[datetime],
+        unrecognized_fields: Optional[Mapping[str, Any]],
     ):
+        if spec_version is None:
+            spec_version = ".".join(SPECIFICATION_VERSION)
         # Accept semver (X.Y.Z) but also X.Y for legacy compatibility
         spec_list = spec_version.split(".")
         if len(spec_list) not in [2, 3] or not all(
@@ -497,11 +499,15 @@ def __init__(
             raise ValueError(f"Unsupported spec_version {spec_version}")
 
         self.spec_version = spec_version
-        self.expires = expires
 
-        if version <= 0:
+        self.expires = expires or datetime.utcnow() + timedelta(days=1)
+
+        if version is None:
+            version = 1
+        elif version <= 0:
             raise ValueError(f"version must be > 0, got {version}")
         self.version = version
+
         self.unrecognized_fields: Mapping[str, Any] = unrecognized_fields or {}
 
     def __eq__(self, other: Any) -> bool:
@@ -838,20 +844,22 @@ class Root(Signed):
     # pylint: disable=too-many-arguments
     def __init__(
         self,
-        version: int,
-        spec_version: str,
-        expires: datetime,
-        keys: Dict[str, Key],
-        roles: Mapping[str, Role],
-        consistent_snapshot: Optional[bool] = None,
+        version: Optional[int] = None,
+        spec_version: Optional[str] = None,
+        expires: Optional[datetime] = None,
+        keys: Optional[Dict[str, Key]] = None,
+        roles: Optional[Mapping[str, Role]] = None,
+        consistent_snapshot: Optional[bool] = True,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ):
         super().__init__(version, spec_version, expires, unrecognized_fields)
         self.consistent_snapshot = consistent_snapshot
-        self.keys = keys
-        if set(roles) != TOP_LEVEL_ROLE_NAMES:
-            raise ValueError("Role names must be the top-level metadata roles")
+        self.keys = keys or {}
 
+        if roles is None:
+            roles = {r: Role([], 1) for r in TOP_LEVEL_ROLE_NAMES}
+        elif set(roles) != TOP_LEVEL_ROLE_NAMES:
+            raise ValueError("Role names must be the top-level metadata roles")
         self.roles = roles
 
     def __eq__(self, other: Any) -> bool:
@@ -1129,14 +1137,14 @@ class Timestamp(Signed):
 
     def __init__(
         self,
-        version: int,
-        spec_version: str,
-        expires: datetime,
-        snapshot_meta: MetaFile,
+        version: Optional[int] = None,
+        spec_version: Optional[str] = None,
+        expires: Optional[datetime] = None,
+        snapshot_meta: Optional[MetaFile] = None,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ):
         super().__init__(version, spec_version, expires, unrecognized_fields)
-        self.snapshot_meta = snapshot_meta
+        self.snapshot_meta = snapshot_meta or MetaFile(1)
 
     def __eq__(self, other: Any) -> bool:
         if not isinstance(other, Timestamp):
@@ -1190,14 +1198,14 @@ class Snapshot(Signed):
 
     def __init__(
         self,
-        version: int,
-        spec_version: str,
-        expires: datetime,
-        meta: Dict[str, MetaFile],
+        version: Optional[int] = None,
+        spec_version: Optional[str] = None,
+        expires: Optional[datetime] = None,
+        meta: Optional[Dict[str, MetaFile]] = None,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ):
         super().__init__(version, spec_version, expires, unrecognized_fields)
-        self.meta = meta
+        self.meta = meta if meta is not None else {"targets.json": MetaFile(1)}
 
     def __eq__(self, other: Any) -> bool:
         if not isinstance(other, Snapshot):
@@ -1660,15 +1668,15 @@ class Targets(Signed):
     # pylint: disable=too-many-arguments
     def __init__(
         self,
-        version: int,
-        spec_version: str,
-        expires: datetime,
-        targets: Dict[str, TargetFile],
+        version: Optional[int] = None,
+        spec_version: Optional[str] = None,
+        expires: Optional[datetime] = None,
+        targets: Optional[Dict[str, TargetFile]] = None,
         delegations: Optional[Delegations] = None,
         unrecognized_fields: Optional[Mapping[str, Any]] = None,
     ) -> None:
         super().__init__(version, spec_version, expires, unrecognized_fields)
-        self.targets = targets
+        self.targets = targets or {}
         self.delegations = delegations
 
     def __eq__(self, other: Any) -> bool: